# PWM Controller for Ceramic Heaters?



## gdirwin (Apr 7, 2009)

I am looking for a PWM controller which will take a 200VDC (pack voltage) and an analog input signal (from a digital temperature controller) and generate a variable output voltage to feed ceramic heating elements.

My interior heat problem is that I have an intermediate pack voltage that does not fall nicely into 120 or 240 chunk sizes for standard AC ceramic elements ( see http://www.diyelectriccar.com/forums/showthread.php/ceramic-heater-and-160vdci-38696.html ). If I use 160VDC (which can be 190VDC fully charged) with a 120VAC rms heater, this will draw much higher currents and the wattage will be 250% (it is a function of voltage squared). This is uncomfortable for a device that sits a few inches from your face, and there is little precedence/experience out there for voltages higher than 144V. The inverse resistance characteristic of ceramic elements should work, but looking for alternatives...


Instead, I thought of using an inexpensive digital temperature controller in the dash, something like:









which uses a PID feedback controller (ie set and forget) which would measure and actively regulate the interior temperature. This would output a 0-10V analog output (0V for all heat off, 10V for full heat). This would be used instead of an open loop manual temperature setting dial in the dash.


The trick is to find a PWM controller (or even a DC/DC converter or AC inverter) rated for 200VDC, and 4 - 5000 Watts, which will accept the analog input signal from the temperature controller, and then generate a variable/limited output voltage (AC or DC).

Pros:
- closed loop temperature control (set and forget)
- ability to limit voltages applied to ceramic elements
- all ceramic elements are used at the same time (so will wear more evenly)
- most of the digital temperature controllers can also be interfaced with car computers via RS232 etc...

It may also be cost-effective as compared to using many DC relays or solid state relays (SSR) to switch numerous ceramic stages on/off. I could probably start waving about how many accidents this would save (ie eyes on the road instead of fiddling with temperature controls in cars).


Ideas/suggestions/possible suppliers?

It seems I want a cut-down mini version of a HV DC motor controller (ie using a temperature differential input instead of a throttle input).


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## MJ Monterey (Aug 20, 2009)

I like the concept. Replying so I can lurk quietly in the background.....


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## MJ Monterey (Aug 20, 2009)

Had a second thought on this. 

You may want to have a temp sensor in the heater box to make sure that you do not over temp the materials in the box. 

So two temp sensors. One to not over temp or start a fire in the heater box. Second temp sensor in the cabin for fine control.

Back to fly on the wall mode......


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## gdirwin (Apr 7, 2009)

I agree - an independent safety cut-out is a good idea. I was planning to use the stock thermistor over-temperature cut-out switch that comes with most el-cheapo ceramic heaters - if the ceramic elements get too hot (ie plastic melting hot) they will cut-out on their own (independent of the main interior digital thermometer and controller etc...). Since I will use 2 ceramic elements (1500W each) there will be 2 cut-outs (one for each side). Since everything is wired in parallel, there should be no problem if one cuts-out and the other doesn't.



I have been looking around a bit to try to find a relatively inexpensive off-the-shelf commercial controller:
- that will take an 0 - 10VDC analog input signal from the digital temperature controller
- proportionally generate a HV output signal to the ceramic heaters
- rated for 200VDC and at least 4000W

Nothing pops out yet, but there must be something out there...

Possible devices:
- flyback type chopper
- DC/DC converter
- IGBT/GTO AC inverter
- Buck converter
- possibly a motor controller?

Anyone aware of a reasonably priced controller that will do the job?


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## Tesseract (Sep 27, 2008)

Interesting idea, gdirwin... I've been thinking of making something like this for awhile but to feed a water heater element because it would be far easier - if not necessarily cheaper overall - to reuse the existing heater core. 

Anyway, a couple of observations/opinions I have on this matter: the control input has to be isolated from the pack voltage; consider making the control signal digital (on or off) and only have the converter ensure the heating element receives its rated RMS voltage.

Now, if the battery voltage can ever go below 120V then a simple buck converter might not be the best choice. That said, any topology besides a buck will require much more expensive magnetics and output capacitors. So, how much are you willing to pay for this thus-far unobtainable module, and do you need it to accommodate sub-120V battery packs?


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## gdirwin (Apr 7, 2009)

Thanks for your reply Tesseract - I am hoping to find something off-the-shelf to keep things economic - likely less than $200 for the digital controller and DC switching converter...




So far the simplest/most promising/economic combination seems to be:
- digital PID temperature controller which has direct pulsed or PWM analog output
- using solid state relays (SSR) for switching of the DC load

I have not nailed down the exact temp controller to be used (must have pulsed or PWM output), but they are out there. I would also want a controller which allows the PID gains to be adjustable (almost all seem to be), has an upper limit (which would limit the effective voltage applied to my ceramic heater load) and can comunicate to a computer (future car-pc).

The dc SSRs seem to have a max off time of up to 1 msec, which would limit the PWM switching frequency to about 500 Hz. They are available for 200 or 400 VDC and from 12A to 40A. Multiple parallel units can be used, and would require a heatsink. I believe the ceramic heating load is mainly resistive (and hopefully wiring length is kept short), so surge suppression diodes are probably not required.




If anyone else can keep digging too, that would be helpful...


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## lhogberg (Oct 30, 2009)

If you can do basic soldering and know something about electronic circuits you could use the XR2206 (http://www.newark.com/exar/xr2206cp-f/monolithic-function-generator-ic/dp/24R2365) to generate a triangle wave with an offset proportional to your desired temperature. Then use a comparator to compare the offseted triangle wave with another signal that is proportional to the actual temperature (from the temperature sensor). The output from the comparator will be a PWM signal with the same frequency as the triangle wave. If you use this output to drive a solid state relay (or even better just a MOSFET or IGBT which can be switched over 20kHz, so it won't be audible) that switches the pack voltage to the ceramic element, the duty cycle will be regulated to achieve the desired temperature. You would also want a IR2130 or other MOSFET/IGBT-driver to decrease the switching losses.

I can draw you a schematic if you're interested, I've built similar circuits before, not for temperature control though.


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## Dave Koller (Nov 15, 2008)

gdirwin said:


> I was planning to use the stock thermistor over-temperature cut-out switch that comes with most el-cheapo ceramic heaters - if the ceramic elements get too hot (ie plastic melting hot) they will cut-out on their own (independent of the main interior digital thermometer and controller etc...).


 I was curious - if the problem everyone has with running the AC heaters on DC would fuse the cut-out switch... seems like that has be one of the problems - BUT by PWM would we have LESS of this contact/fuse problem? it is not zero crossing but at points is at zero... Tesseract? .. errr gate it to release at zero in the PWM.


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## gdirwin (Apr 7, 2009)

I would think that by reducing the average output voltage (by using PWM etc..) then the thermal cut-outs would not activate. 

Have their been problems with thermal cut-outs with high DC voltages? The inverse resistance should increase R as the temperature rises, thus self-limiting the current...



As Lhogberg pointed out, one potential drawback to directly using the pulsed output of a digital temperature controller into a solid state relay, is the reduced switching frequency... 500 Hz is in the audible range - but I am not sure if this would result in a whining noise or not...

The Curtis motor controller is famous for its whine at startup... 

Any thoughts - high frequency switching of SSRs - whiny or not?


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## Dave Koller (Nov 15, 2008)

I believe the temperature cut-outs are just bimetallic type contact switches. I think on just DC they may work and arc and fuse.. Same old thing we see with relays on high voltage/high current DC.. 

I run robots at 100 HZ and I don't hear much whine or tone from the motors.. Think about the 60 cycles from AC on the ceramic heaters - they are not giving off massive 60 cycle hum are they ?

I think the low frequencies on a huge motor might make noise but not so much on an element from a heater! 

P.S. don't the thermal cutouts work on the temp of the air across them NOT the current thru the wire?


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## Tesseract (Sep 27, 2008)

Dave Koller said:


> I was curious - if the problem everyone has with running the AC heaters on DC would fuse the cut-out switch... seems like that has be one of the problems - BUT by PWM would we have LESS of this contact/fuse problem? it is not zero crossing but at points is at zero... Tesseract? .. errr gate it to release at zero in the PWM.


Yep - driving the heater elements with PWM'ed DC (ie- not using an LC filter to convert back to pure DC) uhhh... "rectifies" the arcing problem that otherwise destroys mechanical switches used on DC. (bad pun, I know... sorry)

W/r/t to whining from the elements... there is likely going to be a frequency that the elements will resonate at, but as long as you don't PWM them at that particularly frequency you won't hear a thing from them. After all, they don't hum at 60Hz, do they?

BTW - the reason the motor in an EV might hum (like the infamous Curtis whine at 1.5khz) is because you are pumping a lot more current through the windings, and the strength of the vibrations in the windings is proportional to the current flowing through them. If you could cram 500A through the ceramic heater without immediately destroying it I'm sure it'd sing just as loudly as a motor, too.


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## gdirwin (Apr 7, 2009)

I am giving up - spent way too much Google time trying to find a suitable digital temperature controller.

If anyone wants to keep going, you should look for the following characteristics:
- PID digital temperature controller
- Pulsed Voltage Output to SSR
- -40C operating temperature
- 12V DC input power supply
- RS232 communication and free software
- 1/16 DIN (48x48mm = 1.9" x 1.9")
- 1/32 DIN (24x48mm = 0.94" x 1.9")

There are many models which would work with the SSRs via pulsed output, however the big problem is that these are all process controllers which are meant to work in a nice warm factory - they are not rated to operate at -40C (which is when you really want heat!).

Another possible route is to go to a junkyard and look for fancy temp controllers in BMW/Audi/Mercedes/Lexus etc... and see how they do it... Likely these connect to mechanical doors to change air flow etc.., so maybe not much help.


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## lhogberg (Oct 30, 2009)

I've heard cheramic heater elements switched with IGBTs at between 100 and 500 Hz with voltages from 100v to 300. And they sure make noise even at small currents like 1 amp. This is not an annoying whine because of the low frequency, but still very audible. You cannot compare this with AC-current, the noise occours in the switching moment when the current goes from 0 to something in a microsecond or so. That's the same reason why AC-fans fed with a pure sinus wave are much more quiet than Thyristor-controlled fans.


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## Dave Koller (Nov 15, 2008)

lhogberg said:


> I've heard cheramic heater elements switched with IGBTs at between 100 and 500 Hz with voltages from 100v to 300. And they sure make noise even at small currents like 1 amp. This is not an annoying whine because of the low frequency, but still very audible. You cannot compare this with AC-current, the noise occours in the switching moment when the current goes from 0 to something in a microsecond or so. That's the same reason why AC-fans fed with a pure sinus wave are much more quiet than Thyristor-controlled fans.


Lots of harmonics also - I am sure!


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## lhogberg (Oct 30, 2009)

Dave Koller said:


> Lots of harmonics also - I am sure!


Yea of course, what I'm trying to say is that a resistive load switched at 500 Hz or below is much more pleasant to the ear than something at e.g. 2 kHz.


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## Tesseract (Sep 27, 2008)

lhogberg said:


> ...You cannot compare this with AC-current, the noise occours in the switching moment when the current goes from 0 to something in a microsecond or so....


Good point about the abrupt increase in current being more likely to cause the elements to sing.


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## lhogberg (Oct 30, 2009)

Tesseract said:


> Good point about the abrupt increase in current being more likely to cause the elements to sing.


thanks.

a wild idea.
what is the maximum voltage at the motor side of your motor controller?
Connect a cheramic element with an ordinary thermostat in parallell with the motor. Done! =) You might want to put inductor in series with the element to make the controller happy? Maybe tesseract can comment on that.


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## Tesseract (Sep 27, 2008)

lhogberg said:


> ...
> what is the maximum voltage at the motor side of your motor controller?
> Connect a cheramic element with an ordinary thermostat in parallell with the motor. Done! =) You might want to put inductor in series with the element to make the controller happy? Maybe tesseract can comment on that.


Hmmm... well, you the maximum motor voltage is programmable in the Soliton1, so you certainly could place a heating element in parallel with the motor and protect the element from ever seeing an RMS voltage higher than it is rated for. The additional current required by the element subtracts from the current available to the motor, but few people complain about losing 15-20A out of 500-1000A 

No additional inductance is necessary to make the controller happy. It can still regulate current at less than 1uH (that is, this is the lowest I have tested so far, using coiled copper tubing as the "load").

Hmmm... still, having a dedicated heater output still makes more sense if for no other reason than you really want the heat to work right when you get into the car. In fact, with a tank-style electric heater you could pre-heat the cabin of the car before you even get into it. Maybe use a door-lock signal to start it up with the keyless remote. Hmmm... I'm liking the possibilities here (and I live in a place where Winter barely deserves the name).


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## gdirwin (Apr 7, 2009)

I lied - I did not completely give up...

I ordered the following:
http://www.auberins.com/index.php?main_page=product_info&cPath=14&products_id=83

It will measure the temperature of the car interior, compare to a setpoint (which you can adjust via the controls), go through a PID controller (with adjustable/tuneable parameters), then generate a variable duty-rate signal to a solid state relay (SSR). Not bad for less than 40 bucks!

I will use one of the controllers and feed 2 x 40Amp/200VDC SSR relays (in parallel to get the full current rating with some safety factor). Each SSR will drive a 1500W ceramic heater (3000W total) - should get things toasty quickly.

This method will use all heating elements at the same time (they get cycled on/off/on/off at an adjustable frequency - I will probably use every 2 seconds or so).

It allows continuous and self-regulating temperature control, without having to use numerous SSR relays (ie for low, medium, high etc...).

2 things I am not sure about:
1) How this thing will react to -40 temperatures - I will route a portion of the air from the fan into a tube, and direct it onto the controller in the dash... If it turns on at the beginning, it should be good to go...

2) How a 120VAC (rms) ceramic heater will work with 190V DC (50 TS cells, 3.2 V nominal each, 3.8 fully charged).

Once the parts arrive, I will first test with an AC SSR relay, take some pics etc...


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## Amberwolf (May 29, 2009)

If that temperature controller doesn't work, you can look for controllers meant for "burn-in" of electronics in aircraft use. Those have to measure and control temperatures at least down to -40C and up to +70C, as those are typical temperature levels aircraft equipment is required to be factory tested at.

I don't remember who made the ones we used at Honeywell/Sperry CFSG back in the early 90s, but they were probably made in the 70s. I'm sure there are updated versions of them available these days.http://www.girlcamfriend.com/webcam/shaved/


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## madderscience (Jun 28, 2008)

Or the stupid simple approach to dealing with the problem of making a heater core work with an intermediate voltage pack:

add an electrical tap. (crimp, spot weld, or high-temp solder) 

I've seen it done with ceramic heaters and of course it would be trivial with a nichrome wire based heater like the toaster I am using.

Also, I bet you'd still get useful heat out of two 120V ceramic cores in series, running off of 200V. Ceramic heat elements are safe to run at lower voltages due to their temperature/resistance curve.


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## gdirwin (Apr 7, 2009)

So far so good! Here is my AC test setup...









The controller regulates the temperature in the car to an exact set-point, and will ramp up or down according to the measured temperature. The brown cable out the top is the temperature sensor. 12V power input to the controller. Cables from the controller go into the input of the AC SSR relay (3-12V). The SSR relay is in series with the AC loop with the heater (be careful about which side you put it on to avoid shocks - see the SSR relay manual).

The test setup is hooked up to an AC 120V SSR (I am still waiting for my batteries to arrive, but eventually will use 2 SSR DC relays - one for each heater element, but controlled by the same signal from the controller).

I will add another DC SSR relay in the output loop from the controller to the main SSR relays - this 2nd relay will be controlled by the fan motor - this way the heater will turn on only if the fan motor is on at least low (SSR relays normally turn on if the input voltage is above 3V DC). Unfortunately this means two more DC SSR relays (but very low-rated ones).

Observations:
- controller works well, and operates off of 12V supply
- PID gains, settings can all be changed
- auto-tune feature puts a step onto the temperature set-point (have not tried this yet - will wait until it is in the car).
- tried a K Sensor (1 deg accuracy) but also have a PT100 RTD sensor (0.1 deg accuracy)
- it ramps up from 0 (mostly off) when you first turn it on
- totally silent (no clicks from the controller of the SSR relay)
- PWM frequency adjustable from 0.5 Hz and lower... I have it set to 0.1 Hz (every 10 seconds the on-off pattern repeats)
- power is remembered when the 12V DC power is lost
- also has a simpler limit controller with adjustable settings (although PWM SSR control is better).
- Temperature sensor is quite sensitive - breathing on it will show a response in 1 or 2 seconds...
- digital read-out shows measured temperature (C or F) - press an arrow and the set-point is shown flashing and can be moved up/down - 3 seconds later the flashing stops and it reverts back to displaying measured temp.
- a light on the controller comes on whenever the output is triggered - this maybe a bit annoying showing continuous flashing, but is handy - if the light is always on you know that you are drawing lots of power from the main pack and may want to put on long-undies and reduce the set-point.

I think this is an excellent solution instead of manual high,med,low type controllers - this way all elements are used at the same time. There is enough "thermal inertia" with a 10 second window so that the heat does not appear to be on/off/on/off - it seems relatively constant...

All comments appreciated - can't wait for my batteries to arrive and to try this out... Still not sure about how this will work at -40C (guess I could put it in the freezer as a test), nor how the ceramic elements will work with a full charged pack (190V max, 160V nominal).


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## CFreeman54 (Jan 14, 2009)

How did all this work out? I'm also running a 50 LiFePO4 cell pack at 160V nominal, and just bought a ceramic core heater and deconstructed it. I was planning on wiring it in with a standard 12v 40A automotive relay, but now I am wondering if that relay can handle the voltage/current the heater will pull. Thoughts?


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## gdirwin (Apr 7, 2009)

I am re-visiting this controller (now that I am ready to actually install it).

The only problem with this small unit 
http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=83
is that it does not have a manual control mode - say you just want to crank the heat to defrost a window.

I found another similar (but bigger unit) that has manual control, and will display both the original and set-point temperature - here:
http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=132

This one also drives SSR relays, and will work off of 12V.

The three heater control methods I was toying with are:

1) Rotary switch (replacing the dash heater temperature) - off, 1 on, 2 on. Advantage: Simple, integrates well with the dash. Disadvantage: I think it will be too hot even with the fan on low - have to manually turn on/off/on/off. Needs 2 SSR relays.

2) PWM temperature controller - 1 or 2 SSR relays needed - the PWM controller has an LED display, with buttons to adjust the desired temperature set-point - a PI controller continually regulates the amount of time it is on vs off. 

3) I thought of an extension to 1), which used a 9 position rotary switch - each PWM element has 4 sub-elements, each of which can be controlled separately. This would probably be ideal, but you need 8 SSR relays...

The big advantage of the fancy controller is that it can automatically modulate how much time the controller is on vs off - ie it can lower the output to the desired level (like a continuous switch).

Anyone else want to share their experiences with heater controls? Is it a pain using one 1500 watt element - do you find yourself always turning it on/off/on/off?


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## rwaudio (May 22, 2008)

I've been contemplating the same type of thing, my '86 Porsche as an "analog" type of automatic climate control with a temperature sensor on the dash and a vacuum mechanism that adjusts the coolant flow through the heater core to regulate cabin temperature. Since I will be keeping the core and using a fluid heater I thought that the vacuum valve would be a good starting point for a "pot box" type control that would vary the input signal to a pwm type controller that fed the fluid heaters. So instead of changing the coolant flow it would change the power to the fluid heaters. 

As for the PWM for a 50/60hz AC heater it made me think about my background which is high power car stereo systems. Years ago there were guys who would power blenders and other small appliances off large audio amplifiers when fed with a 60hz tone. The hardest part with an audio amplifier is the power source, but most of us already have a huge DC setup that could eliminate the smps power supply. There are many high power class D amplifiers out there 3-10kw that are pretty efficient and if you found one with a blown power supply it might be able to adapt to run across the full pack voltage but with a tap at the center to create the +/- 60 to 144v rails (just make sure the amplifier is capable of those voltage rails). That way a simple control of a 50/60hz input signal could power these AC heaters with a nice clean AC waveform (not that they require it, but the safety features should remain functional).


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## tomofreno (Mar 3, 2009)

> They are available for 200 or 400 VDC and from 12A to 40A.


 Would you supply a link for these relays? Looking for a 120V/30A DC for my heater.


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## gdirwin (Apr 7, 2009)

tomofreno said:


> Would you supply a link for these relays? Looking for a 120V/30A DC for my heater.


I got my 40Amp/200VDC SSRs from here:
http://www.futurlec.com/Relays/SSRDC200V40A.shtml - I think there were about $28 each.

Crydom also has (D2D40) but they are quite a bit more I think...
http://www.crydom.com/en/Products/Catalog/1_dc.pdf

Note to get the full 40Amp rating, you have to mount to a heatsink (mine are mounted to a large chunk of Aluminum).


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## tomofreno (Mar 3, 2009)

Thanks for the link! That is what I was looking for!


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## tomofreno (Mar 3, 2009)

> ...is that it does not have a manual control mode - say you just want to crank the heat to defrost a window.


 Couldn't you just set the temperature 5-10 degree higher to keep it running? Thanks for a nice report! I think I may go this way to improve control of my heater and eliminate the P&B mechanical relay.


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## gdirwin (Apr 7, 2009)

Now that it is -30C outside, and I NEED heat, I thought I would expand further on my experience with this PWM temperature controller... I have been driving my RAV4 for a few months now, in some pretty cold temps...

More observations:
- when it gets too cold (maybe -10C?), the unit just flashes EEEE and hangs up - no heat. If I am near an AC plug, I have an interior heater that I can point at it for a few minutes, then it starts up okay. Not acceptable.

- I find that the PID gains cannot be increased enough - I find it too slow to ramp from 0% to 100% heat (takes a few minutes), even with a large temp differential. When I start, I end up putting it in manual control, cranking the heat to 100%, then leaving it on full (remember this is Winnipeg with -30 to -40C in January).

- 2 ceramic heating elements are minimal here - they are enough to keep the windows clear/clean, but will not keep you toasty warm the way a fully warmed up ICE will.

- two SSRs (one into each ceramic heating element) is a nice setup - just need to get the right controller.

I am now looking to design a simple manual controller which uses an analog pot input (connected to the heat blue/red dial) and then generates a PWM output signal... There are some 555 timer schemes I have seen, as well as chip-based circuits, but still not sure how these will work when it is -40.

Ideally it would have a manual mode (analog voltage input) and an auto mode (with temperature feedback), and would talk to a USB/serial device (eventually I will design a dash-pc and it would be good to have the full climate control included).

In short, I would not recommend this particular controller unless you are in warmer climates.

Back to the Google drawing board/search...


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## Roy Von Rogers (Mar 21, 2009)

I'm curious have you tried two ceramic units in series, and two of those in parallel, another words four in total.

Use a kilovac relay to energize, and if needed, use an aftermarket thermostat, used on add on electric radiator fans available in most auto stores for temp control.


Roy


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## gdirwin (Apr 7, 2009)

Roy Von Rogers said:


> I'm curious have you tried two ceramic units in series, and two of those in parallel, another words four in total.
> 
> Use a kilovac relay to energize, and if needed, use an aftermarket thermostat, used on add on electric radiator fans available in most auto stores for temp control.
> 
> ...


Hi Roy:

I only have room for 2 ceramic elements (in the place where my heater core was) - if I had room for more, I would put them all in parallel as my DC voltage is just a bit higher than 120*sqrt(2) (peak AC). If I used some in series, there would be much less current and less heat.

I thought of using a mechanical relay, but the SSR is more elegant/quieter, and allows me to use a continuous type control (instead of bang/bang on/off). If you have a temperature feedback through a thermostat etc... then a mech relay would be switching too often...

In hindsight though, it would have been just as practical and would have worked well with a simple off/med/hi switch (no need for a thermostat).


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## EVfun (Mar 14, 2010)

Please make sure a real relay is in the circuit somewhere, SSRs usually fail on!

I don't think you will find much need to PWM the heater core. Those elements really tend to target a given temperature because the way the resistance ramps sharply up at that point. They also pack some thermal mass. Perhaps one cycle every 10 seconds or so to cut the heat output, but turning down the fan also sharply cuts the heat output.


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## gdirwin (Apr 7, 2009)

EVfun said:


> Please make sure a real relay is in the circuit somewhere, SSRs usually fail on!
> 
> I don't think you will find much need to PWM the heater core. Those elements really tend to target a given temperature because the way the resistance ramps sharply up at that point. They also pack some thermal mass. Perhaps one cycle every 10 seconds or so to cut the heat output, but turning down the fan also sharply cuts the heat output.


The 0.1 Hz PWM signal is exactly what I use (SSRs take some time to turn off, so there is a limit on the max frequency). I agree that a "always on" scheme would also work, and to use the fan output to control the amount of heat (this is especially true for short trips where you rarely get the car to the temps you desire).

I agree with the "real relay" - I do not use one installed, and so far have only the SSRs. My fan control has an output when it is on (Low, Med or High) and I use this with a 12V relay to enable/disable the PWM signal - it would be wise to use the same signal to energize a higher power DC relay of sorts to ensure a turn off if a SSR fails.

I think I will stick to PWM using the OEM temperature dial/control - this way you can control the amount of heat and the fan separately (intuitive and similar to the way the stock controls work). 

If I can find a fancy device that accepts an analog Vin, then I can use the OEM controls or control it from a dash PC via an D/A out...


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## EVfun (Mar 14, 2010)

gdirwin said:


> I think I will stick to PWM using the OEM temperature dial/control - this way you can control the amount of heat and the fan separately (intuitive and similar to the way the stock controls work).
> 
> If I can find a fancy device that accepts an analog Vin, then I can use the OEM controls or control it from a dash PC via an D/A out...


You may find you need to control the PWM with a log pot to get the desired control. The ceramic heaters will try not to decrease power very much as you pull back from 100%. Dialing the heat knob (with a linear PWM control) back to 75% might get you 90% of the heat of full power because a slight drop in element temperature causes a sharper decrease in element resistance. You end up on 75% of the time but perhaps drawing 20% more amps during the on time (rough estimate on the effect.)


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## DIYguy (Sep 18, 2008)

Bump. I've been kicking this around a bit. I came to a similar conclusion as what I am reading here. 

I have a fluid heater, which is basically a tractor block heating element in my fluid loop to my heater core. I separated the thermal switch wiring and run this on 12 volts in series with the control side of a kilovac relay. My pack voltage is 211 nominal. I use an RV water pump (second pump version, very nice) to circulate. This system has worked flawlessly since I hooked it up (other than replacing the noisy pump). It draws about 22 amps therefore puts out about 4.5kw or a little more. The thermal switch works fine to turn off the element if it gets too hot, but it's really just for over temp control as I see it.

This amount of heat in a small truck cab is wonderful in the Canadian winter. . but other than fan speed control, I can't cut it back to save energy and reduce/control cabin temp well enough.

I want open loop PWM control to the heater element, and I want this to work from my OEM temp knob on the heater controls. This knob currently controls a mix of fresh air which I don't really want to use due to wasted energy.

So. . . I'm toying with the idea of using this http://www.ebay.com/itm/Constant-Current-PWM-HHO-PCU-250-Volt-/250875201338#ht_2645wt_1413

I'm just having some correspondence now to better understand the "constant current" description. . . as I can live with the current as determined by the circuit resistance, as long as I can PWM.

I'm interested to hear any feedback on this device linked to above for my application. 

Thanks.
Gary


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## m38mike (Dec 27, 2008)

gdirwin said:


> Hi Roy:
> 
> I only have room for 2 ceramic elements (in the place where my heater core was) - if I had room for more, I would put them all in parallel as my DC voltage is just a bit higher than 120*sqrt(2) (peak AC). If I used some in series, there would be much less current and less heat.
> 
> ...


As I look at your posts, and read some of the other comments, I'm encouraged that my simple system might work for me. I'm looking for some added heat, but want to keep the system as basic and simple as I can. Heat on or heat off. I'm thinking that I could use a 160+ VDC relay to turn the single ceramic element on and off, that I would put a 12vdc fan in parrallel with the relay so it would always be on when the relay was closed, and call it good. I've got a 144vdc pack. I've also thought about putting a 160v 15 amp fuse before the relay. I'm happy with simple manual control of the heat. I don't need any auto sensors. After all, this is for a 1952 Army Jeep, not a nice sports car.

I'm not as smart about the possible problems that this simple set up might face as most of you are, so I'd appreciate any comments on what I might need to change.


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## DIYguy (Sep 18, 2008)

Ok, sounds like it will do 30 amps at 250 volts at max setting. Should work fine I guess. I'm thinking to give it a try. A bit pricey tho. I would prefer to mount it under the hood to keep the high voltage out of the cabin, remove the pot and run wires to it's new home behind the temp knob. 



DIYguy said:


> Bump. I've been kicking this around a bit. I came to a similar conclusion as what I am reading here.
> 
> I have a fluid heater, which is basically a tractor block heating element in my fluid loop to my heater core. I separated the thermal switch wiring and run this on 12 volts in series with the control side of a kilovac relay. My pack voltage is 211 nominal. I use an RV water pump (second pump version, very nice) to circulate. This system has worked flawlessly since I hooked it up (other than replacing the noisy pump). It draws about 22 amps therefore puts out about 4.5kw or a little more. The thermal switch works fine to turn off the element if it gets too hot, but it's really just for over temp control as I see it.
> 
> ...


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## gdirwin (Apr 7, 2009)

m38mike said:


> I'm looking for some added heat, but want to keep the system as basic and simple as I can. Heat on or heat off. I'm thinking that I could use a 160+ VDC relay to turn the single ceramic element on and off, that I would put a 12vdc fan in parrallel with the relay so it would always be on when the relay was closed, and call it good...


My current setup taps off of the signal at the fan switch (which can be less than 12V), into a relay (with a low DC voltage on threshold), then into the SSR relays - ie the heat is full-on whenever the fan is on. This has been working well - I have only short drives and pre-heat the cabin, so use the heat just to keep the windows clear (or it has been sitting, in which case full heat is good too). I never drive for long enough so it gets too hot and need to back off the heat via PWM.

I kind-of gave up on the PWM temperature relay concept - the temperature controller I used freaks out when it gets too cold (although I heard from another Winnipegger that it is a problem with the sensor I used - I will dig a bit more). When you first start the car, the PWM controller starts out at a 0 duty cycle output, then the PI controller has to ramp up to full output - even at the highest gains (lowest time constant) it still takes a few minutes to get to 100% duty cycle. If it had a "initial output" setting this would be ideal...

I have driven my car for 1 full winter (every day, including the -40 days) as well as this winter - so far so good!


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## gdirwin (Apr 7, 2009)

DIYguy said:


> Bump. I've been kicking this around a bit. I came to a similar conclusion as what I am reading here.
> 
> I have a fluid heater, which is basically a tractor block heating element in my fluid loop to my heater core. I separated the thermal switch wiring and run this on 12 volts in series with the control side of a kilovac relay. My pack voltage is 211 nominal. I use an RV water pump (second pump version, very nice) to circulate. This system has worked flawlessly since I hooked it up (other than replacing the noisy pump). It draws about 22 amps therefore puts out about 4.5kw or a little more. The thermal switch works fine to turn off the element if it gets too hot, but it's really just for over temp control as I see it.
> 
> ...


Hi Gary - if you drive for long enough such that the fluid (and cabin) gets too hot, then I can see the need for an adjustable heat system... The SSR relays I have been using have been flawless - quiet/reliable - I think these are better then mechanical relays.

That device looks great, but you could probably do it for less with an SSR relay and a low-power 12V manual controller to generate the PWM signal.

You could use the PWM controller I found, with a fluid temperature sensor - this way you get a digital readout of the temperature, as well as feedback control of the temperature (ie set and forget). If you are interested in this unit, I will find out more about the ultra-cold problems I was having with it (I believe it was due to the wrong sensor). Another Winnipegger uses this controller in his car (he drives to/from the city on the highway, and modulates 6kW of electric heat) and told me he fixed the problem I was seeing with the PWM controller freaking out...


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