# PWM Heater



## EVfun (Mar 14, 2010)

Ceramic heaters don't really need voltage control in most applications. Their resistance drastically increases as they heat up, so the fan speed does a decent job of controlling the heat output.

If you do need to PWM them the duty cycle can be very slow. They have enough thermal mass than that you shouldn't need more than 10 cycles per minute. It could help if your pack voltage is a bit high for the elements, and be used to lower the vent air temperature at low fan speeds.


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## steelneck (Apr 19, 2013)

EVfun said:


> Ceramic heaters don't really need voltage control in most applications. Their resistance drastically increases as they heat up, so the fan speed does a decent job of controlling the heat output.


That's right, but the folks in the car certainly need it. With 4kW of heat when it is +5C outside, then the driver need to act PWM for him self and the passengers. If there is heating power enough for the coldest day, then there is a need for some sort of control every other day of the year.

Yes it could be done by having, say 4 PTC elements, and turn them on as needed, but it would be a much more beautiful solution if they all could be controlled stepless, no need for wiring a contactor for every single PTC element. Just connect pack and 12V control voltage and a potentiometer.



> If you do need to PWM them the duty cycle can be very slow. They have enough thermal mass than that you shouldn't need more than 10 cycles per minute.


Yes, this device have very low demands regarding the PWM part, almost whatever will do.

Would it be even simpler and cheaper to use MOSFETs instead of an IGBT to do the switching of the power? As far as i know IGBTs are more efficient, but in this case it could be made so that is does not matter if the unit is placed in the air stream that is going to be heated, any waste heat would be used anyway. But would MOSFETs simplify the PWM part?


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## kennybobby (Aug 10, 2012)

No the IGBT or MOSFET is simply the big switch or power amplifier of the low-level logic signal coming from the PWM oscillator. The potentiometer could be used to vary the pulse width of the Gate driver signal to the power amplifier device. Some sort of current measurement or temperature sensor may be required if you want it to regulate. To me the simplest approach would be an op amp oscillator summed with a voltage divider from the pot, then feed this into another op amp summed with the current sense signal. The output of the second op amp then drives the IGBT or FET. An inductor coil may be needed in the load path to help smooth out the current.


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## gdirwin (Apr 7, 2009)

You can use a DC rated SSR (solid state relay) - and there are inexpensive off-the-shelf PI pwm controllers...

See thread here:
http://www.diyelectriccar.com/forums/showthread.php?t=39023

My controller freaked out when it got to -40 - I know of others who use it with no problems with 6 kW of ceramics though...

I ended up chucking the controller and just turn on the SSRs in 2 stages. I start with a source wire from the fan control (so heat is on only if the fan is on), pass it through a multi-position rotary switch (on the blue/red dash heater control), on to the 2 Crydom DC rated SSRs , and finally to 2x1500W ceramics... My pack sits at 150VDC (peak 180) and all is well with 120VAC ceramics...

This is a simple system which let's me turn on 0, 1, or 2 elements with any fan speed... No need for a closed loop PI based PWM controller IMO. Works well up here in the great white north (the average temperature in Dec 2013 was -20C).

If you have more ceramics, then assign the "red" rotary switch contacts appropriately. Of course the "blue" range contacts are open (so you can use the fan or AC in summer). I added diodes on the rotary contacts for the higher heat stages which turn on the first stage etc... 

I have fiddled with just about everything (pwm etc...) but am finally happy with this simple setup...

Hope this helps!


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## mora (Nov 11, 2009)

I have thought about controlling my PTC elements the same way that steelneck wants. I believe he wants to control amps instead of voltage. This would mean max amp limit so elements wouldn't run as hot and be as powerful as they could be. Of course elements are self-adjusting but I would love to have similar control device too.

If anyone comes up a good plan I'd be willing to test it using my existing setup. Currently I have two solid state relays controlling two sets of 2x 1500W 110VAC elements. 6kW of heating power. Even half of that is enough for me when outside temps are around zero celcius. Sometimes I'd like to adjust it even lower but one element wouldn't be enough. Adjusting blower speed also affects output temperature but I can't run even half of my elements at lowest blower speed setting. Output air is too hot. Hehehe.


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## steelneck (Apr 19, 2013)

Now i am throwing out this simple case and i want to now if this will work, remember i am a mechanics guy, i do not know much about electronics. Let's say i take a very simple 555 based PWM circuit capable of driving a small 5W 12V electric motor, then just connect its output to the gate of an IGBT. Would that work?

Some how i bet that there is something i have missed here.. And how about the isolation in this case?

(edit) The Solid State Relay path, as suggested by "gdirwin" is maybe better, i suppose those are isolated like mechanical relays.


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## mk4gti (May 6, 2011)

Would a cheap DC motor controller work in this case (using a 0-5K pot to control current) ?


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## spoland (Aug 9, 2012)

My plan for heating is to use three DC SSRs to control three domestic heating elements at 220VDC. The heaters heat the coolant instead of the air. The SSRs are controlled by the PWM output of an Arduino computer that is used as a Vehicle Control Unit (VCC). A temp sensor in the coolant is input to the controller which is implemented in software in the Arduino. It keeps the coolant temperature at a steady 80C or lower depending on outside temp. The passenger compartment temperature is then controlled by the stock ACC.
I don't know if this is applicable to your setup, but I thought it might inspire you. 
Anyhow. The SSRs seems like components that are really easy to deal with. Found my at Ebay from Hong Kong.
Check out the wiring diagram: http://www.diyelectriccar.com/forums/showthread.php/comments-wiring-audi-a2-conversion-92227.html


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## steelneck (Apr 19, 2013)

spoland said:


> ... The SSRs are controlled by the PWM output of an Arduino computer ..http://www.diyelectriccar.com/forums/showthread.php/comments-wiring-audi-a2-conversion-92227.html. I don't know if this is applicable to your setup, but I thought it might inspire you.


Not in my case, of course it would work but it would be grossly overkill to use a whole computer just to get PWM to control a heater. In you case it is probably another thing since the Arduino already is there solving other things.

Long time since we heard of you spoland BTW.


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## spoland (Aug 9, 2012)

steelneck said:


> Long time since we heard of you spoland BTW.


All work and no play...


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## EVfun (Mar 14, 2010)

gdirwin said:


> I ended up chucking the controller and just turn on the SSRs in 2 stages. I start with a source wire from the fan control (so heat is on only if the fan is on), pass it through a multi-position rotary switch (on the blue/red dash heater control), on to the 2 Crydom DC rated SSRs , and finally to 2x1500W ceramics... My pack sits at 150VDC (peak 180) and all is well with 120VAC ceramics...
> 
> This is a simple system which let's me turn on 0, 1, or 2 elements with any fan speed... No need for a closed loop PI based PWM controller IMO. Works well up here in the great white north (the average temperature in Dec 2013 was -20C).


Sounds similar to what I did in VW Pickup. I ran 2, 1500 watt ceramic heaters. I had 2 relays to turn them on. Low was every other row of elements in each heater on, high was all rows on. I used 2 Potter and Brumfield KUEP-3D15-12 relays. The relays could only turn on if the heater fan was running. The heater temperature slide was hooked to a switch to select 0, 1, or 2 relays on.


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## cts_casemod (Aug 23, 2012)

Two main questions will you want a coolant or air based system? 

*Air based:*
Much smaller hysteresis. It would be beneficial to use PWM to have a steady temperature, rather than a on-off approach.
Quicker to warm up the cabin with higher efficiencies on shorter journeys
Requires modification of the existing air supply by adding the electric heater

*Coolant based:*
Higher hysteresis. A simple thermostat may be used to set up the coolant temperature within +/- 10C. 
If not previously heated it will take a bit more to reach operating temperature
Might be more efficient if you can have a 20L coolant reservoir and you can pre-heat while stationary or directly using the regenerative energy, if applicable. 
Requires a circulation pump and a reservoir, so that you can use the existing heater matrix and controls without modifications.


*Thermostat (Electronic) versus PWM*

Thermostatic systems may correct your temperature every 10 seconds or higher. They can be bought inexpensively and are easy to set up. This may result is a slight difference in temperatures if using an air based system taking air from the outside.

With a true PWM the temperature difference should not even be noticed, since the correction can be made a few times per second, giving precisions below 1C. Complexity is higher tough. 
Thermostatic systems can either use a relay (Not recommended) a Solid State relay or an IGBT. PWM controllers need an IGBT or similar.

A small IGB brick (50-100Amp @ 400V) can be had for $10 or 20. I am guessing the heater will not draw more than 20/30Amps. Link is for a 1200V 100Amp.
http://www.ebay.com/itm/Toshiba-MG1...942?pt=LH_DefaultDomain_0&hash=item2a21b6e9b6

A PWM controller can be used attached to an optocopler right from your 12V supply with a PTC/NTC acting as feedback. For the high side of the IGBT you can use an isolated DC-DC to supply power. If running at a low frequency (Say 100Hz) you don't even need a driver. Just use a 12V supply from the isolated DC-DC, a pull down resistor and the optocopler with a transistor or opamp to feed 12V trough the gate with a suitable pull down. Say 30mA for a large IGBT (330Ohm 0.5W) . Don't use the optocoupler with a pull up resistor otherwise the heater is always on if there is no signal form the electronics.

http://www.ebay.co.uk/itm/12v-36v-4...ther_Computing_Networking&hash=item2c73449f65

The above link shows a PWM controller that can be used up to 10Amps at 160V (I have one).
With little modifications this can be used with an IGBT for higher voltages/Power levels.

Be aware of AC controllers. They wont work on DC. I'll make a tutorial about modifying the above controller for operation at higher voltages for the purpose of using with a heater. 
If you are interested send me a PM and maybe Ill speed things a bit.


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## steelneck (Apr 19, 2013)

@cts_casemod:I was thinking about the most simple PWM control of PTC elements, no thermostat, just controlling the heat. Think the temp control of a car from the sixties. 
So this is "air based".

I think a simple 555 based circuit for PWM driving a SSR could do it just fine. From what i have seen there are a lot of SSRs that are fully opto-isolated, the more powerful ones are IGBT based (not cheap) and the cheaper ones are based on MOSFETs, and both can be used in parallel to handle whatever amps needed.

Then of course the most basic PWM control circuit could be extended with temp control, even with multiple sensors for both inside and outside temp, or timers to turn the heat on X minutes before you expect to drive away, and so on..


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## cts_casemod (Aug 23, 2012)

steelneck said:


> @cts_casemod:I was thinking about the most simple PWM control of PTC elements, no thermostat, just controlling the heat. Think the temp control of a car from the sixties.
> So this is "air based".
> 
> I think a simple 555 based circuit for PWM driving a SSR could do it just fine. From what i have seen there are a lot of SSRs that are fully opto-isolated, the more powerful ones are IGBT based (not cheap) and the cheaper ones are based on MOSFETs, and both can be used in parallel to handle whatever amps needed.
> ...


I see. 

For best efficiency keep the air temperature at say 30-40C and change the fan speed as required.

Remember that unlike older cars (which have the heating core at a "constant" temperature) if you use a duty cycle to control the heater each time you change the fan speed you have to change the duty cycle as well, otherwise you get super hot air when the fan is slowed down.

A 555 alone cannot change duty cycle from 0-100%. 
There are workarounds:

Get a triangular wave from the 555 charge capacitor and an op-amp to trigger the output with a POT from a reference voltage. The circuit needs to be fed from a regulated supply like a 7809 or 05.

Another one you cant beat is a ICL4017. This is a decade counter. Using 1 10 position switch you could increase the duty cycle in 10% steps from 0 to 100. This is just what you want but wouldn't be good if later you decided to have the temperature feedback. Could be nice for the fan tough.

I used an IC that drives a PWM from 0-100 using a single 0 to 5V voltage on my Pre-Charge circuit. Ill come back when I remember which one it was. However it is a surface mount package.

Any reason to use a SSR or mosfets? I for one would stay well away, when used IGBTs are as affordable nowadays.


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## steelneck (Apr 19, 2013)

cts_casemod said:


> Remember that unlike older cars (which have the heating core at a "constant" temperature) if you use a duty cycle to control the heater each time you change the fan speed you have to change the duty cycle as well, otherwise you get super hot air when the fan is slowed down.


No, older cars do not have "constant temperature". They just have a valve controlling the flow of hot water. But yes, low fan speed will result in hotter air.



> A 555 alone cannot change duty cycle from 0-100%.
> There are workarounds:
> 
> Get a triangular wave from the 555 charge capacitor and an op-amp to trigger the output with a POT from a reference voltage. The circuit needs to be fed from a regulated supply like a 7809 or 05.


Regulated supply? What i want is to drive the PMW with 12V from the aux battery, isnt that regulated enough?

Do you have a circuit diagram that would work to drive a SSR?



> Any reason to use a SSR or mosfets? I for one would stay well away, when used IGBTs are as affordable nowadays.


I have no clue from where i could get used IGBTs, and from what i have understood there is more to it than just connect a PWM to the IGBT gate. From what i have undersood, if i give minus voltage to the gate it comes on, and then it stays on until it gets + voltage on the gate. A simple PWM would only swing between 0 and +, not minus. But as i said, i am a mechanic, not an EE.


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## cts_casemod (Aug 23, 2012)

steelneck said:


> No, older cars do not have "constant temperature". They just have a valve controlling the flow of hot water. But yes, low fan speed will result in hotter air.


Such valve is a thermostat whose function, when operating properly is to block the flow of water to the radiator, hence keeping the engine temperature more or less constant after warm up. Did I forgot something?



steelneck said:


> Regulated supply? What i want is to drive the PMW with 12V from the aux battery, isnt that regulated enough?


The electronics need a regulated supply to keep the PWM duty cycle constant. Otherwise the PWM will vary with the car supply that can range from 11 to 14.5V



steelneck said:


> Do you have a circuit diagram that would work to drive a SSR?


A SSR is as solid state relay. Such as a relay, PWM is way to fast for it, so it simply wouldn't work.



steelneck said:


> I have no clue from where i could get used IGBTs, and from what i have understood there is more to it than just connect a PWM to the IGBT gate. From what i have undersood, if i give minus voltage to the gate it comes on, and then it stays on until it gets + voltage on the gate. A simple PWM would only swing between 0 and +, not minus. But as i said, i am a mechanic, not an EE.


eBay?
An P Channel Power IGBT conducts with a gate voltage of 15V and cuts with a gate voltage of 0V. High performance, high frequency applications take the off voltage to -10V in order to shutdown faster, and hence reduce linear losses and overcome gate capacitance. At 100Hz the gate acts as an open circuit so you should not have such problems with switch on/off

You need an isolated circuit for the gate driver with a regulated 15V supply.

Unfortunately using electronics requires a certain level of knowledge and research. If you want to keep it simple I suggest using a different approach with two smaller heaters controlled with a relay for a low/high unregulated setting.

Regards


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## cts_casemod (Aug 23, 2012)

There you have it 

http://cds.linear.com/docs/en/datasheet/69921234fc.pdf

SMD in SOT23 Package, should not be very hard to solder.


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## steelneck (Apr 19, 2013)

cts_casemod said:


> Such valve is a thermostat whose function, when operating properly is to block the flow of water to the radiator, hence keeping the engine temperature more or less constant after warm up. Did I forgot something?


Yes. The valve restricting flow of hot water to the cabin heating element, do just that. It is _not_ there to keep the engine temperature constant, that is the function of the engine thermostat.



> The electronics need a regulated supply to keep the PWM duty cycle constant. Otherwise the PWM will vary with the car supply that can range from 11 to 14.5V


Will that voltage difference make that much of a difference? 20% up or down in this case does not matter, just as 80 or 95 degree C coolant temp do make a whole lot of difference in whatever old car, the heat knob on the dash will work just the same at both temps.



> A SSR is as solid state relay. Such as a relay, PWM is way to fast for it, so it simply wouldn't work.


Would 1 Hz be too fast, or deoes it require something like 0.1Hz? PTC elements are quite slow, i think a modulation width of 10 seconds would be quite enough. The quality of the PWM signal is almost insignificant in this case. What i am talking about here is just some automation of the driver turning the heat on and off to at least not feel cold or hot in the car. Again, think a car from the sixties or even older than that. As the saying goes, KISS, Keep It Simple Stupid. That is my motto.



> An P Channel Power IGBT conducts with a gate voltage of 15V and cuts with a gate voltage of 0V. High performance, high frequency applications take the off voltage to -10V in order to shutdown faster, and hence reduce linear losses and overcome gate capacitance. At 100Hz the gate acts as an open circuit so you should not have such problems with switch on/off


Now that was some useful information. Thanks. But i think this is exactly what i get with an IGBT based SSR, even if i would find such a SSR used at ebay.



> You need an isolated circuit for the gate driver with a regulated 15V supply.


Do you have a circuit diagram for that? I have no clue, i am a mechanic, not an EE.



> Unfortunately using electronics requires a certain level of knowledge and research. If you want to keep it simple I suggest using a different approach with two smaller heaters controlled with a relay for a low/high unregulated setting.


Yes that is an option, using the driver of the car as the PWM module. But the reason for me starting thread, and possibly other mechanics like me, was to gain some knowledge to be able to put something together that will make it at least as as easy to use as the temp control of a 40 year old car. Remember, this DIY thing of converting cars to electric is built upon the knowledge sharing of mechanics and people knowing electronics. For me it is knowledge-wise very easy to build a car completely from scratch (with an ICE), but as soon as it comes to electronics (not simple old car electrics, switches and lightbulbs), people like me need help, and of course the other way around.


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## cts_casemod (Aug 23, 2012)

steelneck said:


> Yes. The valve restricting flow of hot water to the cabin heating element, do just that. It is _not_ there to keep the engine temperature constant, that is the function of the engine thermostat.


To Clarify, I was always referring to the engine thermostat and engine coolant as *"*constant*" *temperature. Cars nowadays don't have that valve, coolant always flows trough the heater core and a flap mixes hot air from the heater core with cold air from the inlet to avoid the problems you mentioned. (Or they have an electrically operated valve (BMW's, etc) This might be an issue because if you install the heater on the inlet you need full electric control, there is no "cold air" supply.



steelneck said:


> Will that voltage difference make that much of a difference? 20% up or down in this case does not matter, just as 80 or 95 degree C coolant temp do make a whole lot of difference in whatever old car, the heat knob on the dash will work just the same at both temps.


I would always use a regulated supply for my electronics for smooth operation. On mechanic terms, same as using a fuel injected car with no fuel rail pressure valve. Even if it would work, would you?



steelneck said:


> Would 1 Hz be too fast, or deoes it require something like 0.1Hz? PTC elements are quite slow, i think a modulation width of 10 seconds would be quite enough. The quality of the PWM signal is almost insignificant in this case. What i am talking about here is just some automation of the driver turning the heat on and off to at least not feel cold or hot in the car. Again, think a car from the sixties or even older than that. As the saying goes, KISS, Keep It Simple Stupid. That is my motto.


1Hz with 10% duty cycle is 0.1Hz switching. 10 second per cycle would work, but you need a custom made PWM controller for working at such a low frequency the relay reliability at such cycle is unknown.



steelneck said:


> But i think this is exactly what i get with an IGBT based SSR, even if i would find such a SSR used at ebay.


I can not tell you they will work reliably, so try it at your own risk and make sure its opto isolated. Maybe someone using one show up
You need a DC relay. AC types wont work.



steelneck said:


> Do you have a circuit diagram for that? I have no clue, i am a mechanic, not an EE.


No, I am going to build one sometime, but the principle is easy enough. Look online for DC motor controller and You'll find lots of info. Use the IC on the datasheet above as your starter.


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## steelneck (Apr 19, 2013)

I have read through this thread again and thought a bit more, looking at various diagrams out there on the wild wild net, refreshed my memory on some of the most basic stuff (30 years since i soldered any electronics..), reading some component datasheets and so on. The simplified drawing below is what i have come up with, in general terms, so far.

I will get back with a more detailed and complete diagram of the whole circuit. Any comments are welcomed.


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## steelneck (Apr 19, 2013)

Here comes a more complete circuit diagram. Those of you who are more used to electronics, please do comment. Will this work?

The voltage regulating part based on the 7806 puts out 6V, but i think it will work with 9V too (with a 7809), but then the car voltage can never go under 11.5 volts.

The 10K Thermistor (V1) is supposed to be located in the hot air stream behind the PTC elements, to regulate air temperature. The 10K potentiometer (VR1) is supposed to be mounted in the dash, replacing the original temp control. The 22K trimpot. is there to adjust the control. (the voltage divider controlling the PWM)

I am a bit unsure about the thermistor (V1). The MIC502 has a voltage range of 30% to 70% of the driving voltage to produce 0 to 100% duty cycle, in this case at 6V i need the voltage divider, made of V1, VR1 and VR2, to be able to produce voltages between 1.8 to 4.2 volts.

The PWM part with the MIC502 is happy with voltages from 4.5 to 13.2V. It is mainly designed for fan management in computers and is meant to produce a quite low PWM speed. I have chosen to drive it at 10Hz with the cap C3. It can output 25mA.

The SSR is an opto-isolated unit that has an operating voltage up to 300V and 20A, it can handle PWM signals up to around 80Hz at 20A (700Hz at 5A). It draws maximum 12mA on the input and has a control voltage range of 4 to 32 volts. This is the expensive part.

Last there is a fly-back diode, i have not yet looked in to this part, how to calculate what is needed. I suppose it is needed switching power to PTC elements.

So, will this work?


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## cts_casemod (Aug 23, 2012)

steelneck said:


> Here comes a more complete circuit diagram. Those of you who are more used to electronics, please do comment. Will this work?
> 
> The voltage regulating part based on the 7806 puts out 6V, but i think it will work with 9V too (with a 7809), but then the car voltage can never go under 11.5 volts.
> 
> ...


Should work.

11.5V is not a problem. If your battery runs that low something is very wrong and you should not be using the heater anyway. The 12V Fan draws in 20-30Amps at full power.

Never use 100% duty cycle. 95% or less is required to make sure any arc generated is quickly extinguished. Preferably the relay should be as close to the Battery as possible and the supply taken to the heater with a dedicated cable using this PWM waveform.

A 120V element is desired to work with 70% or less duty cycle. As such, should the relay get stuck/shorted the higher current flow will burn the fuse, if properly designed. This is because the internal safety switch is not rated for DC operation, unless it is used to drive a dedicated contactor.

The flyback diode is to clamp inductive loads (ie. The fan motor) it can be withdrawn if the load is resistive.

That relay goes for £86 + VAT on Farnell!? Ouch...!
http://uk.farnell.com/crydom/dc400d20c/ssr-20a-400vdc/dp/2061275


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## steelneck (Apr 19, 2013)

I have basically two issues.

I did forget to write that the thermistor was PTC, not NTC, so its resistance rise with temperature. But i am quite unsure about this voltage divider part giving input to the MIC502, i really do not know how to chose thermistor. Of course it could also be turned up side down and then use an NTC instead. 

I could also go back to my original thought, only regulating power to the heating elements without any kind of temp sensor, replacing the thermistor in the circuit with a resistor around 7-10K. Just as in the old car where heating power is regulated by restricting hot water flow to the heating radiator. The PTC elements are self regulating, so there is only need for being able to dial down the power.



cts_casemod said:


> Should work.
> 
> 11.5V is not a problem. If your battery runs that low something is very wrong and you should not be using the heater anyway.


Yes, i guess the motor controller also will fail if the 12V supply goes under 11,5V. So i could just as well chose 9V



> Never use 100% duty cycle. 95% or less is required to make sure any arc generated is quickly extinguished. Preferably the relay should be as close to the Battery as possible and the supply taken to the heater with a dedicated cable using this PWM waveform.


That part i fail to understand? The SSR is a relay, as far as i understand it is meant to be 100% on or off, could as well be controlled with a manual switch. Just like when the driver act as a very slow PWM turning the heat on or off.



> That relay goes for £86 + VAT on Farnell!? Ouch...!
> http://uk.farnell.com/crydom/dc400d20c/ssr-20a-400vdc/dp/2061275


This is my second issue, the price is a bit on the "ouch" side. I bet this can be done a lot cheaper, somehow..


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## cts_casemod (Aug 23, 2012)

steelneck said:


> I have basically two issues.
> 
> I did forget to write that the thermistor was PTC, not NTC, so its resistance rise with temperature. But i am quite unsure about this voltage divider part giving input to the MIC502, i really do not know how to chose thermistor. Of course it could also be turned up side down and then use an NTC instead.
> 
> ...



Yes I imagine that. I deleted that bit afterwards.

You make a voltage divider, hence the need for a regulated supply. Let me know the maximum air temperature you desire, the maximum duty circle ( depends on you pack and heater voltage) and the minimum air temperature. I recommend minimum 20C and max 50C. I also need the PTC resistance at those temperatures. You will need a trimmer to GND and another at VCC to define both values.

The motor controller is a multi wind resistor on most cars. You should get that replaced as well but thats another story.

The SCR is a Diode. Its voltage controlled.
That is another subject related to physics.

On an DC current if an arc is allowed to form for whatever reason if won't extinguish because DC is Direct current so the arc is supplied continuously just like an HID bulb. Depending on the arc current the fuse may or not blow. In other terms that's a welder ready to set fire to your carpets or a hole in your car or anything Else. So... It is desirable to convert DC into pulsating DC which will be as safe as AC. The arc will extinguish and unless the conditions are favourable (which means you have a BIG problem) it won't re-arm.

Surely, you came up with such a nice circuit I am sure you can get a better solution. Not sure why you keep insisting in the SSR


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## kennybobby (Aug 10, 2012)

*Test it on the bench.*

Build it up and test it.

For The voltage divider you need to get the thermistor curve R vs Temp and determine the range of resistance you will expect to use from coldest to hottest, then you can set the bottom 22k pot to trim it such that at full CCW of VR1 pot you have heat off and at full CW you have heat on full blast.

For example with thermistor at 10k at the top, 10k pot VR1 and set 10k on the bottom pot, then with 6 volt supply you will drop 2 volts across each component. When the vr1 is ccw the voltage at the wiper is 2 volts and at full cw it goes up to 4 volts. This fits nicely with your 30-70% range for 0 to 100%. But of course the thermistor resistance won't be 10k all the time, it depends upon the temperature--that's why you need the spec sheet curve and test it on the bench.


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## steelneck (Apr 19, 2013)

*Re: Test it on the bench.*



cts_casemod said:


> Surely, you came up with such a nice circuit I am sure you can get a better solution. Not sure why you keep insisting in the SSR


Ohh, i am not insisting, this is out of ignorance, i simply do not know how to make it isolated, how to chose the right IGBT or power transistors and how to connect those things. The SSR i can treat almost like a mechanical relay, the kind i understand. I have a lot of reading up to do, i do not even know what knowledge i miss, i am really on my own here.

I did some some basic learning about electronics in my teens, but that was almost 30 years ago.



kennybobby said:


> Build it up and test it.


Yes, probably. For me nothing beats "learn by doing". I will build the PWM part, for the last part switching the pack voltage i need to read some more. But it would help if someone pops in to this thread with some nice diagrams on how to do it with a lower price tag than that "Ouch SSR"..


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## cts_casemod (Aug 23, 2012)

Lets get some work done, to be different from every other pointless comment so far 

PTC's are not the most common thing, so I replaced that with a 2.2K NTC

Nominal Resistance (at 21C) - 2.2K
http://uk.farnell.com/vishay-bc-com...ntc-2k2-5-radial/dp/1187025?Ntt=NTCLE100E3222

-55 to 150C

PWM delivered at 5C - 80%
PWM Delivered at 80C - 10%

No adjustment is needed.

Looking at the schematic on page one of the datasheet:
http://www.farnell.com/datasheets/87906.pdf


R2 is a 1KOhm Resistor in Parallel with the 2.2K NTC
R1 is a 470Ohm Resistor

The Fan must always be on for the circuit to operate.

With the component values provided the system only protects the heating element and the ducts from excessive temperature. Control is therefore manual, similar to an old car running the heating on maximum. Maximum power is delivered at 10C. Cut-off at 80C. 

To keep a constant temperature the voltage divider consisting of R3 and R4 should be set (I can do that later). For now leave it grounded.

The Circuit below is an isolated IGBT Driver.










A 10Ohm Gate resistor is recommended (not on the schematic). 
15VDC power supply for the high side. For operation at a few hundred hertz a 1W DC-DC is enough. An SMPS AC-DC adapter running from the main pack can also be used.

Pin 7 of the MIC502 goes into the input pin 2 of the opto driver through a 220Ohm resistor.

http://uk.farnell.com/murata-power-solutions/mev3s0515sc/converter-dc-dc-3w-5-to-15v-3kv/dp/1818378

5 Input, 15 Output. Can be powered by the regulated 5V supply to avoid oscillations (Models with wide input voltage (9-18V) also available but more expensive.
A 1Kohm load resistor is required to keep output regulation

http://uk.farnell.com/vishay/vo3120-x007t/optocoupler-igbt-driver-25a/dp/2251532?Ntt=vo3120

And the driver.

As always, use a suitable fuse.


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## zaxxon (Jul 11, 2009)

cts_casemod said:


> Lets get some work done, to be different from every other pointless comment so far
> 
> PTC's are not the most common thing, so I replaced that with a 2.2K NTC
> 
> ...


I think you forgot this is a DC application not AC. Once the SCR is triggered it well not turn-off.


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## subcooledheatpump (Mar 5, 2012)

I was just about to say the same thing, saw this in casemod's other thread. Am I missing something?

Why not use an IGBT with PWM to control the heater, seems simple enough. 

At one time I did use a simple 555 timer with an IGBT to control a heater, seems to work decently. You could even use a microcontroller and wire in feedback from a thermocouple or PTC, NTC or thermostat to turn off the gate signal incase of overtemperature


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## cts_casemod (Aug 23, 2012)

Thanks. 
I got confused for a bit (yes, stupid when I advised earlier that AC SSR wont work for that same reason )

Ill be updating the drawing with an IGBT instead.


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## cts_casemod (Aug 23, 2012)

Here's the revised schematic.
VR is a 100Ohm Pot for temperature regulation










Happy prototyping


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## steelneck (Apr 19, 2013)

Just a qick comment (i am almost off to buy my donor car). What is the IGBT? Is it everything after the 220Ohm resistor or is it just the part between ground and the heating element. You also had two IGBTs specified there, then i got to have something wrong. Or is it that the big IC in the diagram is some sort of optocoupler? I guess so.. Then what is the two IGBTs? And why two IGBTs?

Why the two resistors and the 100 VR (100 Ohms?) on the leftmost of the diagram, i see that it is some sort sort of voltage divider going to the VSLP input of the MIC502, but i do not know why?


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## cts_casemod (Aug 23, 2012)

steelneck said:


> Just a qick comment (i am almost off to buy my donor car). What is the IGBT? Is it everything after the 220Ohm resistor or is it just the part between ground and the heating element. You also had two IGBTs specified there, then i got to have something wrong. Or is it that the big IC in the diagram is some sort of optocoupler? I guess so.. Then what is the two IGBTs? And why two IGBTs?
> 
> Why the two resistors and the 100 VR (100 Ohms?) on the leftmost of the diagram, i see that it is some sort sort of voltage divider going to the VSLP input of the MIC502, but i do not know why?


Yes. The circuit description is on page 3.
The leftmost of the diagram is the temperature regulation


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