# Help with heater wiring - 3-way switch to a contactor



## corbin (Apr 6, 2010)

Hi all!
I'm having trouble with wiring up my ceramic heater. I bought the Universal Heater Kit from Randy at Canadian EV (http://canev.com). There is a 3-way switch to control the blower speed. I wan't the ceramic heater's contactor to only be on when the fan is on. I can't figure out how to take the output from the one of the three speed input wires and make it go into the contactor to turn it on. Randy said to use a diode, however, I haven't ever used any diodes and I'm not sure how to use them, exactly where to use them, and what kind. 

Referenced below is my wiring diagram with the ??? part that I don't understand. 










So, how do I split one of the three wires to go to the contactor?
What kind of diode do I use, and how are they installed?
Should I have any other types of diodes (or other things) on the contactor?

Thanks all!
corbin


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## samborambo (Aug 27, 2008)

What you have controlling your fan is probably a 3 speed / heat switch, not a 3 way switch. There is a difference. 3 heat switches are used for fan heaters, electric blankets, etc. The principle is very simple. The switch controls two load elements and switches them from (both) off -> single element (low) -> both elements in series (med) -> both elements in parallel (high).

Your goal is to turn on the HV contactor when the fan is on low, medium or high but not when off. Tap the output terminals to supply the HV contactor coil. The output terminals of the 3 speed switch will have continuity to the input terminal at various positions. You need to ensure that you always have 12V (continuity to the input terminal) to supply the coil when in low, med, high and 0V (no continuity or ground) when in the off position. You may find that none of the output terminals satisfy this logic. The way to solve this problem is to join two or three of the output terminals signals in order to drive the coil in the right states. Of course you can't simply bond the output terminals together as that would upset the operation of the 3 speed switch. Tapping the output terminal signals through diodes will ensure that the coil current doesn't interfere with the fan.

Diodes are very simple devices - just a one way valve. They allow current to flow in one direction but not the other. Diodes have two important parameters in this situation - current handling and reverse bias voltage or blocking voltage. The RB voltage should be at least as high as the voltage you're working with plus a safety factor - eg: 12VDC plus some potential for voltage spikes from the fan. I'd suggest 60V at a minimum. The current you need to pass is limited by the HV contactor coil. Find out how much current the coil draws.

Lastly, I'd recommend you quench the freewheeling current from the coil when it opens. A reverse biased diode around the coil with the same ratings as the others will do the job.

Sam.


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## samborambo (Aug 27, 2008)

See attached diagram for my last post.


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## gdirwin (Apr 7, 2009)

If you are converting a newer car, look for a "control wire" in the factory wiring diagrams - my 2001 Toyota had a pink wire which was open-circuit when the heat was off, but was grounded whenever the fan switch was on any other position. This allowed my to directly control a relay, which only allowed the ceramic heater to come on when the fan was on (any position).

Also be aware that if you tap the switch outputs via diodes, you should check the output voltage for each fan position. The fan switch goes through big resistors to slow down the fan motor, which results in a low voltage output when the fan is on the lowest position. This voltage may be too low to turn on a conventional relay, in which case your ceramic heat would only work on some fan speeds, but not all positions. You can use a SSR (solid state relay) or a conventional relay with a low turn on voltage to solve this (or use one of these small relays to drive your bigger relay with 12V).


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## corbin (Apr 6, 2010)

Hi sam,
Thanks for the info! I had come to pretty much the same drawing that you gave.



samborambo said:


> What you have controlling your fan is probably a 3 speed / heat switch, not a 3 way switch. There is a difference. 3 heat switches are used for fan heaters, electric blankets, etc. The principle is very simple. The switch controls two load elements and switches them from (both) off -> single element (low) -> both elements in series (med) -> both elements in parallel (high).


The heater element only has two wires wired into it; I didn't see any way it could control a low or high setting on it. 

Is there any online source for diodes? 

I'll probably go by Fry's Electronics tomorrow night; I'm pretty sure they will have them, and I'll look for one that is in the right voltage and current rating. I guess they way they work is you cut the wire and solder them in place. I haven't seen any pictures of diodes being used, so I'm not entirely sure on that part. 

Thanks for the diagram -- that does clear things up a bit. I don't understand why you have a diode off the HV Contactor Coil that comes back to the input and also goes to ground. What is the purpose of doing that?



samborambo said:


> Lastly, I'd recommend you quench the freewheeling current from the coil when it opens. A reverse biased diode around the coil with the same ratings as the others will do the job.


What is the purpose of doing that? (Maybe this is what you had drawn in the diagram).

Thanks again for the help!
--corbin


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## corbin (Apr 6, 2010)

gdirwin said:


> If you are converting a newer car....


Hi gdirwin -- it is a '69 bug, and doesn't have any heater fan. I'm using the complete kit from CanEV.com, which includes a fan and element. 

-corbin


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## samborambo (Aug 27, 2008)

gdirwin said:


> If you are converting a newer car, look for a "control wire" in the factory wiring diagrams - my 2001 Toyota had a pink wire which was open-circuit when the heat was off, but was grounded whenever the fan switch was on any other position. This allowed my to directly control a relay, which only allowed the ceramic heater to come on when the fan was on (any position).
> 
> Also be aware that if you tap the switch outputs via diodes, you should check the output voltage for each fan position. The fan switch goes through big resistors to slow down the fan motor, which results in a low voltage output when the fan is on the lowest position. This voltage may be too low to turn on a conventional relay, in which case your ceramic heat would only work on some fan speeds, but not all positions. You can use a SSR (solid state relay) or a conventional relay with a low turn on voltage to solve this (or use one of these small relays to drive your bigger relay with 12V).


The fan switch shouldn't be using resistors. It will switch the windings through single, series and parallel to get low medium and high. When in series you'll see half the voltage on one terminal. That's the reason why you use the diodes. Only the diode with the highest input voltage will conduct.

Sam.


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## samborambo (Aug 27, 2008)

corbin said:


> Hi sam,
> Thanks for the info! I had come to pretty much the same drawing that you gave.
> 
> 
> ...


You're correct. The diode going to ground is the freewheeling diode. Basically the coil is a big inductor so it has stored energy that is released as a very high reverse voltage when the current is stopped. The diode allows the current to keep flowing until the coil energy is gone. See this page for a better explanation and lots of math:

http://en.wikipedia.org/wiki/Flyback_diode

Once you find your diodes, remember that they are polarised - meaning it matters which way around they go. Every diode I've seen has a bar or stripe at one end corresponding to the bar end (not the triangle) in the schematic. 

Sam.


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## corbin (Apr 6, 2010)

samborambo said:


> You're correct. The diode going to ground is the freewheeling diode. Basically the coil is a big inductor so it has stored energy that is released as a very high reverse voltage when the current is stopped. The diode allows the current to keep flowing until the coil energy is gone. See this page for a better explanation and lots of math:
> 
> http://en.wikipedia.org/wiki/Flyback_diode
> 
> ...


That makes a lot of sense, thanks for the reference link!

corbin


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## DC Braveheart (Oct 12, 2008)

Very useful thread.

I'm looking to do a similar thing but using a Kat's water heater instead of a ceramic heater. Also, if I'm understanding the wiring diagrams for my '92 Miata correctly, while my setup is the same from the perspective of the multiple wires going through a block of resistors to control the blower motor speed, it appears that Mazda have a different wiring scheme to the one here - the fan switch in the dash connects one of the resistor paths to ground instead of (if I read the diagrams in this thread correctly) to power. 

I don't think it really makes much difference but if someone more knowledgeable wouldn't mind casting an eye over the attached schematic to let me know if I've got anything wrong I'd appreciate it (the black elements are direct from the Miata wiring schematics with some extra stuff removed for clarity - the added components are in red).

I know I'm missing a freewheeling diode (or two ... should there been one on both the relay and the contactor)?

The way I think this works is:

- Setting the fan switch to positions 1,2,3 or 4 will connect one side of the Radio Shack 275-017 SPDT switch to ground via it's diode
- that switch will be turned on by it's lever when the the heater control is moved to the 'hot' region 
- also inline with that is the thermostat switch from the Kat's heater
- so when the fan is on, the thermostat is closed and the heater controls say heat is needed the relay's coil is connected to ground and closes it's 2 sets of contacts
- one set of contacts turn on power to a 12V pump to circulate 'coolant' through the heater core and the Kat's heater
- the other set contacts initiates the contactor that provides 144VDC from the traction battery to the Kat's heater.

Let me know if you see anything wrong with this plan.

One final thought ... do I really need to worry about knowing whether or not the fan is running? While that seems like a perfectly necessary thing when using a creamic heater core I'm not sure it makes sense with a fluid-based heater. The wiring is a lot simpler if I don't connect up all those diodes and with the contact switch I have a way of turning the heat on and off. That way I can get the water up to temp faster in the morning by turning on the heat and NOT running the fan.

Thanks


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## samborambo (Aug 27, 2008)

DC Braveheart said:


> Very useful thread.
> 
> I'm looking to do a similar thing but using a Kat's water heater instead of a ceramic heater. Also, if I'm understanding the wiring diagrams for my '92 Miata correctly, while my setup is the same from the perspective of the multiple wires going through a block of resistors to control the blower motor speed, it appears that Mazda have a different wiring scheme to the one here - the fan switch in the dash connects one of the resistor paths to ground instead of (if I read the diagrams in this thread correctly) to power.
> 
> ...


Your schematic looks good.

I agree, where you're monitoring the heater fluid temperature with the thermostat, it's not really worth ensuring that the heater fan is on.

Make sure you fuse the HV source!

Free wheeling diodes are less critical if you're only switch low voltage mechanical switches. If there's semiconductors and/or high currents involved, make sure that you use FWD where necessary.

Sam.


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## DC Braveheart (Oct 12, 2008)

samborambo said:


> Your schematic looks good.
> 
> I agree, where you're monitoring the heater fluid temperature with the thermostat, it's not really worth ensuring that the heater fan is on.
> 
> ...


Thanks Sam! And yes, I did realize after I posted the picture that I was missing the fuse!


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## geosynch (Oct 30, 2012)

I too have found this thread useful; and I need help. First, some background.

I have a 1999 Miata EV that I am in the process of completing. I drive it most days and working out a few bugs. One of the "bugs" is a lack of a heater. 

I decided to go the route of putting in a ceramic heater core where the old hot water heater core was. That is now complete and I can confirm that when I put 120 VDC across the heating elements, hot air comes out of the ducts. Picky me, I want to be able to easily turn the heater and on off.

To switch the heater on only when the fan is blowing, I decided to make use of the Miata's factory installed wiring. The AC has been removed completely and the button in the dash that controls the AC circuit only comes on when the fan is on. Perfect! I decided to use that AC button to control my heater relay. Pushing the AC button in the dash does nothing unless the fan is also on.

I figured I'd get 12V off of the AC button and use it to control a relay. Problem is, there is only 300 mV coming off the wire that comes from the AC button.

This is where I need some help.

I bought a relay kit from KTA. I should be able to use a transistor as a switch as shown in the attached schematic. It's been too long since I've taken a EE class and I don't know how to pick out the right kind of transistor. Anyone willing to help an ME?  The values shown in the schematic are what I have measured. The 300 mV and 130 mA at the base are from the AC button when it is "on" and are with respect to chassis ground. The 66 Ohms is across the coil of the P&B relay, which would be about 180 mA when supplied 12V

Here are my question:

1- What kind of transistor do I need (part #)?
2- Do I need a resistor at the base? If so, what value?
3- I don't think I need resistors at the collector or the emitter, but would appreciate confirmation.

Thanks in advance.
geosynch


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## dtbaker (Jan 5, 2008)

geosynch said:


> I figured I'd get 12V off of the AC button and use it to control a relay. Problem is, there is only 300 mV coming off the wire that comes from the AC button.



I think you've got your AC switch on the wrong side of the relay coil. The 12volt, 300mV from your AC switch should be enough to drive the coil of the relay (the 12v+). The other side goes straight to chassis ground 12- as shown originally without your switch on that side.


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## geosynch (Oct 30, 2012)

I thought I'd update my efforts on getting a heater in the Miata. I did get it working, but not without a few mishaps, and not exactly how I wanted it to work. Details to follow.

First, I had to completely give up on the idea of using a transistor to switch the relay on. It shouldn't matter which side of the relay I put the transistor -- that wasn't the issue. but, most circuit diagrams I found on the web that used a transistor as a switch for a relay put the transistor on the ground side of the relay. Like in this tutorial (scroll 1/3 of the way down the page and lok for the heading "Basic NPN Transistor Switching Circuit").

The problem was the 300 mV coming off of the AC button that I wanted to use. 300 mV just wasn't enough to saturate the base to the emitter -- the typical transistor requires about 700 mV to saturate.

So, no go there.

As a reminder, i wanted to run the heater off of the AC switch for a couple of reasons. I felt that using the AC button helped maintain the look and feel of the stock Miata dash but more importantly, the AC button was powered only when the fan was operating, which is a safety consideration. And since the AC had been removed from the car, the once useless switch could be re-purposed to run my heater and my life would be a wee bit happier.

Truth be told, I was lazy looking for 12V out of the AC switch. There was in fact 12V at the switch, but I had to tear the dash apart to get at it. But, the lazy side of me saw an opportunity. There is a temperature sensor in the evaporator that is only powered when the AC is on; that sensor's wiring is easily accessible and with no load, did in fact have 12V. Reference the wiring diagram for the switch. 










Problem was, when a load was applied, the voltage fell to 300 mV. Which set me down the path of looking for a way to amplify the 300 mV, using a transistor as a switch, yada, yada. It wasn't meant to be.

So, I tore the dash apart and tapped in to the pink and black/blue wires in the diagram above at the back of the AC switch, got my 12 V and now my life is a wee bit happier with my now functioning heater.

Except my life isn't a wee bit happier, which is REALLY why I'm writing this -- to serve as a warning to those of you, who, like me, think you have control over a situation only to find out in the end, you don't!

I don't know how many times I have actuated the temperature controls in a ICE-mobile. Twist the knob to red, and the air blows warm. Twist the knob to blue, the air blows cold. I honestly never gave it much thought, but in my defense, in retrospect I must of thought that the knob controlled the water flow through the heater core. Except, that it doesn't work that way at all. The knob controls the flow of air through the heater core.

So, i set up my heater to be idiot proof. The new electric heater core, now physically residing where the old hot-water heater core used to be, could only be electrically powered when the fan was actually blowing air. And it does in fact work that way.

But the problem is, that because of that effing temperature control knob, depending on the knob setting, the fan may be blowing air, but it may not be blowing air through the heater core. So, it is possible for the heater core to be electrically energized, no air blowing through it, and it is just getting hotter, and hotter and hotter yet cold air is what is coming through the vents.

Remember, the idea was to make this idiot proof and not just because my wife and kid also drive the car, but because this is how accidents happen -- you forget or accidentally push a button without realizing you pushed it.

There is a thermal fuse on the heater core, but that's just not good enough for a belts and suspenders guy like myself.

So, for now, the temperature control knob has been physically removed so the fan is always blowing air through the heater core. But, I will eventually rip the dash out AGAIN and remove the linkage from the temperature control knob to the flap the directs the flow of air.

Which should once and for all, make the heater idiot-proof and fail-safe.

geosynch


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## dtbaker (Jan 5, 2008)

the guy that did most of the original conversion on my Miata removed all the ductwork, and the fan even.... used the control area for the EVdisplay screen, and put in two marine bilge blowers without any heating elements at all just to move the air around. Fine for San Diego, but not so good for Santa Fe.

Now that you are an expert in pulling the dash..... how big of a job is it? An hour, or a whole day? 

I'm contemplating adding heat.... but not crazy about pulling the dash to get to everything. I had to pull the instrument hood cover to fill the holes where the fuel, oil, and water gauges were, and that was actually pretty easy; but I have not attempted to pull the main dash to get to the area behind the console and glove box....


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## geosynch (Oct 30, 2012)

Hi Dan,

I don't know if you recall, but I bought a Manzanita Micro PFC-20 from you about a year ago. Works great, but now I'm wishing it was a PFC-40  Charging is slo-o-o-o-ow. I digress.

Too bad that the duct work is ripped out of your Miata. Just a guess, but it may be worth going to a junkyard and getting the duct out of another Miata and trying to re-install....?

I now have to confess I haven't ripped out the _entire _dash out of a Miata. Removing the glovebox is about a 10 minute job and if the AC condenser is removed, there is a bunch of room under there. Removing the center console (the armrest and cubby hole, along with the trim around the gear shift, is also straightforward -- about an hour. Then with the radio removed, there is actually a fair amount of room in which to work, for the purpose of swapping out the hot water heating element for an electrical heating element.

That is assuming you have the original duct work.

There is a bit of a trick to getting out the original heater core with a length of twine. Searching through Miata forums and you'll find it. Without said trick, you may think you have to rip out the entire dash all the way to frame. That is reported to be an 8 hour job.

Unfortunately, that trick is going to be useless for you if all your original ductwork is gone.

I sorta failed at making a build blog for the Miata, but all my photos are online:

http://higham.smugmug.com/Cars/Miata/Miata-EV-conversion/21622093_rqcn5t#!i=1724315397&k=RTqb6M2

Ciao,

geosynch


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## Russco (Dec 23, 2008)

geosynch said:


> I thought I'd update my efforts on getting a heater in the Miata. I did get it working, but not without a few mishaps, and not exactly how I wanted it to work. Details to follow.
> 
> 
> Which should once and for all, make the heater idiot-proof and fail-safe.
> ...


The ONLY safe way to provide heat in an EV is by circulating hot coolant through the OEM heater core. Those PTC solid state heater cores still draw a couple of amps when no air is circulating through them; that's a couple of hundred of watts!

I have melted/distorted the plastic heater ducting/dampers many times trying the PTC cores. Even with the fan running on LOW, it's melt time.

I only use hydronic heating in my EV.

I know, I used to manufacture EV heaters in the past. Sold hundreds of them. Even my current EV has a 20 year old Russco iiquid heater.

No chance of melt downs.  

And no need to tear the dash apart.


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