[corbin]

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

[samborambo]

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.

[samborambo]

See attached diagram for my last post.

[gdirwin]

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).

[corbin]

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

Originally Posted by samborambo

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?

Originally Posted by samborambo

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

[corbin]

Originally Posted by gdirwin

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

[samborambo]

Originally Posted by gdirwin

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.

[samborambo]

Originally Posted by corbin

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



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?



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

Thanks again for the help!
--corbin

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.

[corbin]

Originally Posted by samborambo

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.

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

corbin

[DC Braveheart]

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