Wiring question (relays and diodes)

[dogmush]

OK, Brain trust, help me out here, I'm questioning myself.

My problem:  I am puttingheated/cooled seats in my truck, and it's a homebrew project.  Each seat has two modules.  Each module has a fan and a heating element.  For cooled seats, the fan has to be on.  For heated sats the heater AND the fan have to be on.  I've already wired the seats, and have them run to plugs.  Each seat plug has three pairs of wires.  Cooling fan, heater 1, heater 2.  Each heater is about 8 amps, and the fan draw is miniscule.  I'm running the power to a set of relays (per seat). One relay for cool, one relay for hot.  (relay's are rated at 40 amps).  I have a blue button, and a red button.  I want it wired so that when I push the blue button, the cool relay closes, and whe I press the red button BOTH relays close.

My solution:  Wire blue button to cooling relay.  Wire red putton to heating relay then jump from trigger pin of heat relay to trigger pin of cool relay with a diode "pointing" towards the cool relay.  That way when the heat relay is triggered it will also close the cool relay, but when the cool relay is triggered the diode will keep voltage from "backfeeding" and closeing the heat relay.

Right?  This should work?  Diodes (that I can get at radio shack) can handle indefinite use at the amperage that closes relays?

[Nick1911]

Are the relays spst?

(And yes, that should work.  You'll take a voltage drop across the diode, but the relay won't care.  But, there might be an easier way to do this.  Also, do you have it wired so this only works with the ignition on?  Otherwise if you forget to turn it off, you'll have a dead battery when you come back...)

[RocketMan]

Also, how do you turn the heating and cooling off? Are the relays wired to latch?  Are there [Off] buttons?

[Nick1911]

And, is the cooling *just* fan powered?  Are you sure that the heating elements aren't actually thermoelectric heat pumps, which can actively cool when the polarity is reversed?

[dogmush]

Are the relays spst?

(And yes, that should work.  You'll take a voltage drop across the diode, but the relay won't care.  But, there might be an easier way to do this.  Also, do you have it wired so this only works with the ignition on?  Otherwise if you forget to turn it off, you'll have a dead battery when you come back...)

SPDT.  And yes, the power that runs the trigger side of the relays comes from a switched 12V source.

Also, how do you turn the heating and cooling off? Are the relays wired to latch?  Are there [Off] buttons?

The buttons latch.  Press once, closed, press again, open. The idea is I can leave "cool" on, hit the remote start on the truck and have the seats and truck AC com on.  then in 5-10 minutes I can get in a nice cool truck with cool leather.

And, is the cooling *just* fan powered?  Are you sure that the heating elements aren't actually thermoelectric heat pumps, which can actively cool when the polarity is reversed?

  It is now.  I can't say for sure that the lincoln I stole the modules from was, although I'm pretty sure it was.  The heat strips in the Lincoln modules were computer controlled and needed wierd voltages to work, so I gutted the modules, replaced the heaters with 12VDC Ceramic ones and wired everything back up.  "Cool" just grabs A/C'ed air from under the seats and pumps it up through them to cool and ventilate.  "Heat" turns on the heat strips. "Cool actually works pretty well just sitting in my garage and moving air.  Moving actual cold air aught to be pretty good.

[castle key]

You may get an invite to the White House....

[dogmush]

I have an LCD display around here somewhere I could throw on it.

[TommyGunn]

Do you plan on encorporating detcord into the design?

[Firethorn]

Nah, what he needs is some oil-filled cylinders to moderate the heating elements - just run the element right through the middle.  4-5 of them should be good.   

[Triphammer]

If you wire the fan to Common on one of the relays, run the N.O. to the switched 12 v feed. the blue button get wired to pull in the cooling relay. Wire the heaters through the other relay C in NO out. feed from the NO of the heat relay to the NC on the cool relay. When you hit the cool relay the fan comes on through NO. When you hit the red button the heat relay pulls in and feeds the fan through the NC contact.
You can even run a two speed fan. slow for heat fast for cool. You can run the fan in fast for heat by pushing both red & blue.