Background: I have been working on car seat heater circuit. I purchased the universal seat heater with manual 2 level (high and low) switch. Instead of manual switch, I re-wired the seat heater circuit to electronic control relay with trigger latch to set either high and low setting using car oem button and mechanical relay(that fits in dash).

My current question is relating to diode use on high output terminal from electronic relay.

The high output terminal has ~14V when ON and is connected directly in parallel to seat heater pads (2; for seat cushion and back). The low output when ON is connected as series to the same pads and it uses the high output connection to establish a series between pads. In Low level state, i get ~7v of backfeeded electricity in high terminal which i want to block using diode.

What type/size diode is needed to do this job? The high level wire is 16g stranded and forward electricity is 14v (amps at max ~15A---but a mechanical relay in circuit has rating of 40A) the back current which i want to block is ~7v (amps i am unsure). And I want it to be blocked to correctly set high and low level LEDs.

This is the general diagram. My diode point is yellow wire at the switch to block reverse current when the circuit is in low level (i.e white wire is +12v)

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A. 1 press of switch will trigger T1 ON/T2 OFF via flip flop latch (interlocking). T1 = HIGH setting, will output 12V via YELLOW wire to both heater pads and the black from upper heater wire is earth and red wire from lower heater pad is connected to earth via relay pin 30-->87a connection. One LED light in the switch will turn ON.

B. 2 press of switch will trigger T2 ON/T1 OFF via flip flop latch (interlocking). T2 = LOW setting, will output 12V via PINK wire complete relay circuit by charging 12V to pin 86 (pin 85 connected to earth). Pin 87(12V) will connect to pin 30 sending 12v down the lower heater pad and the circuit is then completed by yellow wire connecting both heater pads in series. Both LED lights in the switch will turn ON (This is what i do not want). Both LED turns on because red wire of relay 2(low) is 12v and yellow wire of relay 1 (high) has 7v (because of series connection). The switch LED has input voltage range of 5v-12v (ideally 5v)and is connected via resistor which drops voltage around LED to ~2v.

What i want is: a. 1 press>high setting activated> 2 LED on (not one) b. 2 press> low setting activated> 1 LED on (not two)

HOW? I need help! I was thinking of adding diode at yellow wire at relay 1 insertion with banded side away from relay so current can move forward but blocked reverse. Then i will take 2 output wires from both ends of diode to my switch. What it will do is a. 1 press> high setting> both LEDs glow ( as both are connected to yellow wire and current is flowing in that direction across diode b. 2 press> low setting> one LED will glow ( the 7v at the banded side of diode on yellow wire will glow the LED but the LED on the blocked side will not.

Any thoughts??

  • 3
    \$\begingroup\$ You'll need to post a schematic, I can't make head nor tail of your description. If you just want to select a diode though, the parameters you care about are: max forward current (probably about 20A in your case) and max reverse voltage (about 30V is good enough for 12V car stuff). \$\endgroup\$ – William Brodie-Tyrrell Feb 11 '16 at 0:03
  • \$\begingroup\$ [link} (s185.photobucket.com/user/busyguy360f1/media/…) This is the general diagram. My diode point is yellow wire at the switch to block reverse current when the circuit is in low level (i.e white wire is +12v) \$\endgroup\$ – Shahab Feb 11 '16 at 0:21
  • \$\begingroup\$ You already said that. You need to post a real schematic. \$\endgroup\$ – EM Fields Feb 11 '16 at 1:09
  • \$\begingroup\$ @ EM Fields. Sorry, i do not have real schematic and i do not know how to draw one. I am physician by profession. All i can make is a drawing like one above. I just do these things as a hobby. \$\endgroup\$ – Shahab Feb 11 '16 at 1:53
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    \$\begingroup\$ We really need a schematic showing how your modified circuit, including the "electronic control relay" is connected to be able to make any informed comment, or even to see why you feel you need this diode. \$\endgroup\$ – Peter Bennett Feb 11 '16 at 2:06

Expected Diode current is 7.5 Amp.You will need Say a 10 Amp shottky rated at say 40V . This needs a heatsink .

  • \$\begingroup\$ I do not know how to attach picture but i have added link in comments above. Do i really need this big a diode? the seat heater pads draws more current than i would expect it to be present in yellow wire at the switch when the white wire is at 12V and yellow wire is just providing series connection between pads. \$\endgroup\$ – Shahab Feb 11 '16 at 0:29

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