Hi all,

I'm working on a circuit for a motorsport application and wanted to get some eyes on it before I build it. I'd appreciate any feedback on whether this will work as intended or if I've missed anything.


**The Goal**
I have a motor controller that outputs ~12–15V (automotive battery voltage) and reverses polarity as a control signal. I want to use that polarity reversal to switch between two DC-DC converters — a 24V and a 16V — feeding a single output load rated at up to 40A.

- Polarity + → 24V converter connects to output
- Polarity − → 16V converter connects to output
- Only one supply should ever be connected at a time



**The Circuit**
I'm using two identical Panasonic CB1-P-12V relays (12V coil, 40A SPDT).

Each relay coil is driven from the motor controller output via a 1N5404 diode in series — one diode forward, one reversed — so each relay only activates on one polarity.

A 1N5404 flyback diode is placed across each coil (cathode toward Pin 1 / controller side) to protect the controller from inductive spikes.

On the contact side, Pin 4 (COM) on each relay is fed from its respective converter through a 40A fuse. Pin 2 (NO) on both relays joins together at the output. Pin 3 (NC) is left unconnected on both.

All grounds — controller negative, both converter negatives, and load negative — share a common ground point.

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I guess my question is to see if there is something alarming that I am not thinking about, or any recommendations anyone could give.

Thanks in advance!

 

The 1N5404 3A diode current rating is overkill for the small relay coil current of 117mA, as a 1N4007 would be more than sufficient, but that doesn't hurt anything.
The diodes across the coils reduce the relay turn-off time, so there could conceivably be an instant where both relays are connected to their sources, which would not be good.
EDIT: Adding a resistor in series with each diode (e.g. 100Ω), should eliminate this as a potential problem.

Do you know how the controller reverses polarity?

 

The diode current rating is overkill for the small relay coil current, as a 1N4007 would be more than sufficient, but that doesn't hurt anything.
The diodes across the coils reduce the relay turn-off time, so there could conceivably be an instant where both relays are connected to their sources, which would not be good.
A slowdown of the change in the polarity reversal voltage would eliminate this as a problem.

Do you know how the controller reverses polarity?

Unfortunately I do not :/ all I know is that it does haha. I will look more into how it does it itself, but it is all enclosed electronics that are given to the teams. Should be a Cosworth part, I'll take a deeper look!

 

Note my edit to my post.

 

Note my edit to my post.

Got it. thank you!