Protecting a Velleman relay board (K8090) for automotive use

[VinzC]

Hi all.

I have a Velleman relay board that I need to protect somehow for use in automotive environment – battery nominal voltage is 12V. To cut a long story short such a board was installed in a vehicle and I need to do some post-sales maintenance and occasionally some repair. From what I've been told about the failure, I suspect the 5V regulator might be blown – the board still responds to the USB commands but the relays don't move, which typically happens when the power supply is disconnected... or something bad happened to the 7805. The board sinks up to 350 mA when all relays are on. The board is protected with a 5 Amp fuse in a dedicated fuse box.

I'm considering some transient protection for the board but there's no obvious way I could hook something to it.



I thought about replacing the diode rectifier bridge (D11-D14) with unidirectional TSV diodes, such as the TP6KE20A or TP6KE18A from Littlefuse. The diodes maximum current is about 22 Amp and I'd like to clamp the input voltage of the 5V regulator to something like 20 volts due to the 1000µF/25V electrolytic capacitor.

The TSV diodes aren't rated for power protection but the power protection diodes I found on digi-key are all SOT23 packages rated for 5V power lines, definitely not what I'm looking for. So is it worth replacing the 1N4007 (see page 10) with those TSV unidirectional diodes? Is the breakdown voltage OK or should I chose a higher value?

Any hint/advice welcome.