Electrical – Reverse Polarity & Load Dump Protection (TVS)

Circuit protection isn't my forte but since I was involved in the development on Relay for Intermittent Wiper Function I will chip in.

Is the 100mA PTC fuse appropriate for the circuit?

It seems appropriate.

Is the TVS a good idea or unnecessary with the LM2931 already having 60V load dump protection, -50V reverse transient protection, internal current limiting, and temporary mirror-image protection?

I don't know.

Is the 6.3A fuse appropriate? (The motor power from M_LO and M_HI are on a 20A fuse, much too high for the PCB). The motor can pull over 2A at start and about 1A while in operation.

If your measurements are correct it's fine. The 2 A start current sounds a bit low. What happens if the wipers are loaded with snow or leaves?

Are the emitter resistors (R4 and R8) appropriate? I did not have them in the original circuit but believe there should be some value there to limit emitter current to ground.

No.

1. The datasheet says that the relays are 53 mA at 12 V. The most you can get through 1k at 12 V is 12 mA so the relays won't pick up.
2. They will cause the emitter voltage to rise up to the Tiny's output voltage and will cause Q2 and 3 to be only half-on. You have designed a kind of negative feedback current limiter.

Just bridge them out or put in wire links if you've made a PCB.

Are the snubbers necessary for a 28V/2A motor relay? Selection of snubber components acceptable?

They can only help. The motor is inductive and they should protect the contacts - although you've only snubbed the NO side.

Would any components apart from the TVS diode and relays need to be substituted when building for 12V (~14V) use rather than 24V (~28V) use?

Looks OK.

The only other comment I have is that D6, 1N4148, is rated at 100 mA (from memory) and might be a little puny for the relay. A 1N4001 (1 A) might be safer for D6, D4 and D5. Take others' advice on that.

I have not checked the relay logic matches the design worked out in the original question.

I know this question is several months old but I couldn't help but respond.

I had a nearly identical problem with a project at work recently. The TVS diodes originally used on a board were too small (too low of a breakdown voltage) and would fail short-circuit, taking out the entire board. The solution for me involved sourcing a new diode, and there were two parts to this:

1. Spec a diode with a breakdown voltage as high as possible while maintaining a reasonable clamping voltage (the clamping voltage must be below the absolute maximum ratings of the devices it is protecting). My project ran on a maximum of 15 volts and the chip being protected had an absolute maximum input voltage of 35 volts. I sized the diode to have a breakdown of 18 volts and a clamping voltage of 30 volts. Keep in mind that the breakdown voltage is nominal and the actual voltage may vary, so having it so close to 12 volts could be a problem. I would recommend a diode with a 15V breakdown voltage, but again, make sure the clamping voltage is lower than the absolute maximum voltage of the next device down the line.
2. Make sure the diode is suitably sized to handle the power properly during a transient event (especially important during nearby lightning strikes but is still applicable for ESD). Instead of using an SMA, try using an SMB or SMC. It looks in your image like you have enough room, so shoot for an SMC.

Something else I notice, you have your reverse polarity protection (D10) going straight between the power and ground. This is very bad form as it will be shorted and damaged in the event that the polarity is reversed. You should consider using a series diode instead, or even better a MOSFET-based reverse polarity protection circuit (these are very easy to implement).