24V Circuit Protection - Fuse, TVS Diode, RC Snubber

Question

I have pored over data sheets and read numerous articles on circuit protection. I understand conceptually, but I don’t trust my level of knowledge to implement it optimally. I want to be as specific as possible to meet forum guidelines so I’m asking for critical analysis of the circuit protection in particular, both for the LM2931 and the relays. Specific components selected below with rationale for selection. Data sheet links included. The circuit provides intermittent operation of a wiper motor.

Power Supply. LM2931 accepts up to 40V (suitable for both 12V and 24V systems), is “designed primarily to survive in the harsh automotive environment,” and the TO-220 package generates much less heat than a TO-92. Per the data sheet, C1 is a ceramic 10µF cap and C2 is a low-ESR 47µF aluminum cap. Providing some protection is D1 for reverse-polarity, the P6KE TVS diode – P6KE20 (17V standoff) for the 12V and a P6KE39 (33V standoff) for the 24V system – and a PTC 100mA fuse. Load on the 2931 is an ATtiny85 and two 2N2222 NPNs; around 10-15mA.

Relays. Switching is through two 20A Panasonic automotive relays: CP1-12V or CP1-24V as appropriate. Relays have a diode across the coil and a snubber across the C and NO contacts. The snubber is a 0.1µF metallized film cap and 100Ω carbon 1/2Watt resistor. Relays are controlled by a 2N2222 NPN with 1kΩ on the emitter reducing current to ground and 1kΩ between the base and the ATtiny. [D6 ensures K1 is energized if K2 is energized. This prevents both M_LO and M_HI grounded simultaneously.] Power to the motor enters the board from either M_LO or M_HI and exits through a 6.3A PCB fuse to M_GND. The PCB traces for the motor current are 200mil (100mil on top and 100mil on bottom with a via mid-length.) capable of carrying over 8A. The longest motor trace is 17mm.

I have operated the circuit with a different TO-92 regulator and without the TVS, fuses, and snubbers and it works correctly. Before I build another, I would appreciate some critical scrutiny and constructive criticism on the incorporated protection.

1. Is the 100mA PTC fuse appropriate for the circuit?
2. 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?
3. 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.
4. 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.
5. Are the snubbers necessary for a 28V/2A motor relay? Selection of snubber components acceptable?
6. 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?

LM2931

P6KE TVS

CP1-12V relay

Answer 1

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.

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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.