I have a PCB that is giving occasional problems.

The circuit is a remote receiver, based on the Heltec Cubcell Module. Basically, it will turn up to 3 relays on, depending on the signal it receives from the transmitters.

The circuit is powered from 12VDC, supplied by a lead acid battery, which is charged by a stationary engine - similar to a rideon lawn mower setup.

2 of the 3 relays power up an electric clutch (max current draw 4.7A), while the 3rd relay powers up a small solenoid (~2A draw)

Early on, we were having issues with the DC-DC converter blowing up (suspect reverse voltage from switching of inductive load), and the occasional relay sticking on. To fix the relay issue, I added diodes across the inductive loads. To fix the DC-DC issue, I added a TVS (D9) between Vcc and GND to reduce voltage spikes, as well as a diode (D12) between Bat -ve and GND to prevent any reverse voltage. This has worked quite succesfully, with perhaps 1% of boards failing since. However, we have recently had a few boards where the TVS has short circuited after a few days of use. All of these boards have been on a machine powered by a diesel engine instead of petrol engine, so I suspect the larger starter motor is introducing larger voltage spikes that are overwhelming the TVS.

Other than using a TVS rated for more power, is there something else I should be looking at to make the circuit more robust?

Thanks

 

I assume that the circuit itself doesn’t take much current, just a few relay coils. Try some resistance in series with the supply, so the TVS can do its stuff Without blowing up.

 

Hi,
-add C-L-C filter for Vcc ,1000uF, 1mH, 1000uF .
-add 1,5 kOhm resistors in series with diodes D1,D2,D3. This will reduce relay switch -off time. Replace transistor D882 to another, with higher Vce, ( 2SD2096 or similar)
-resistors of 10 Ohm 1W can be added in series with D6,D7,D8 to reduce current decay time

 

Hi,
-add C-L-C filter for Vcc ,1000uF, 1mH, 1000uF .
-add 1,5 kOhm resistors in series with diodes D1,D2,D3. This will reduce relay switch -off time. Replace transistor D882 to another, with higher Vce, ( 2SD2096 or similar)
-resistors of 10 Ohm 1W can be added in series with D6,D7,D8 to reduce current decay time

Would you add the CLC filter before or after the supply to the inductive loads?

 

I assume that the circuit itself doesn’t take much current, just a few relay coils. Try some resistance in series with the supply, so the TVS can do its stuff Without blowing up.

What value R would you recommend?

 

Ten ohms could be a reasonable resistance. The trouble with many TVS devices is that once they trigger and start conducting and continue conducting. .

 

What value R would you recommend?

How much current does your circuit require? How much power supply overhead do you have? How much efficiency can you afford to lose?

 

How much current does your circuit require? How much power supply overhead do you have? How much efficiency can you afford to lose?

Without the load on the relays, the circuit requires less than 500mA
With the load on the relays, the circuit requires up to 8A.
The circuit is a nominal 12V circuit, however, it typically runs at 14.2V when the engine is running, so have got a good 2.2V spare

 

Would you add the CLC filter before or after the supply to the inductive loads?

CLC filter for AP63203 only. And relocate diode D5 in parallel to TVS D9

 

Without the load on the relays, the circuit requires less than 500mA
With the load on the relays, the circuit requires up to 8A.
The circuit is a nominal 12V circuit, however, it typically runs at 14.2V when the engine is running, so have got a good 2.2V spare

So connect the loads before the TVS and drop 2V across a resistor. 2V/500mA=4Ω 0.5W. Use a 15V TVS and all should be well.
If it still fails, you must be getting some longer term over-voltages, which you should be able to find the cause of.