I am currently trying to work out whether a particular PCB is faulty. The board consists of a low capacitance a.c filter in parallel across the line and neutral. The circuit also consists of potentiometer and transistor and few other smaller components which im not sure what they are for. The function of the board is to increase and decrease the power for a motor (A.C). The power goes into the PCB then out and down to spade connectors which connect to the motor.

I measure roughly 50 Mega ohms of resistance across the PCB which is why firstly i assumed it may be faulty, but then when I measured voltage at the spade connectors by the motor I was getting 240 volts. Surely with 50 mega ohms I would have a large voltage drop and not read the full 240v.

So if the PCB is fine and functioning correctly is it perhaps due to the transistor acting as a switch which is why i'm reading such a high resistance while it is not energised.

Thanks if anyone can share any light on the situation.

  • 2
    \$\begingroup\$ It is almost impossible to answer such a question without a schematic. \$\endgroup\$
    – Macit
    Apr 14, 2017 at 17:15
  • 2
    \$\begingroup\$ DRAW. SCHEMATIC. \$\endgroup\$
    – winny
    Apr 14, 2017 at 17:32
  • \$\begingroup\$ Indeed, but I bet you will find that that 'transistor' is a TRIAC and not a transistor at all, look up the number printed on the package. \$\endgroup\$
    – Dan Mills
    Apr 15, 2017 at 13:11

1 Answer 1


The key to fault finding electronic circuits is to understand how they are supposed to work. Otherwise you can only use the 'shotgun' approach of testing and replacing individual components until it (hopefully) works.

The procedure I generally follow is:-

  1. Visual inspection - check for burnt or cracked components, dry joints etc.

  2. Identify components by part number etc. and read the datasheets of devices I am not familiar with.

  3. Trace the circuit and draw a schematic.

  4. Make resistance/voltage/current measurements.

  5. Figure out how the circuit is supposed to work, and compare theory to actual measurements to isolate where it is going wrong.

Sometimes the fault becomes apparent in steps 1 or 2, but when it doesn't then drawing a schematic is essential for all but the simplest circuits. While it may take a little time to do, it saves time in the long run. It can also be combined with the earlier steps.

When you are not sure what certain components are for, it's time to find out! Think of it as not just fixing a fault, but also gaining general knowledge for the future.

Your 'transistor' is probably an SCR or TRIAC. You should read and understand its datasheet (if available, or look at similar devices) and the circuits it is typically used in. The 50MΩ resistance may just be due to leakage through active components. Most meters use a low voltage to measure resistance, which is not enough to fully turn on diodes etc. At 240V the situation could be very different.

Mains power is dangerous. Don't attempt a repair until you are sure about what you are doing! Be very careful when testing the device under power. Use an isolating transformer if possible, and clip leads or solder wires onto the PCB rather than touching the meter probes to it. Power the device through a light bulb to limit current in case of a short.


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