Low current to transistor base pin

I am having a problem with a pcb that is having issues with low relay coil voltage. It is a 12v relay and i get 12v on the supply side of coil but on the ground side i am getting 6v…it reads 6v if i probe across it…

It is working and turning on the compressor (wine cooler)…but after a while it starts sparking…and rattling…

I checked everything i could think of so far…replaced transistor…i have 2.28 dc volts coming from the display board into the pcb…from there it goes to a 10k ohm resistor and that knocks it down to .5v which is not enough current to the transistor to fully turn it on resulting in a weak corcuit for the relay coil to work right…

Im tempted to just replace the resistor with a smaller one and get it so it works any way i can but i dont know if it will work with shutting it off too
…plus i just want to find the problem now

Hello macknumber9 ,

Thank you for contacting DigiKey TechForum. We are looking into this for you and will let you know what we find out.

Thank you

Hello macknumber9,

This schematic presents my understanding of your circuit. Your concern about the resistor is correct. You are likely starving the transistor base.

For circuits such as this, the transistor is typically saturated (turned on hard) so that there is minimal voltage on the collector with respect to the emitter. One common way to do this is to place the transistor into a “forced beta condition.” Under these conditions the resistor values are selected so that the base current is 1/10 of the collector current.

Unfortunately, I can’t provide a specific recommendation as I don’t know what type of coil you are driving. However, even small relays will demand around 50 mA. Using the forced beta calculation, this would require a resistor in the neighborhood of 500 Ohms.

CAUTION: I cannot recommend this resistance without knowing more about the circuit that is driving the transistor. You are also correct to be concerned about lowering the resistance as it could damage the driving circuitry.


  • Use a Darlington transistor. While it’s likely over-specced for your application, the TIP122G may provide an acceptable stopgap solution. Know that the Darlington has significantly higher gain and will not demand as much input current.

  • Use a MOSFET. Again, it’s likely over-specced for your application, but a logic level MOSFET such as the IRLZ14PBF may be suitable. Once it is turned on it will require very little current from the driver.

  • Determine the current required by the relay coil, use the forced beta condition, calculate a suitable resistor, and then determine if the driver circuitry can support the base current.

Best wishes on your design. Please let us know if you were successful. Pictures are most welcome.