I currently have a circuit which consists of two different boards.
In the main board, there is a \$230\,\mathrm{V}\$ ac supply, a \$230\,\mathrm{V}\$ to \$12\,\mathrm{V}\$ dc power supply, a 7805 linear voltage regulator and other components as transistors biased on their collectors by the \$12\,\mathrm{V}\$ dc generated in the main board itself. On the another board there is a controller, powered from the \$5\,\mathrm{V}\$ supply generated by the main one: this controller drives the transistor located on the main board.
Now I want to use a separated \$230\,\mathrm{V}\$ to \$12\,\mathrm{V}\$ dc power supply for the controller board, so I'll be able to power my controller board from a \$3.3\,\mathrm{V}\$ power supply produced by using another regulator. So there will be two \$230\,\mathrm{V}\$ to \$12\,\mathrm{V}\$ dc regulators, one located on the main board and another on the controller board. Now my question is:

Is it OK for the controller to drive transistors whose \$12\,\mathrm{V}\$ collector bias is generated from a different, separated regulator, however sharing the same \$230\,\mathrm{V}\$ dc grid voltage,?
I am also using an ADC which is measuring the output of a temperature sensor powered by the main board \$5\,\mathrm{V}\$ supply and sending its output to the controller board: could this create any problem? should I short the gnd terminals of both boards? Do I need to do anything else? Is this operation safe?

Block schematics of the circuit

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1 Answer 1


You have identified one issue correctly: both need the same ground reference, tying the grounds together will do that.

The other issue is whether your circuitry will work if only one of the supplies is on.

In your case, if the controller is powered, pulls the base of the transistor high, but the supply on the collector is off, then the other circuit will be powered through the base-collector diode of the transistor, i.e. the relay gets 2.6V (3.3V minus Vf of that diode) from a microcontroller GPIO, and the internal resistance of the relay will not be high enough to limit the current to what is safe for the microcontroller.

So, if the top power supply fails, your microcontroller will be fried if it tries to pull the transistor base high.

The simple fix would be a current limiting resistor on the base (which needs to be large enough to keep the current below the maximum for the GPIO, yet small enough so the transistor can fully open; whether such a value exists depends on the 𝛽 of the transistor and the design current through it).

A probably better fix would be to replace that BJT with a FET.

Side note: are you using that transistor to short 12V to ground to turn off the relay? You probably want that in-line with the relay, and protected with a flyback diode, and possibly also add a capacitor and current limiting resistor for the relay.

  • \$\begingroup\$ yes, i added 10k resistor between Controller pin and base of BJT also relay have flyback diode on Coil. for replacing BJT with FET not possible. because i do not want to change main AC board i can make changes in controller board. will it be safe if i add resistor to base and flyback on relay? not adc will create any problem as supplied from another regulator? \$\endgroup\$
    – Tjtech2
    Dec 15, 2018 at 11:25
  • \$\begingroup\$ If the relay already has a flyback, that should be fine then. Not sure if the transistor will fully open with 10k, this is something you have to find out. You can probably go as low as 135Ω, assuming 20mA current limit. The ADC will probably be fine, assuming there is an Op-Amp on the input, but you might want to check the "absolute maximum ratings" page of the datasheet to see if voltage limits are relative to GND or to Vcc there. \$\endgroup\$ Dec 15, 2018 at 11:44
  • \$\begingroup\$ Yes i Changed resistor on Base of transistor to 2.2K and also it has divider on ADC pin to fall withing range. still i am not clear, whether it will run or have any problem. i also attached relay circuit. \$\endgroup\$
    – Tjtech2
    Dec 17, 2018 at 4:52
  • \$\begingroup\$ anyone have tried simillar thing before? \$\endgroup\$
    – Tjtech2
    Dec 17, 2018 at 5:02

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