I'd like to switch a boiler on and off electronically. It is now connected to a thermostat. The first thing I did was measuring the current flowing, closing the circuit with a multimeter set to DC. It is 0.44mA. Then I switched to voltage and measured 90V. I didn't expect such a high voltage.

I was wondering if I could use BC547 transistor, but I saw that limit on collector voltage is 50V. So I think it will not fit. However the current is very small so I don't know.

Do you have any suggestion ? May I still use this transistor, or you have another kind of transistor to suggest? I would use NPN transistors because they are the easiest to use. Would in this case be needed a different kind of transistor?

UPDATE @ Dave Tweed: Good point the AC. I closed the circuit with my multimeter set to AC. I got 120V and 0.6mA. Now I don't know any more if it's DC or AC.

@ markrages: NPN transistors, as long as I have read are easier compared to PNP. What do you mean when you say they are not easy? Compared to what? I'm afraid I don't get the sense of your sentence.

@ George Herold and @ Peter Bennett: A relay is easy to intall and certainly working for my project. This is why I'm asking about transistors. However, I didn't consider the circuit isolation point, which I consider very valuable information, since I really don't know anything about the remote circuit. I just have 2 cables and I know that if I connect them the boiler powers on. The reason why I am asking about transistors as an alternative to relay are the following Relays are more expensive, more noisy (chatters), larger, more power demanding.

So I renew my question: Does it make any sense to use a transistor to electronically turn on and off a remote circuit with the current characteristics outlined above? In case nobody has any suggestion for this question I'll certainly use a realy as suggested.

  • 2
    \$\begingroup\$ Did you try any AC measurements? Boiler control circuits often run on 24 VAC (not DC). \$\endgroup\$
    – Dave Tweed
    Feb 12, 2015 at 20:19
  • \$\begingroup\$ NPN transistors aren't easiest to use, especially in a circuit that needs a different type. \$\endgroup\$
    – markrages
    Feb 12, 2015 at 20:24
  • \$\begingroup\$ How about a solid state relay? \$\endgroup\$ Feb 12, 2015 at 20:55

2 Answers 2


I would use a relay for this. A relay provides electrical isolation between the controlling circuit and the controlled circuit. You would not need to know anything about the controlled circuit: whether either side is grounded, polarity, voltage, AC/DC, etc., as long as the relay contacts are rated for the voltage and current they will need to handle.

  • \$\begingroup\$ I don't see why he should not use a transistor if he is switching DC currents. It is much more durable than a relay. \$\endgroup\$
    – gstorto
    Feb 13, 2015 at 9:54
  • 1
    \$\begingroup\$ @GleisonStorto galvanic separation is a good practice where you have no knowledge about the remote circuit. You may be creating a huge ground loop causing intermittent behavior. \$\endgroup\$
    – jippie
    Feb 13, 2015 at 20:21

I agree a relay would be a better choice (though my transistor knowledge is not great) your current requirements are small so you can use very small relays that make no noise and will handle the voltage whether 12V or 120V and provide the isolation.


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