NPN and relay

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Posts: 6
Joined: Tue Dec 18, 2018 7:58 am

NPN and relay

Post by kymyc » Wed Dec 26, 2018 8:59 am

Hi, i have a need to turn on a [url]relay[ ... il/1295104] but because of where everything is sited/currently used the transistor needs to be before it (i.e. between the +ve and the relay coil). I have a 2N3904 NPN but it doesnt work. If i swop to a PNP will tht solve my problem. Also where would be best to connect the protection diode?


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Joined: Sun Oct 04, 2015 6:18 pm

Re: NPN and relay

Post by David » Fri Dec 28, 2018 11:47 am

You can't just replace a NPN transistor with a PNP one (and vice versa) and expect it to work. The circuitry would need to be changed to accommodate the altered polarity. You will need to find out the chacteristics of the faulty transistor (current and voltage handling, gain and dissipation are the main ones) are and get a similar tranny. The diode goes straight across the relay coil, connected so that it is reverse biased when the relay is powered.

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