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relay not activating

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gjpollitt

New Member
CAn someone please tell me why the relay would not trip on this circuit?

This is a simulated circuit with a 5volt relay

The output from pin 7 of the lm311 is activated with 8.3 volts to the relay yet it does not change state. Does there have to be a certain amount of current and if so how can i increase it to the relay as changing r2 doesnt make any difference with regards to making the relay trip

thanks
graham
 

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Roff

Well-Known Member
See the schematic below. If your relay coil draws more than 50ma or so, you will need a separate transistor to drive it.
The reason your circuit isn't working is you have 2k ohms in series with the coil when it is trying to turn on. You need to measure the voltage across the coil to see if you have enough voltage to trip it. BTW, I can't see how that circuit could give you 8.3v at pin 7 in any case.

I reversed the + and - pins on the inputs, which I think is what you will need to do, because the relay will turn on when the output is low.

You seriously need to figure out how to post your schematics here. These zipped Word docs are a pain. All you need to do is figure out how to save them as a .GIF or a .JPG file. If you can't do it from your schematic capture program, try a screen capture program like this one.
 

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  • relay_dvr_302.gif
    relay_dvr_302.gif
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Sebi

Active Member
Apply an NPN transistor for relay driving. If You need only a lamp, connect it direct the transistor collector, without relay.
 

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