Relay Driver Q

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ripple_fx1

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Joined
Dec 29, 2008
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2
Hello - searched to no avail....sorry.

I've run out of +5vdc current, and need to activate my non-polarized 100mw relay coils w/-24vdc in a circuit with both of these rails sharing a common ground.
logic goes hi, opens a transistor, which closes the relay.
easy with a bipolar if both Vcc's are +, but I'm not sure what type of discrete transistor to use in this case. National DS3680 would work for an integrated approach, but due to layout, I'd rather not....

Thanks!



 
When the base of an NPN transistor is made positive it turns on. Reverse is applicable to PNP. You tie the base of a PNP (BC558?) to the ground (with a resistor) which is positive, so it stays turned off. When you apply negative to the base it turns on.  

Or an expensive method would be to use two NPN (BC548?). First one drives the second one. Base of the first one is tied to ground (positive) and is turned on, second one is turned off. When the base of the first is pulled negative it turns off, this time second turns on.

Edit. I have just noticed Abbey's reply.  Iwas not sure whether you want to turn it with -24V but anyhow you have  a bit of an explanation of the operation.
 
Abbey,

Would it not be better to omit the series resistor on emitter of PNP and put the second resistor in series with the base of NPN after the one that ties its  base to ground?
 
sahib said:
Abbey,

Would it not be better to omit the series resistor on emitter of PNP
No, because it will clamp the logic high to 0.7V, which may be a risk.
and put the second resistor in series with the base of NPN after the one that ties its  base to ground?
That's a possibility. May ne necessary if logic low is high enough to slightly turn on the PNP. Obviously, this is a sketch illustrating the principle. Should work perfectly in a perfect world where logic low is zero and logic high is always really high...in real world, needs some tweaking, as always. After all, we are perfect, it's our world that needs tweaking  :D
 
fancy abbey - thank you all for putting your big brains on this - i'll give it a spin and let you know how it fares with a suppression 1N and led....
 

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