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Trying to understand transistors

Rob_K

Sep 20, 2013
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Sep 20, 2013
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Dear All,

I found this circuit for a latching switch using transistors and it works brilliantly, I've had it up and running.

Simple transistor latch

Now, I can see how it works, no current moves through the circuit until NPN transistor T1 is triggered, thus pulling current through the base of the PNP transistor T2 allowing current to flow and activate the relay whilst also sending current through R3 back to the base of T1. Also I totally see where the capacitor comes in, the circuit just doesn't work without it, it just trips as soon as you connect power.

Two little questions to help me understand this. Firstly, I have used an electrolytic capacitor, as I couldn't find a ceramic one amongst my gubbins, but it seems to me the capacitor could potentially receive current from either pole, is this correct or am I barking up the wrong tree?

Also, I am having difficulty understanding why R4 is in this circuit and what it's purpose is.

Some clarity would be marvellous. Thanks in advance.

Rob
 

Harald Kapp

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Nov 17, 2011
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An electrolytic capacitor is fine as long as the anode is connected to the positive voltage rail. How could it possibly receive a reverse voltage? I can't see a way.

R4 is there to ensure that T2 is definitely off unless triggered by T1. R4 holds the base of T2 at positive potential. Only a negative potential (negative with respect to the emitter) will turn on T2.
 

Rob_K

Sep 20, 2013
59
Joined
Sep 20, 2013
Messages
59
Yes that makes perfect sense in both cases, thanks Harold for clearing this up for me.
 
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