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# This email is very technical

A couple of weeks ago I received an email.

It’s a bit technical, but here goes:

[Tom’s question]:

Hi Oyvind,

I have a simple question I can’t figure out. It’s about the relay circuit and it’s behaviour. The circuit seems to work for me but the rate of switching between the NO and NC is strange. I had assumed it would be using the time constant = RxC, and would switch after approx. 3 or 4 tau. but it does not. My circuit uses 233ohms and a 2200uf cap, tau = .51 sec approx.. .What happens is that Initializing( powering up ) the circuit takes 9 seconds thereafter switching every 4 seconds. Is this correct ?

thanks for any help you could give me to explain this strange behavior.

Hi,

That’s a good question!

The fact that you are not experiencing the same in your experiment as in your calculations is likely because of your relay.

There are two things in the relay that affects the circuit:

-The voltage at which it triggers
-The resistance in its coil

A 9V relay triggers before it gets to 9V. Let’s say you had a relay that triggered when it’s coil voltage was 6V. The RC time delay is the delay for the voltage to become 63% of it’s full value. So in your case, the relay should trigger after one tau (0.51s) right?

But because the coil of the relay has a resistance, the R2 and the resistance of your coil make up a voltage divider (you can ignore the capacitor in this case). You can read about voltage dividers in the book at page 72-73, or here

Because of the voltage divider, the maximum voltage is no longer 9V. The maximum voltage is now maybe 7V (decided by the voltage divider output). 63% of 7V is only 4.4V, which is not enough to trigger the relay, so it would take longer to trigger it.

Does this make sense?

Cheers!
Oyvind