# delay circuit with relay , resistor and capacitor

Discussion in 'The Projects Forum' started by Safar.na.zizi, Jan 30, 2016.

1. ### Safar.na.zizi Thread Starter New Member

Jan 30, 2016
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0
How to calculate this diagram..what is this circuit called? Rc circuit or voltage divider circuit?

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2. ### andre_teprom Member

Jan 17, 2016
31
9
Both.

Anyway, setting the R value splitted into R1/R2 is useless, unless due to power rating constraints.

3. ### Safar.na.zizi Thread Starter New Member

Jan 30, 2016
4
0
Im using 3kohm resistor and 10u f capacitor.. if my power supply is dc110v and relay rated voltage is dc48v..with coil resistance 2.6k ohm..i can calculate the delay time with this equation?
T=-rc ln (1-Vc/Vs)?

4. ### Safar.na.zizi Thread Starter New Member

Jan 30, 2016
4
0
Im not sure what reference of R for this calculation..

5. ### andre_teprom Member

Jan 17, 2016
31
9
R = R1//R2 = (R1*R2)/( R1+R2 )

6. ### Safar.na.zizi Thread Starter New Member

Jan 30, 2016
4
0
Not needed to consider coil resistance of the relay?

7. ### andre_teprom Member

Jan 17, 2016
31
9
No, the electrical design of the relay is done in such a way that you can simply apply a voltage on its bobin terminals, and the current limit is done by it bobin wire's resistence..

8. ### AnalogKid Distinguished Member

Aug 1, 2013
4,686
1,297
Yes.

If you are operating a 48 V relay with a 110 V power source, then the relay coil resistance is critical. The first thing to do is to calculate the resistors necessary to protect the relay. The delay time calculation canot be started until this step is completed.

First calculate the current through the relay coil at 48 V using the relay coil resistance, and call that Irelay.
Next, subtrace 48 V from your 110 V source, and call that Vr.
Next calculate the total resistor value Rtot based on Vr and Irelay.
Next, calculate the total resistor power Pr, based on Rtot and Irelay.
Now you know the total resistance needed to protect the relay, and the total power that resistance has to dissipate. You want to use a resistor rated for at lease twice that power level for long-term reliability. If you can't buy a single resistor to do the job, you can use two resistors in parallel. Each resistor must have twice your calculated Rtot value.

After all of that is done, you can calculate the capacitor value needed to get the time delay you want. How long a time delay do you need?

ak

9. ### andre_teprom Member

Jan 17, 2016
31
9
In fact, I was not considering the circuit that Safar originally posted there. Although can work with no accuracy, it is not too usual set delay time based on its coil resistence, which a priori could vary a lot, because is not one of the parameters that define its main features

10. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,968
1,135
Why do you want your relay contacts to short the supply when they close?

11. ### AnalogKid Distinguished Member

Aug 1, 2013
4,686
1,297
I think that is an amateur drafting mistake, and the two arrows to the left, not connected together, connect to a load circuit or device.

ak