# Coil equations

Discussion in 'General Electronics Discussion' started by Sravanthi, Apr 24, 2015.

1. ### Sravanthi

72
1
Feb 4, 2015
HI....

Can anyone give the idea about what is the mathematical equations of energizing and de-energizing equations of electromagnetic coil in relay.

Thank you.....

5,165
1,087
Dec 18, 2013
Can you be a bit more specific in what you want?
Thanks

3. ### Sravanthi

72
1
Feb 4, 2015
Hi...

During the energization relay contacts are closed.
Relay during de-energization current equation how to derive???
In that equation how to consider the inductance variation due to environmental change between the contacts of the relay when opening the contacts.

5,165
1,087
Dec 18, 2013
Hello
I have not got a clue what your talking about. Sorry. I guess this is homework, can you post the question from your course work

Last edited: Apr 25, 2015
5. ### Sravanthi

72
1
Feb 4, 2015
Hi..

Its not my course work.
I am doing a project.In this I am not able to get the equations.
Here I am attaching the coil de energization waveform.Across the coil i added reverse bias diode and series resistor.
This waveform is the voltage across the series resistor(100Ω).
For this waveform I want to get a mathematical equation.
There is a dip in waveform because of inductance variation while opening the contacts.
So give the de energizing equation by considering the inductance in that flux path.

#### Attached Files:

• ###### coil de energization.png
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5,165
1,087
Dec 18, 2013
Because of the lack of a circuit diagram and any information I can only give you an example. You will need to add the different values of resistance an inductance of your relay. You will also have to include the capacitance of the relay coil for better accuracy. But this will give you an approximate answer.

Example:
Using a 2N2222 NPN transistor connected up as a common emitter switch with the relay connected to the collector and +12 Volts supply. A 100 Ohm resistor connected in series with a MURS320 reversed biased diode and connected across the relay coil. The transistor is switched on for 100 ms every 500 ms and the input to the base resistor is a pulse with 5 Volts amplitude and a 10 us rise and fall time.

Coil resistance 10 Ohms
Coil inductance 1 mH
Q1 on resistance very small so will be ignored.
Rise and fall time 10us

Formula:
Coil current = 12 Volts / 10 Ohms = 1.2 Amps

Voltage across R1 = L*(1.2 Amps / 10 us) -12.75 Volts = 107.25 Volts

The waveform shows 110.15 Volts. That's not far off.

Fig1: Circuit Diagram

Fig2: Voltage waveform across R1