# Using a relay, calculating impedence

Discussion in 'General Electronics Chat' started by poopscoop, Jul 1, 2013.

1. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
Energizing a relay coil and I need to know the current so I don't smoke my 2N3904. Is it really as simple as DC resistance for the coil impedance?

Also, considering the coil is separated from the controller by a transistor, is there anything to be gained by putting a reverse biased diode across the coil to keep the spikes low?

2. ### w2aew Member

Jan 3, 2012
219
64
Yes, it is as simple as the coil's DC resistance. And yes, definitely out a reverse biased diode in parallel with the coil to avoid damaging the transistor when you turn the relay off.

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3. ### Dodgydave Distinguished Member

Jun 22, 2012
4,967
744
Like this circuit, just measure the dc resistance of the coil, will prob be abut 100 to 200 ohms, for most 5v to 24V coils

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4. ### Stuntman Active Member

Mar 28, 2011
181
47
Generally, the DC relay coil you are planning to use should be spec'd for the supply voltage with a current (or power) rating. Often, the same relay model can be ordered with whichever coil meets your application (5v coil, or 12V coil, or maybe a 24V coil)

To better clarify, here is a good example from an OMRON relay (See pg.1 under "Ratings"):

http://www.omron.com/ecb/products/pdf/en-g5v_1.pdf

Notice that each coil is setup for a specific voltage, however, they all have the same power consumption. Consequently, a 12V coil will use twice as much current as a 24V unit. (P=V*I)

The point here is although measuring resistance should be fine, your coil should have a rated current.

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