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?

 

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?

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.

 

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

 

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.