On Fri, 17 Oct 2014 06:38:27 -0700, DaveC wrote:

I have a 12V SPST cube relay in my junque box. The diagram molded into
the
relay¹s housing includes a component connected in parallel with the coil
which looks suspiciously like a resistor.

It¹s not a back-emf diode: I connected the relay coil using both
polarities
(using a current-limited power supply) and the same current draw is
measured.


What is the purpose of this resistor?

Thanks,
Dave


Don't know about that specific case, BUT envision current flowing through
the inductor, then you turn it off and the current has to go somewhere, it
goes through a resistor. Since you knew the current through the coil, you
know the voltage that can 'pop' across the resistor [equal to the drive
current times the resistance, but reverse polarity]

This technique is sometimes used to 'dump' the current out of an inductor
faster. A small catch diode can take a LONG time, but the higher voltage
of the current going through the resistor dumps the energy pretty fast.
The area under the curves is pretty constant. High voltage is fast, low
voltage takes a long time.