Automotive relay question

Started by DaveC October 17, 2014
I have a 12V SPST cube relay in my junque box. The diagram molded into the 
relay�s housing includes a component connected in parallel with the coil

which looks suspiciously like a resistor. 

It�s 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




On Fri, 17 Oct 2014 06:38:27 -0700, DaveC <invalid@invalid.net> wrote:

> I have a 12V SPST cube relay in my junque box. The diagram molded into > the > relay&#2013266105;s housing includes a component connected in parallel with the
coil
> which looks suspiciously like a resistor. > > It&#2013266105;s 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 >
And what voltage is across your coil as you measured this 'same' current?
On Fri, 17 Oct 2014 06:38:27 -0700, DaveC <invalid@invalid.net> wrote:

> I have a 12V SPST cube relay in my junque box. The diagram molded into > the > relay&#2013266105;s housing includes a component connected in parallel with the
coil
> which looks suspiciously like a resistor. > > It&#2013266105;s 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.