Automotive relay question

[DaveC]

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

Itšs 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

 

[RobertMacy]

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
relayšs housing includes a component connected in parallel with the coil
which looks suspiciously like a resistor.

Itšs 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?

 

[RobertMacy]

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
relayšs housing includes a component connected in parallel with the coil
which looks suspiciously like a resistor.

Itšs 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.

 

[Sjouke Burry]

On 17.10.14 15:38, DaveC wrote:

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

Itšs 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

Congratulations.
You found the perfect way of killing a protection diode.
After this the diode will bother you no more.
Dont touch my car please.

 

[Maynard A. Philbrook Jr.]

In article <544176f3$0$20406$703f8584@textnews.kpn.nl>,
burrynulnulfour@ppllaanneett.nnll says...

On 17.10.14 15:38, DaveC wrote:
I have a 12V SPST cube relay in my junque box. The diagram molded into the
relayšs housing includes a component connected in parallel with the coil
which looks suspiciously like a resistor.

Itšs 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




Congratulations.
You found the perfect way of killing a protection diode.
After this the diode will bother you no more.
Dont touch my car please.

Not really, could be a bidirectional TVS of sorts.

Jamie