Automotive relay question

[DaveC]

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

[DaveC]

I have a 12V SPST cube relay in my junque box. The diagram molded into the

relayï¿œs housing includes a component connected in parallel with the coil

which looks suspiciously like a resistor.

Itï¿œs not a back-emf diode: I connected the relay coil using both polarities

[RobertMacy]

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

And what voltage is across your coil as you measured this 'same' current?

[RobertMacy]

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

[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

> 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
>
>
>
>

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 <0001HW.D0666062...@news.eternal-september.org>,
inv...@invalid.net says...

more than likely a snubber network, they normally have a
resistor and cap in a single component..

Many times, they simply show it as a R across the coil because
it's in series with a cap.

http://en.wikipedia.org/wiki/Snubber

[Maynard A. Philbrook Jr.]

In article <0001HW.D0666062...@news.eternal-september.org>,
inv...@invalid.net says...
>

Also, it could be a bidirectional TVS diode, those will clamp
either polarity when ever the voltage exceeds the coil rating.

Jamie

[Maynard A. Philbrook Jr.]

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

Not really, could be a bidirectional TVS of sorts.

Jamie

[Kevin McMurtrie]

In article <MPG.2eab6a6c6...@news.eternal-september.org>,

"Maynard A. Philbrook Jr." <jamie_...@charter.net> wrote:

> In article <0001HW.D0666062...@news.eternal-september.org>,
> inv...@invalid.net says...
> >
> > 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
>
> more than likely a snubber network, they normally have a
> resistor and cap in a single component..
>
> Many times, they simply show it as a R across the coil because
> it's in series with a cap.
>
> http://en.wikipedia.org/wiki/Snubber

I haven't ever seen a snubber in a relay. It usually has the iron
designed to short-circuit fast field changes.

--
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.

[mrob...@att.net]

Followups set to sci.electronics.components .

In sci.electronics.components DaveC <inv...@invalid.net> wrote:
> 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. [...]

>
> What is the purpose of this resistor?

It suppresses the inductive spike when the relay turns off. See page
3 of the Tyco application note
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=Automotive_Relay_Applications&DocType=CS&DocLang=EN
or
<http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=Automotive_Relay_Applications&DocType=CS&DocLang=EN>
or even
http://is.gd/uGsYcA

I knew you could get automotive relays with nothing, a diode, or a
resistor across the coil, but I'd never heard before that any of these
shorten the life of the relay. I wonder what the mechanism is... does
it damage the coil winding insulation, or does it try to move the
armature a little, or what?

Matt Roberds

[Maynard A. Philbrook Jr.]

In article <mcmurtrie-0C54D...@news.sonic.net>,
mcmu...@pixelmemory.us says...

> >
> > more than likely a snubber network, they normally have a
> > resistor and cap in a single component..
> >
> > Many times, they simply show it as a R across the coil because
> > it's in series with a cap.
> >
> > http://en.wikipedia.org/wiki/Snubber
>
> I haven't ever seen a snubber in a relay. It usually has the iron
> designed to short-circuit fast field changes.
>
> --
>
>

We buy 24VDC relays with snubber networks in side on the coil, it
removes the need to put one on the terminals. It comes in handy when
you're doing a few rows of relays with PLCs and micro controllers
involved. They don't like the little pulse noise in the lines.

Those with diodes in them are ok for driver component protection but
they still can generate a noise pulse, just not a damaging one.

When you have bundles of wires tightly packed together, in
race ways and wire harnesses, like in cars, it can cross talk very well.


Jamie

[Arfa Daily]

"Maynard A. Philbrook Jr." <jamie_...@charter.net> wrote in message
news:MPG.2eab6a6c6...@news.eternal-september.org...

Looking at a data sheet for an Omron automotive relay that shows this
'unknown device' across the coil, it appears to detail it in the specs as
"1.1k suppression". It gives the coil resistance with and without this
included. If there was any series C involved, then the coil resistance would
not change with or without the device. This leads me to believe that it is
nothing more or less than a simple current dumping resistor with a value of
1.1k

Arfa
>
> http://en.wikipedia.org/wiki/Snubber
>

[Tom Biasi]

It's usually a suppression resistor. Bosch used them a lot.

[DaveC]

> It's usually a suppression resistor. Bosch used them a lot.

So the resistor dissipates the coil energy as the magnetic field collapses?
You still get back-emf; more than with a diode but less than without
anything?

This one is a Bosch.

Thanks.

[Tom Biasi]

Pretty much.

[Maynard A. Philbrook Jr.]

In article <ZTZ0w.212161$_r4....@fx25.iad>, tomb...@optonline.net
says...

:)
Jamie

[Ian Field]

"DaveC" <inv...@invalid.net> wrote in message
news:0001HW.D0666062...@news.eternal-september.org...

There are such things as bidirectional transient suppressor diodes.

Although I've never seen it on a car relay - a resistor isn't impossible, it
could act as a "Q-spoiler" to damp inductive ringing.

Or a bit more likely - a VDR.

[Phil Hobbs]

Think of it as an RC snubber without the C. The capacitance will be
whatever the coil and wiring supply.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[Arfa Daily]

>
> Think of it as an RC snubber without the C. The capacitance will be
> whatever the coil and wiring supply.
>
> Cheers
>
> Phil Hobbs
>

Don't you think that's starting to get a bit complicated Phil ? I would have
thought that the amount of C in the coil and associated wiring was
comparatively small - even given a car wiring loom. I reckon that its just a
simple damping load across the L to encourage it not to 'ring' so fiercely
as the field collapses ??

Arfa

[Phil Hobbs]

The reason you care about the ringing is primarily insulation
degradation due to the peak voltage. If the parallel resistor is six
times the coil resistance, the peak voltage kick will be close to six
times the drive voltage of the relay (because the coil current wants to
be constant).

That's how RC snubbers work, except that since they don't represent a DC
load, you can use lower value resistors (with a tradeoff in switching
speed, of course). The speed of the ringing is set mostly by the
capacitance of the winding, unless the snubber resistor is very small.