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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 Friday, October 17, 2014 5:57:54 AM UTC-7, DaveC wrote:
> I have a 12V SPST cube relay in my junque box. The diagram molded into th=
e=20
>=20 > relay=B9s housing includes a component connected in parallel with the coi=
l=20
>=20 > which looks suspiciously like a resistor.=20 >=20 >=20 >=20 > It=B9s not a back-emf diode: I connected the relay coil using both polari=
ties=20
>=20 > (using a current-limited power supply) and the same current draw is measu=
red.=20 So, perhaps it's a varistor. That's a ceramic gizmo, looks sometimes like = a disk capacitor, that has symmetric forward and reverse breakdown. It'd be a goo= d surge protector, just like a catch diode, but isn't polarity sensitive (so = works with AC or any polarity DC coil excitation).
In article <0001HW.D066606206DB1E51B05399BF@news.eternal-september.org>, 
invalid@invalid.net says...
> > 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
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
In article <0001HW.D066606206DB1E51B05399BF@news.eternal-september.org>, 
invalid@invalid.net says...
> > 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
Also, it could be a bidirectional TVS diode, those will clamp either polarity when ever the voltage exceeds the coil rating. Jamie
In article <MPG.2eab6a6c6cc7d5bc989ac2@news.eternal-september.org>,
 "Maynard A. Philbrook Jr." <jamie_ka1lpa@charter.net> wrote:

> In article <0001HW.D066606206DB1E51B05399BF@news.eternal-september.org>, > invalid@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.
Followups set to sci.electronics.components .

In sci.electronics.components DaveC <invalid@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
In article <mcmurtrie-0C54DC.19172817102014@news.sonic.net>, 
mcmurtrie@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

"Maynard A. Philbrook Jr." <jamie_ka1lpa@charter.net> wrote in message 
news:MPG.2eab6a6c6cc7d5bc989ac2@news.eternal-september.org...
> In article <0001HW.D066606206DB1E51B05399BF@news.eternal-september.org>, > invalid@invalid.net says... >> >> 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 > > 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.
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 >
On 10/17/2014 8:57 AM, DaveC 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 > > > >
It's usually a suppression resistor. Bosch used them a lot.
> 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.