All About Circuits Forum 12v SPDT relay Works only on 12Volts?
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#11
04-27-2013, 06:12 PM
 k7elp60 Senior Member Join Date: Nov 2008 Location: Utah, USA Posts: 368

Quote:
 Originally Posted by strantor on the inside, a relay is like a spring loaded light switch with an electromagnet attached to it. as you increase the voltage to the coil, the current in the electromagnet increases and the magnetic field gets stronger, which brings the contacts closer to touching eachother. At some point (7V apparently) the electromagnetism has increased sufficiently to juuuust barely make the contacts start to touch eachother, and you start to detect current flowing through the contacts. This is not a great place to stop. you need to continue all the way to 12V. You need to have a strong magnetism to hold the contacts firmly together. If you do not (if you leave it at 7V) then you will have sparking and arcing inside between the contacts which will cause them to wear prematurely or possibly even weld themselves together if your load draws enough current. You can observe this with a household light switch. If you try to hold the switch in the center position and move it back and forth in tiny increments you can hear the arcing inside; if you remove the cover, on some switches, you may even be able to see the sparks.
I agree with strantor. If the relay doesn't have the normal operating voltage on the coil the contacts are not fully making contact and the contact resistance can be affected. This could cause more voltage drop across the contacts than anticipated.
#12
04-27-2013, 06:31 PM
 ErnieM Senior Member Join Date: Apr 2011 Location: Lon Guyland, Noo Yawk Posts: 5,317

A relay coil has various voltage ratings:

Code:
``` - Nominal voltage, or the "design" midpoint.
5V, 9V, 12V, 15V, 24V are commonly found.

- maximum voltage (so you don't burn up the coil)

- pick up (pull in) voltage: the minimum guaranteed voltage to
make the contact close. Obviously, it is designed to work at a
lower voltage to insure making the spec.

- release voltage: the maximum voltage a closed contact will release.
Again, it is designed to release at a voltage greater
to meet this spec.```
Select All
I've seen engineers select a 15V relay for a 12V system where they are depending on the maximum pull in voltage to be met by their driver, and also are using the higher coil resistance of the higher voltage rated part to lower the current consumption.

"Nominal voltage" is only a suggestion. It is the other numbers that make or break the design.
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 The Following User Says Thank You to ErnieM For This Useful Post: Tajiknomi (04-28-2013)
#13
04-28-2013, 01:44 AM
 THE_RB Senior Member Join Date: Feb 2008 Posts: 5,377

Quote:
 Originally Posted by k7elp60 I agree with strantor. If the relay doesn't have the normal operating voltage on the coil the contacts are not fully making contact and the contact resistance can be affected. This could cause more voltage drop across the contacts than anticipated.
I'll agree with ErnieM and disagree with you two.

With relays the voltage is needed primarily for pull-in, because the least efficient magnetic path for the armature is at the start.

Once the relay pulls-in, the metals parts of the amrature are touching, or very close to touching, and the magnetic path is so much more efficient they only need a fraction of the current to remain closed and keep the contacts closed.

So operating a relay from a reduced voltage would first start to give the symptom of failing to close altogether, long before you got the symptom of "increased contact resistance".

Example; 12v relay;
12v = 100mA coil current
pull-in current might be 70mA
hold current might be 30mA
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 The Following User Says Thank You to THE_RB For This Useful Post: strantor (04-28-2013)
#14
04-28-2013, 02:10 AM
 strantor Senior Member Join Date: Oct 2010 Location: Houston tx Posts: 3,588

Quote:
 Originally Posted by THE_RB I'll agree with ErnieM and disagree with you two. With relays the voltage is needed primarily for pull-in, because the least efficient magnetic path for the armature is at the start. Once the relay pulls-in, the metals parts of the amrature are touching, or very close to touching, and the magnetic path is so much more efficient they only need a fraction of the current to remain closed and keep the contacts closed. So operating a relay from a reduced voltage would first start to give the symptom of failing to close altogether, long before you got the symptom of "increased contact resistance". Example; 12v relay; 12v = 100mA coil current pull-in current might be 70mA hold current might be 30mA
I'll disagree with myself, and agree with you. At least on the theory part about the efficiency of the magnetic path and actuation current vs. holding current. But I stick to my story about using the proper voltage. In practice, I have witness the things that I described (chatter, arcing, welded contacts) and correlated them with low supply voltage. If I could go back in time I might find though, that the problems were caused by fluctuating supply voltage and not just low supply voltage.
#15
04-28-2013, 11:12 AM
 Tajiknomi Junior Member Join Date: Mar 2010 Posts: 13

@Strantor : I got that point now, Coz i have tried another Relay , Which is 6Volts but it active on 3.5V (According to my DMM) But as you have said ,it isn't firm Contact.. So one should make a relay working on its Described voltage, not below that voltage Coz of Arcing..
Thanks to all for your kind and helpful replies

 Tags 12v, 12volts, relay, spdt, works

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