Relay coil resistance

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When current flows through relay coil it creates the necessary magnetic field which then opens or closes the relay contacts.
Coil resistance is thus very crucial because if its too high it will limit the current and the relay may not work at all.
Is there any other reason as to why relay coil resistance is important?
Also how does relay coil resistance apply to electrical design?

thanks
 

templdl

Senior Member
Location
Wisconsin
When current flows through relay coil it creates the necessary magnetic field which then opens or closes the relay contacts.
Coil resistance is thus very crucial because if its too high it will limit the current and the relay may not work at all.
Is there any other reason as to why relay coil resistance is important?
Also how does relay coil resistance apply to electrical design?

thanks
Interesting. Wouldn't you specify if the relay had an AC or DC coil as both coils assemblies are different. With an AC coil the coils resistasns may be one thing but with an actual AC current applied to that coil you will have inductive reactance, impedance which is not present with a coil assembly designed for DC current.
A come for an AC coil must be a laminated core where the cvore for DC coil can be solid.
 

Upnorth

Member
Location
NH
Is there any other reason as to why relay coil resistance is important?
Also how does relay coil resistance apply to electrical design?

thanks

One practical reason for using a higher impedance coil might be to reduce the power budget when you're controlling a lot of relays with the same power supply. If the controller has a small 24VAC transformer rated for 20 Watts, it may not be very happy pulling and holding a dozen 200-ohm relay coils all day when you could have used 1700-ohm coils to do the same function with 1/8 the power.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150412-2360 EDT

Upnorth:

In general you can not just use a relay with a higher resistance coil to reduce power dissipation and do the same mechanical job (work).

Within some limited range the coil of a relay will be of a physical size that has the power to move the contacts that can handle a given switched load. For continuous duty purposes I suspect that the coil power required won't differ by more than a factor of two for a specific relay design between different manufacturers. An AC coil should use less power than a DC coil in the same relay.

Once a voltage has been specified, then based on the power needs to move the relay contacts the resistance is pretty much defined.

.
 

moonshineJ

Member
Location
USA
When current flows through relay coil it creates the necessary magnetic field which then opens or closes the relay contacts.
Coil resistance is thus very crucial because if its too high it will limit the current and the relay may not work at all.
Is there any other reason as to why relay coil resistance is important?
Also how does relay coil resistance apply to electrical design?

thanks

Some relay families have many different option (AC coil, DC coil, different number of contact, different material of contacts, etc.). Some coils wound with many turns of fine wire and have quite high resistance. On one occasion, when I took resistance across the coil with a multimeter, I had OL. After some extra time spent, I grabbed another model multimeter, and it shown something around 15k. Either the first Fluke had batteries frozen, or whatnot. Any way, the finer wire, more resistance has coil, higher volts it's rated for, and less current per volt will flow. The field is obtained rather for having thousands of turns of wire around core. Coils having wire of larger gauge would have less resistance, fewer turns, higher current, and lower coil voltage. So for some relays will be listed in catalog with different coil voltages, e.g. 28 V, 24 V, 12 V, etc. Others, with heavier gauge wire may be listed for operating current of coil, not voltage. They too will have a voltage drop across the coil, but again, they can be listed by current thru coil necessary to operate the realy.
Probably those who design circuits can answer better, when it's more advantageous to have relays with voltage coils, and when it's better to use current (by the way car starter is an oversized example of current-coil relay...600A, etc.)

Since transmitted power is product of voltage and current, the same power can be achieved either by raising voltage or by increasing current. Power transmissions use prefer higher voltage, as increasing current will require heavier gauge conductors > more copper > more bucks. I don't see the same reasoning being true for miniature relays, as fine wire costs more than heavy wire per unit of weight. I guess, there may be some other considerations as what type of coil to use (besides the available voltage).
 

templdl

Senior Member
Location
Wisconsin
I'm intrigued that the are attempts to address the OP question by having no knowledge of the relay being referred to. It's like shooting at shadows with no additional input from the OP.
 

mgookin

Senior Member
Location
Fort Myers, FL
When current flows through relay coil it creates the necessary magnetic field which then opens or closes the relay contacts.
Coil resistance is thus very crucial because if its too high it will limit the current and the relay may not work at all.
Is there any other reason as to why relay coil resistance is important?
Also how does relay coil resistance apply to electrical design?

thanks

The resistance in the coil tells you how much power you need to move (or hold) that relay.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150415-1702 EDT

jason619:

Go to the following location and download the datasheet. This is for the P&B KUP relay family.

http://www.google.com/url?url=http:...sQFjAA&usg=AFQjCNH3yVFeNlUX8M607sTfQXmIVRg4ig

Consider the coil specifications for the 1, 2, or 3 pole DC relay. Note the DC coil power is essentially the same for all the different coil voltages. Power is the important criteria. The coil resistance will be what it is based upon the voltage of operation and the required coil power.

Also note that when going to a 4 pole contact configuration that more power is required because this is necessary to maintain adequate contact pressure. Because of greater power dissipation in the 4 pole arrangement the maximum operating ambient temperature allowed is lower.

The coil resistance will change by about a ratio based on the square of the nominal applied voltage.

.
 

Upnorth

Member
Location
NH
150412-2360 EDT

Upnorth:

In general you can not just use a relay with a higher resistance coil to reduce power dissipation and do the same mechanical job (work).

Within some limited range the coil of a relay will be of a physical size that has the power to move the contacts that can handle a given switched load. For continuous duty purposes I suspect that the coil power required won't differ by more than a factor of two for a specific relay design between different manufacturers. An AC coil should use less power than a DC coil in the same relay.

Once a voltage has been specified, then based on the power needs to move the relay contacts the resistance is pretty much defined.

.

Agreed: the power needs of the relay are largely driven by the physical mass and motion of the armature, if that's what's in your relay. The point was that coil resistance (and power budget) can be a significant factor in multi-relay electrical designs.

I guess I was trying to suggest you could select a different type of relay, with a higher coil resistance to do the same (switching) job with much less coil current.

That scenario illustrates an answer to OP's Q on "how coil resistance applies to electrical design" of a SYSTEM when you have limited current available and lots of relays to hold. I've heard stories alleging that E = IR, so if you have a fixed E and need to limit the cumulative I as you add more resistors in a parallel circuit, you're going to have to consider a much larger R when you have many resistors, in this case, relay coils.

One electro-magnetic coil relay might switch 8A@250VAC contacts using 40 mA in a 600 ohm coil. A reed relay might switch the same rating contacts using only 10mA in a 2400 ohm coil. A solid-state relay might use only 1 mA, giving us a derived resistance of 24,000 ohms, and could switch 40A at 480 VAC.

Sorry if I was being too fuzzy; I only just got here.:blink:
 
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