# Which relay to use?

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#### Electroenthusiast

##### Member
I have seen that relays with Low resistance draw more current,
and relays with high resistance draw less current;

If the source voltage cannot provide a sufficient(big) current, all of us incline to use a High resistance relay, because it draws low current.
If high resistance relays draw less current then why cant we use it for all applications? Why do we require Low resistance relays?

If i have a 5V (1A) Source, i can use any of these relays;

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#### dknguyen

##### Well-Known Member
For a relay switch and spring of a particular size, you need the a certain amount of magnetic force to be generated by the coil to overcome the spring force and close the switch. You can get the same amount of magnetic force in two ways:

1. Make the coil have lots of turns and have low current (high resistance because more turns).
2. Make the coil have few turns but have lots of current (low resistance because fewer turns).

So by changing the number of turns in the relay coil, we can make the same relay work at high voltage or low voltage. Either way will generate the magnetic field strength that we need, except one requires high voltage low current and the other requires low voltage high current.

The magnetic field strength of the relay coil depends on the current. The only thing the resistance of the relay affects is what voltage you can directly apply to it to get that current. If the relay's coil resistance is too low and your voltage is too much current flows making the magnetic field strength even stronger which isn't a problem but the extra heat might burn out the coil. In that case you can just add a series resistor in if you want to run it off a higher voltage. If your supply voltage is too low and the coil resistance is too high to get the required operating current, there's nothing you can do except to get a higher supply voltage or a lower coil resistance.

If high resistance relays draw less current then why cant we use it for all applications? Why do we require Low resistance relays?
So why do we require low resistance relays? THe answer is simple- so we have relays that work at lower voltages. A relay coil with a resistance that requires 1000V uses even less current. Why don't we use those everywhere? Well, not everyone has 1000V available. Different applications have different voltages available to them.

THen you might ask, well why don't we use low resistance relays for everything since we can use them with higher voltages just by putting a resistor in series with the relay coil? Well the answer there is that wastes power (sometimes a lot of power if the supply voltage is much higher than it needs to be). The power being dissipated is not being used to generate the magnetic field in the coil that closes the relay. Another reason is big heavy relays needs lots of magnetic force which means a lot of power. We can use lots of current at low voltage to get that high power, but because of I^2R it makes a lot more heat than if we used a higher voltage with less current.

As for what relay you can use, we need to know both the relay coil resistance AND operating current to know what voltage you can use. It's a simple V=IR problem. For example,
If your coil needs 1A to run, and has a resistance of 10ohms, then you need V=IR=(1A)(10ohms) = 10V. If the voltage you have is lower than 10V, you need more voltage or a new relay. If you have a voltage higher than 10V, you can get a relay that at 10V or you can put a resistor in series with the coil to limit the current in the coil to what you need so the coil doesn't burn out.

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#### Electroenthusiast

##### Member
Hi dknguyen,
Make the coil have lots of turns and have low current (high resistance because more turns).
Make the coil have few turns but have lots of current (low resistance because fewer turns).

if i have a 5V,100ohms; and 5V,1Kohms , which one should i use?(both rated for 5V) Re1: 50mA; Re2: 5mA

...then you need V=IR=(1A)(10ohms) = 10V. If the voltage you have is lower than 10V, you need more voltage or a new relay. If you have a voltage higher than 10V, you can get a relay that at 10V or...
i have an 6V, 100ohms relay. can i make use of it for 5V supply?

#### dknguyen

##### Well-Known Member
Oh I see what you mean.You actually have two relays rated that way with no explicit current rating. The 5V,100ohm relay is going to draw 10x more current and generate a much larger magnetizing force than the 5V, 1kohm relay. There must be something different about it. The most common reaosn is that it's a much bigger relay with a higher current rating, and therefore a larger contact and spring to move. But you can usually tell that by looking at it so it should be obvious or you wouldn't be asking. Maybe it has larger distances for between the coil and primary switch or the primary switch opens and closes over a larger distance for higher voltage isolation? Do you have the datasheets for the two relays? If you do, it should be obvious why one of them draws so much more current than the other. Of course, it's also possible that the coil just wasn't built as well and therefore takes up more current (in this case a LOT more current) to produce a similar magnetizing force. There must be something different about it. But if either one is good enough for you, you might as well go with the one that draws less current.

You probably won't get a 6V, 100ohms relay to work reliably with a 5V supply. It might magnetize enough to turn on, or it might not. If it does magnetize enough to turn on, it might turn on every time you try to turn it on.

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#### Electroenthusiast

##### Member
......5V,100ohm relay is going to draw 10x more current and generate a much larger magnetizing force than the 5V, 1kohm relay. There must be something different about it.........Do you have the datasheets for the two relays?......... Of course, it's also possible that the coil just wasn't built as well and therefore takes up more current (in this case a LOT more current) to produce a similar magnetizing force.....There must be something different about it.... But if either one is good enough for you, you might as well go with the one that draws less current.

THANKS dknguyen,
Thats what i meant... 10x more current...
Nope, i dont have a datasheet, i jus went a shop and bought one.; How can i search datasheet for a relay?
My relay, does'nt have any number on it.
All thats written on it is>
INDUSTRIAL RELAYS

SERIES - 202

EDIT>This might be the datasheet:https://www.electro-tech-online.com/custompdfs/2010/06/700s-p.pdf

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#### dknguyen

##### Well-Known Member
THat's no much information to go on to find a datasheet for that relay.

If you bought them from a store you should have at least a current rating for them.

I dunno...is one much heavier or bigger than the other? Does it have more terminals? Maybe the one that draws more current is a multi-pole relay. Maybe it clicks on and off louder or something because the spring is stronger? Maybe it has a larger airgrap between the switch and coil? Or a larger airgap for the switch when it's open for better isolation. Maybe it's just a crappy magnetizing coil.

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