Author Topic: Quick Question About Relays  (Read 432 times)

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Offline MastermimeTopic starter

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Quick Question About Relays
« on: March 16, 2013, 07:22:49 PM »
Hello everyone,

I had a rather confusing debate with a friend on relays and I'm looking for an answer.  Say  you have a 24v system with a load of 50 amps.  If you wanted to use a 12v relay, would the correct rating to look for be 25 amps (divide by 2) or 100 amps (multiply by 2).

I said 25 amps b/c according to Ohm's law, as voltage increases current increases considering the resistance is the same.

I just want to settle this lol

« Last Edit: March 16, 2013, 07:24:47 PM by Mastermime »

Offline billhowl

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Re: Quick Question About Relays
« Reply #1 on: March 17, 2013, 01:48:15 AM »
Relay contacts all have a current rating. This rating is specified by the manufacturer, and it tells you how much current the contacts can safely turn on and off. If you use a relay to control more than its rated current, it will likely go to an early grave.
Relays that switch more than their rated current often end up with their contacts welded together. They appear to be 'on' all the time.

Note that contact ratings are for resistive loads. You must take extra precautions if your relay is switching a circuit with incandescent lamps or inductive loads such as solenoids or another relay coil.

To help your relays live a long happy life, have them switch incandescent lamps that normally consume 1/8th of the contact current rating or less. That's right, 12.5% or less. For example, if a relay is rated for 0.5 amps, only switch an incandescent lamp rated at 62 mA (0.062 Amps) or less.

To maintain UL registration, no rating can be exceeded. A contact rated at 12VDC, 100A cannot be used in a 24VDC, 50A application.

If the rating is 12VDC@100A, I would consider this to be the maximum voltage which could be applied to the relay contact and still expect it to reliably extinguish the arc produced on opening of the contact. To double the voltage would greatly reduce the contact life, even if you are only running half the current.
The contact spacing is the main factor limiting your voltage rating to 12VDC. The contact area will limit the current rating.

Relays have a break rating specified in Watts for DC (apparently 1200 W in your example), but also an absolute max. voltage.
It is always recommended to read the mfg. specs. The major issue at DC is breaking the circuit (quenching the arc - unlike AC, where there is a zero voltage point every half cycle). You may also want to consider the inductive time constant (L/R) of the circuit being switched vs. relay specs, and add a free wheeling diode (in reverse polarity, of course) across the contacts.

Contacts specs (current and voltage) should be looked at as separate specifications and not as a composite power spec. Also if you check the spec on most relays they have different max current specs for AC Vs DC current.
This is because AC can be more easily switched off as the AC voltage zero crossings help keep contact arcing down, where as DC is much harder on contacts at the same ratings. Not a good idea to utilize relay contacts at or near their rated maximum as contact life will be shortened. I would not go above 50% of ratings if reliability is important.

Relay contacts are available in a variety of metals and alloys, sizes and
styles. There is no such thing as a universal contact. The relay user
should select contact materials, ratings, and styles to meet, as precisely
as possible, the requirements of a particular application. Failure to do so
can result in contact problems and even early contact failure.


Offline jwatte

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Re: Quick Question About Relays
« Reply #2 on: March 17, 2013, 03:51:41 PM »
The "voltage" rating of the contacts of the relay means how isolated it is from an electrical safety point of view. If you exceed the voltage, there may be arcing inside the relay, or there may be a touch danger when touching the relay.

The "current" rating of the contacts of the relay measures basically how much heat the contacts will generate, and how much heat it can dissipate to stay within safe bounds. Note that this is not voltage related at all -- the amount of heat (power) created by a contact (or any resistance) is equal to I-squared-R, and thus proportional to the square of the current. A relay rated for 25A is rated for one-quarter the contact heat of one rated for 50A -- thus, trying to use a 25A relay with a 50A load is quite likely to break the relay.

Finally, for the voltage, it's often the case that a "12V, 30A" relay, really is a relay with a *coil* voltage of 12V (meaning ideal voltage to pull the relay is 12V) and a *contact) rating of 30A (meaning the contacts won't break for resistive currents up to 30A) and the safety voltage rating is left out for brevity (thus generally assumed to be at least 115V or 230V.) Make sure to check the data sheet for what the numbers actually mean.

To answer your question: Using a relay rated to switch 12V to switch 24V is unsafe, no matter what. However, if you had to do it, using a relay rated for 100A is possibly the less unsafe option, if such a thing could be said to exist.

Just get a properly rated relay, would be my advise.

Offline MastermimeTopic starter

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Re: Quick Question About Relays
« Reply #3 on: March 19, 2013, 03:30:29 PM »
Thanks for the responses Jwatte and Billhowl! You went beyond what I was looking for answer! Thanks again


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