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Question about Relay Ratings

  1. Nov 21, 2011 #1
    Hello All,

    I'm working on a project to design and build an experimental motor controller that operates on a 36V supply. As part of my test setup, I would like to include a mechanical relay to disconnect the battery supply from the H-Bridge output stage in case of a failure during testing.

    Right now, I have on hand one of the RadioShack 12V 30A automotive relays which I am considering for use in the circuit. However, I'm wondering how important the "12V" rating is to the relay's operation, and what about the maximum rated votlage of a relay limits its use at higher voltages? In my system, I'm not expecting the current to go over 20A on average, though there may be surges as high as 30-40A during startup. How concerned should I be about using a 12V relay in a 36V application?

    Jason O
  2. jcsd
  3. Nov 21, 2011 #2


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    The "12V" will refer to the voltage used to operate the relay. The "30A" will refer to the current that the contacts can carry or break. You would need to look at a more complete spec for the relay to decide whether it would break a circuit with 36V across it. But I can't imagine it would be a problem, unless you intend to break a very inductive circuit, in which higher voltage spikes can be generated when you break the circuit. In any case, a catching diode across the contacts can kill these spikes. The situation could be different if you were dealing with mains voltages.
  4. Nov 21, 2011 #3
    Hello JDO - yes you do need the spec for the relay and look at the contact rating. Since you are looking at using this for "protection" AND this is in the DC part of the circuit - the Contact rating needs to handle the Voltage and the Maximum Fault current - if this is from a battery you may have a lot of DC current available.

    DC protection is a nightmare - as the current does not naturally commutate.
  5. Nov 21, 2011 #4


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    A fuse is pretty bomb proof as long as you can afford to wait for it to blow.
  6. Nov 22, 2011 #5
    Hello everyone,

    thanks for the comments. Yes, the relay that I am thinking to use uses 12V for the coil. My question was mainly understanding how they rate the contacts' voltage rating. for example, they have relays that say they are rated fro 120V AC or 30V DC at blah blah amps. I'm wondering what physical factors of the relay's contacts they are evaluating to come up with these numbers?

    The only thing I can think of is the insulation resistance in case of an inductive spike when the circuit is broken, but if the circuit you want to use it in is adequately protected from this, I'm wondering what else they could be looking at.

    - Jason O
  7. Nov 22, 2011 #6


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    Insulation resistance wouldn't be a problem. What canes relays is arcing when the current is broken. Heavier contacts will overheat less than light contacts and a large amount of travel (big separation), coupled with fast operation can reduce the time that the arc operates each time. As I said earlier, breaking the current through an inductive load can produce hundreds of volts of spike, which is usually suppressed using a diode, connected so that it is normally reverse biased (arrow pointing towards the positive rail). Many of these issues also apply to domestic switches but AC makes life a lot easier because the volts go through zero twice every cycle and arcs are more likely to die.
  8. Nov 22, 2011 #7

    jim hardy

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    if you've ever arc-welded you have an intuitive feel for what goes on at a relay contact when it opens. metal from one contact gets deposited to the other and they might weld together.

    DC is harder to interrupt because there's no natural "sinewave zero crossing". the current has to be brute forced to zero in the arc. that's why the relay's DC rating is so much less than its AC rating.

    so find the datasheet for your relay.

    Perhaps a small series resistance can limit worst case fault current to something your relay can handle, without causing much voltage drop at normal load.

    fuses do an excellent job. good high current ones are filled with sand to quench the arc. little automotive fuses might be able to do your job, check their ratings.

    as somebody mentioned , a diode can absorb an inductive "kick", but traditionally it is wired across the load not the contact.
  9. Nov 24, 2011 #8
    Hello all, first post here...

    I work in the aerospace industry and work with DC circuits quite a bit. I would agree with all the posts above regarding needing the specs for the relay you are looking at.

    I can offer up some generic DC relay contact rating info: typically the contact rating for a DC relay is for a resistive load. If another type of load are being used, you must de-rate. So for a relay rated at 10A (resistive), an inductive load is derated to 8A, motor load is derated to 4A and a lamp load is derated to 2A. Of course, there are many variations to these factors.

    When using a DC voltage higher than the relay rating also calls out for a derating factor. I think there is a formula for this and I don't have that memorized :)


  10. Nov 24, 2011 #9


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    Hi choppersparks

    If it's really important to use the cheapest relay you can get then what you say is correct. otoh, you can just over-engineer and put in something massive and stop worrying about it.

    If you actually have one in your hand then try it (including a fuse, of course) and see how it goes when you stall the motor or give it the highest load that is likely (measuring currents all the time).
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