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I have an Arduino. A suitable relay can be directly connected to an Arduino (with a flyback diode) if its coil is rated at 5 V and 40 mA maximum. However such relays have a limit on the voltages and current they can switch.

Can I exceed the contact rating of such relays if, for example, I was switching-on the relay for a short time of say 0.2 seconds and switching-off the relay for a relatively longer time (say 0.4 seconds)?

It seems that it could switch higher voltages than designed for, if the on-time is very short and there is a relatively long off-time after that quick pulse. This would of course give a cheap way (no relay driver circuit used) to control high voltage.

Is there any real danger of doing this?

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  • \$\begingroup\$ You can. But you'd not do so if you had any wisdom. A relay driver can be as simple as a very cheap and small transistor and perhaps a resistor. There is no gain in doing out of spec things that will cause short lifetimes and likely damage \$\endgroup\$
    – Russell McMahon ♦
    Commented Aug 26, 2016 at 12:38

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No you cannot. The principal failure mode of relays in over-voltage conditions is arcing between the contacts on switch off. It doesn't matter how long the relay has been on, exceeded voltage rating will prevent it from cutting the arc off fast enough. If anything, switching it on and off repeatedly will only kill it faster.

A faulty relay connected to mains voltage can easily catch fire or explode and project molten metal particles to your face (or silently die - don't take this as a recipe for pyrotechnical show). So I'm inclined to say there is a danger.

EDIT: to make my point crystal-clear, once the relay is switched on, it doesn't care about voltage anymore as long as its current rating is respected. It will not heat up or otherwise suffer from over-voltage, since the voltage drop on closed relay contacts is close to zero. You could apply literally millions of Volts to your circuit, and practically all the voltage would be dropped on the load (provided it can cope with millions of Volts). The moment you try to switch the relay off however, it will see the full voltage applied on its contacts, and fail if its spec is exceeded.

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  • \$\begingroup\$ But if you program the arduino so the high voltage side contacts for the minimal time for it to switch on- does that make your answer any different? \$\endgroup\$
    – qwerty10
    Commented Aug 25, 2016 at 17:23
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    \$\begingroup\$ Read the answer again. You have not understood what is very clearly explained. \$\endgroup\$
    – Transistor
    Commented Aug 25, 2016 at 17:42
  • \$\begingroup\$ it just seemed that keeping the relay on for a long time or maximal time (say days) would make it only switch on once. Because as a guess it may allow time for heat may build up and maybe cause a fire destroying the relay. Thats why I asked about the minimum time in the comment. \$\endgroup\$
    – qwerty10
    Commented Aug 25, 2016 at 18:13
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    \$\begingroup\$ At some level above the switching current rating, the arc across the opened contacts will not extinguish. Current will flow continuously through the arc across the gap between the contacts. In a small relay parts could start melting or burning in a short time. If the relay is only operated once a day, that may not happen the first day or the second, but it will probably happen some day in a week or a month. It might last longer, but no one is going to make any prediction. At rated current the relay will probably last for 100,000 operations or a million at half of rated current. \$\endgroup\$
    – user80875
    Commented Aug 25, 2016 at 18:34

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