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In general, is it advisable/safe to switch a mechanical relay with the following abstract conditions?

  • load voltage << rated switching voltage
  • load current > rated current

Relevant Questions

The current rating is to limit the self-heating of the relay due to its own resistance.

maximum current that the relay can sustain when being opened or closed and not sustain contact damage


My Use Case

I have a mechanical relay (Digi-Key PN 281-6348-ND) with the following specs:

  • Switching Voltage: 250 Vac
  • Contact Current Rating: 10 A

I would like to use it to PWM control a 12 Vdc thermoelectric plate (Peltier module) rated to pull up to 15 A.

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    \$\begingroup\$ Independent of any issues with the relay low speed PWM is a terrible way to control a Peltier module. The I2R losses will make it very inefficient. \$\endgroup\$ – Kevin White Feb 21 '20 at 18:46
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    \$\begingroup\$ So you already found out that if the current is too high, the relay might overheat or the contacts might be damaged (which will stop the relay from opening or closing) and your question is whether these problems are advisable and/or safe? \$\endgroup\$ – user253751 Feb 21 '20 at 18:46
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    \$\begingroup\$ if you want to use the peltier to cool something PWM is a really bad choice, it maximises \$I^2R\$ heating in the module which you doe't want. it also puts more physical stress on the module accelerating wear-out. Use a switching regulator instead. \$\endgroup\$ – Jasen Feb 21 '20 at 19:15
  • \$\begingroup\$ @Jasen thank you for suggesting a better method \$\endgroup\$ – Intrastellar Explorer Feb 25 '20 at 18:53
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The current rating determines the \$I^2R\$ losses of the closed contact hence, operating at 15 amps when the rating is 10 amps is going to result in early failure of the relay. The voltage rating determines characteristics of the contact when it opens. There is usually no trade off.

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Some switches and relays are rated for higher current at lower voltage, however this relay is specifically limited to 10A in the datasheet, even at 24V.

enter image description here

If you exceed the maximum rating, especially by a factor as large as 50%, you can expect unpleasant things to happen.

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  • \$\begingroup\$ Thank you @SpehroPefhany for finding a datasheet, and finding a DC usage spec! \$\endgroup\$ – Intrastellar Explorer Feb 25 '20 at 18:50
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Your correct course is to use a larger relay with a higher current rating. Simple as that.

The issue is thermal, and is caused by current. The voltage doesn't really enter into it. So using a lower than intended voltage doesn't help.

Voltage matters to the relay's ability to interrupt an arc. At your low voltage, that's not the question; it's the contact heat when active.

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  • \$\begingroup\$ Thank you for the simple explanation! \$\endgroup\$ – Intrastellar Explorer Feb 25 '20 at 18:52

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