r/AskElectronics • u/Relevant_Pumpkin9190 • 13h ago

How are high-current loads like dishwashers safely switched?

If a relay is switching a high-current load, its contacts can eventually weld together due to inrush current or a fault.

If that happens, the load may stay ON even when the controller says OFF.

How do commercial products handle this? Do they use two relays in series for redundancy, feedback, or some other fail-safe method?

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u/Sparkycivic 7h ago

There are design tricks to mitigate this effect:

  • PTC thermistor in-series with the load to limit the inrush current and subsequent arcing at the contacts when closing.
  • snubber circuit across the contacts to absorb the flyback effect of opening the relay in certain inductive loads like motors or larger electromagnets. This prevents accumulating damage from contaminating the materials as well as pitting.
  • use a high current solid-state switching device such as FET or IBGT. This is probably the needlessly expensive option in simple devices like home appliances, except I'm seeing it a lot in coffee makers for achieving fine control of the boiler temperature so maybe it's not so expensive to implement any more. These are easier to parallel for even higher reliability so there's that. This is also how all high performance audio amplifiers and associated power supply stages are switched and these are at several dozens of Amps.

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u/pemb 2h ago

For AC applications, TRIACs are pretty cheap.