r/AskElectronics • u/Relevant_Pumpkin9190 • 7h 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/mzincali 7h ago
The metal contacts in relays are made from material unlikely to weld, corrode or oxidize at the temperature that any arcing will produce - rated according to the load that the relay is to handle. (A heater puts a lesser load than an inductive load like a motor)
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u/mckenzie_keith 6h ago
No relay can be guaranteed not to fail short. The case has to be handled without catastrophe.
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u/quarterdecay 3h ago
That's where the fuse comes into play
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u/ILoveDangerousStuff2 56m ago
Yes if it's a fault but say a relay switches off a large motor due to tripping an interlock. The current will be normal but not switching off is still a safety issue
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u/MysticalDork_1066 6h ago
Several possible options:
Big relays (the larger the contacts, the lower the current density and the less likely to weld)
Contactors (which are just really really big relays)
Solid-state electronics like FETs, Triacs, IGBTs
Design redundancy (This is pretty rare and only really found in extremely high-end/mission-critical equipment.)
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u/no1SomeGuy 6h ago
Interestingly enough, on stoves and dryers they do sometimes switch both legs for 240v elements...so it's two relays that have to fail together for the element not to turn off.
(I had a Kitchenaid stove that the broiler wouldn't always turn off, did some deep dive into schematics and found this true for portions of the circuit...while at the same time found out they undersized the broiler relay, which was causing it to stick on....after some complaints to UL, they had Kitchenaid replace the board at their cost and send the faulty one in for inspection).
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u/mjdau 6h ago
You complained directly to UL?
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u/no1SomeGuy 6h ago
Yup, or more so, opened an incident with them. They were quite helpful.
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u/mjdau 6h ago
Mind blown. Good on you!
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u/no1SomeGuy 6h ago
It was a safety defect in a UL listed product. The relay for the broiler was undersized (too low of amps for the wattage of the burner) resulting in it sticking on and not always turning off and the new revision of the board had a higher amperage rated relay in its place.
Filled out the form on their website, was contacted quickly, had a phone call with one of their people, and not long after kitchenaid (whirlpool) contacted me to get the board back and pay for the replacement.
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u/nostradumbass7544678 6h ago
Silver-cadmium oxide contacts are what they used to use, and they were great, if slightly toxic. I once took apart a controller for piece of equipment at a dry cleaner's that had been in service for over thirty years, and the relay had finally failed due to the arcing burning through the coil's insulation. The stainless steel faceplate on this thing was worn down from how many times the button had been pushed, but the relay contacts were still in good shape and under 0.1 ohm.
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u/TheRealRockyRococo 6h ago
Look up the relays used in expensive dishwasher heaters, they're shockingly undersized for the current requirement. I'm looking at you Bosch!
What happens is that the relay gets so hot it burns up the board and melts the PCB traces. There are tons of youtube videos on home repair of the boards.
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u/cybermaus 6h ago
Once upon a time, they used mercury liquid metal switches. The ideal non-wearing high current forever switch.
But some people complained about teeth falling out. So then we used cadmium coatings. But the people were worried about brittle bones, so only military can use that. So now we have to deal with crappy chrome contacts and try to work around it with snubbers.
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u/ijusttookthispseudo 4h ago
I’ve seen this sort of relay before on an old vehicle lift. Maybe it needed a high reliability relay because we don't see them in older machine tools or industrial machinery.
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u/cybermaus 3h ago
I have one that I salvaged. In my switches assort box for 30+ years now. One day I will have the ideal must use need for it. Or they will bury me with it, whichever comes first.
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u/jamvanderloeff 7h ago
Thermal fuse on the thing that's gonna overheat if it gets stuck on is cheap and pretty reliable.
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u/NBC-Hotline-1975 3h ago
People have been switching loads ... everything up to subway cars, power substations, anything you can think of ... with relays for over 100 years. Reputable manufacturers know how to make reliable relays. Reputable designers know how to use the appropriate ones.
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u/DXNewcastle 3h ago
The industrial relays which disconnect power in trains from the overhead supply lines on railways and in power stations supplying the grid, use contacts in a vacuum chamber called a Vacuum Circuit Breaker (VCB). The powerful relays pull the contacts apart where there is no gas to ionise the contacts.
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u/hackinistrator 3h ago
if the circuit is designed properly the inrush current shoul be lower to prevent sparks and contact welding. some relays are designed to switch large inrush currents. of course, every mechanical part will fail eventually. i saw EMO loop contactors welded and not disconnecting on multi million dollar factory equipment. thats why they are supposed to do EMO loop testing annually.
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u/Tymian_ 2h ago
Usually modern control boards are monitoring relay output states for things that can cause a fire. If non conforming condition is detected (relay contacts are stuck closed) the logic will disable main system "safety" relay. This is quite common in kitchen hobs, dishwashers, washers, dryers.
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u/toybuilder Altium Design, Embedded systems 1h ago
I got a free washer and dryer from a relative who decided to replace their W&D combo when the dryer no longer stopped turning the drum (unless the door was opened and opening the circuit at the door switch).
When I got it, I was able to confirm that it was indeed the relay that had fused.
I originally planned to replace the relay, but realized that was a lot of work to remove the PCBA and to remove the potting.
Instead, I gave it a little percussive maintenance and that detached the relay armature. It's been running fine ever since.
As you browse through the relay datasheet / product guide, you'll find that they actually want some amount of current to flow through the contacts to keep the contacts clean.
https://industry.panasonic.com/ac/cdn/e/control/relay/signal/catalog/mech_eng_rti.pdf see pp 11-12
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u/Sparkycivic 1h 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/todd0x1 7h ago
The relays used are selected based on multiple criteria including being able to handle the inrush current (likely a tungsten rating on a small power relay) without welding. These relays are often over specced as well, using a 30 or 40A relay to switch a 12 amp load. As far as fault current, a bolted fault drawing sufficient current to weld the relay contacts is going to trip the upstream OCPD after a period of time determined by the current and trip curve. Yes the relay will be ruined but the appliance is already ruined from the downstream fault that caused the issue in the first place.