I replaced the power switch on an old 70's receiver and added a 40A relay (40mA coil) to preserve the switch from burning out again. After just a few months I started hearing a buzz from the relay and then the contacts chattered, indicating a failing coil. It did it with the load(200VA xformer) disconnected too. Did I make a poor choice for a relay, is it just a defective part, or should I have put a snubber across the coil? I didn't think 40mA would require one, but now I'm not sure.
Probably a bad relay with loose coil connection on the pins. It is possible the power switch contact may be bad.
Test switch full load.
Test switch full load.
Thanks, test the switch, yes. Embarassed to say I didn't because it's new, but lo and behold 40ohms across the contacts. 🙁
What is the minimum rated switch current of the switch you replaced? I have had both power switches and relay contacts go high resistance due to insufficient current through them - not what you would usually expect.
The possibility is that the switch metallurgy is poor quality or inappropriate for switching low currents.
The possibility is that the switch metallurgy is poor quality or inappropriate for switching low currents.
Interesting. So basically a minimum current is needed to burn through the oxidation that might form.
Ironically had you used a relay with a DC coil, resistive dropper, bridge rectifier and perhaps 47uF - 100uF filter capacitance you would probably have gotten away with it, the inrush current at switch closure might be enough to break down the oxide layer. (It also might not be)
I imagine finding a compatible switch with suitable contact metallurgy might be a problem, but that is only one avenue to pursue. The others might be to cobble up a simple circuit with sufficient inrush, but very low current consumption in order to keep the contacts clean. Series RC, bridge/cap/resistor or similar idea.
If you operate the switch repeatedly with no power applied does the contact resistance drop?
I imagine finding a compatible switch with suitable contact metallurgy might be a problem, but that is only one avenue to pursue. The others might be to cobble up a simple circuit with sufficient inrush, but very low current consumption in order to keep the contacts clean. Series RC, bridge/cap/resistor or similar idea.
If you operate the switch repeatedly with no power applied does the contact resistance drop?
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Could that be the problem? Some switches are deliberately designed to have 'wear action' to keep the contacts clean in some applications known to build up oxidation. If that's why you mentioned it, ignore my dumb comment... 🙂
NEVER use a zero crossing SSR (= the usual type) for transformers. Please think why.
Are you sure it isn't the switch that is failing to make good contact? Possibly the switch is not suitable for a tiny 40 mA load. Otherwise I think this relay is chosen too heavy rated for the actual load. Besides that the datasheet mentions "resistive load" a few times too many and it is rated only 5A for "ballast" loads. I think you will do a better choice with an 8..10A rated type suitable for inductive loads.
In effect both the original switch and the relay possibly may not have enough load current to keep them working correctly.
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Good catch. Maybe it's a combination of the switch and the relay. I can't find an alternate switch that works with the original knob so I'm stuck with that. I just don't like the idea of all the current going through that tiny contact. That's why it failed in the first place and it was apparent I wasn't the first one to replace it. I will look for a relay rated for inductive loads which has more coil current.
What is the maximum current of the switch?
Usually they are designed to switch stuff on and off 🙂 It depends a lot on which alloy is used for the contacts.
Usually they are designed to switch stuff on and off 🙂 It depends a lot on which alloy is used for the contacts.
Put the largest rated relay in that package, price difference is tiny, life is enhanced.
Here I see switches rated 3A DC and 10A AC, contacts are small.
So really, see the DC rating on the relay, it will give you a better idea of its current breaking rating or strength.
Also, see if the relay can be sourced from a reputed brand, rather than a 'special' off Aliexpress or their competitors.
Please post a photo of the original switch etc., with the marked ratings clearly visible.
I have a line voltage conditioner, it has tiny 1N4148 diodes on the relay coils, something to do with back EMF?
Jean-Paul, please give your opinion on using SSR and NTC together, the NTC will control inrush current to the transformer.
Might get complicated?
Best to use relay?
Here I see switches rated 3A DC and 10A AC, contacts are small.
So really, see the DC rating on the relay, it will give you a better idea of its current breaking rating or strength.
Also, see if the relay can be sourced from a reputed brand, rather than a 'special' off Aliexpress or their competitors.
Please post a photo of the original switch etc., with the marked ratings clearly visible.
I have a line voltage conditioner, it has tiny 1N4148 diodes on the relay coils, something to do with back EMF?
Jean-Paul, please give your opinion on using SSR and NTC together, the NTC will control inrush current to the transformer.
Might get complicated?
Best to use relay?
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It is not an opinion but switching a transformer with a zero crossing SSR is the worst scenario for a transformer. A switch or relay has the bonus that the moment of pushing it is random compared to the sinus of the grid.
Overdimensioning a relay is a good thing but extreme overdimensioning sometimes does not work out well with regards to the minimum switching current.
Of course if you show a picture of the type possibly someone here will have a replacement that does not break down.
Overdimensioning a relay is a good thing but extreme overdimensioning sometimes does not work out well with regards to the minimum switching current.
Totally missed this post but the switch was indeed high ohmic. If this happened because of too low current pushing it in and out a few times should restore it to normal contact resistance. It then needs more load. Like a 200VA transformer 😀
Of course if you show a picture of the type possibly someone here will have a replacement that does not break down.
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Putting a snubber across the coil would be a good idea, since suddenly opening the circuit on an inductor can cause a high voltage spike that would result in arcing across the relay's contacts during opening. The arcing eventually erodes the metal contacting surfaces, resulting in failure.