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Does any solid state DC relays really disconnect the load?

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David_

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Hello.

I have been designing a dual floating channel lab power supply from times to times and I wanted to have relays to completely disconnect the outputs, I can't recall exactly why but I had a good reason for why I wanted to do that.

The last thing I looked at was to use solid state relays(obviously the DC kind) but it occurs to me now that I am really unsure if such a relay really disconnects the load as a mechanical relay would.

If a MOSFET isn't suitable for disconnecting the output of power supplies due to what can happen if the output terminals would be exposed to some potential generated from another source that could exceed the MOSFETs reverse breakdown(not sure what it is called in MOSFETs but they can be damaged is the drain is exposed to a unsuitable potential).

If so then a solid state relay wouldn't be suitable ether would it?

I am really unsure about this.

Regards
 

If you're worried about applying some high voltage to the output of your power supply (why would you do this?), then you probably don't want to use an SSR.
 

If you're worried about applying some high voltage to the output of your power supply (why would you do this?), then you probably don't want to use an SSR.

And if there is a chances of reverse voltage, definitely, no SSR.

I assume you are running your relay on the high side. (switching power, not ground). This makes reverse voltage a real issue.
If you can tolerate a diode drop, then it's fixed easy enough with a series diode from the output of your SSR.
 

A diode... good idea.

The design is for a general purpose lab supply and since I don't now what situations I might come to face I think I should expect the worst so that the design is as safe as can be.
When I started this design I for some reason really didn't like the idea of using an ordinary mechanical relay but if I can find a bistable relay that can carry the current that might be best, I have no plan for how to deal with the diode drop so this is perhaps the obvious solution.

The 2 channels are fully floating and are supposed to be capable of connecting to each other to create dual rail power supplies.
 

Also if you only want to block one direction you can use a single mosfet with a suitable driver (see photovoltaic opto) rather than an SSR which should be smaller and cheaper than an SSR (which blocks bidirectionally and has a cost associated with that).


But as you seem to understand, mechanical relays have much better performance for a given size and cost and might be the way to go.
 

1) diode. Bad idea. Now you've screwed up your power supply output as it's going to have a nonlinear diode drop. Unless you don't care about accuracy.
2) mosfet. That's the same as using an SSR. Same problems.
 

1) diode. Bad idea. Now you've screwed up your power supply output as it's going to have a nonlinear diode drop. Unless you don't care about accuracy.

In case you can modify the feedback loop of your voltage regulator, maybe you can put the diode (or the SSR) inside the loop so that its non-linear effects become negligible.
 

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