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Old 26th November 2004, 02:55 AM   #1
Magura is offline Magura  Denmark
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Default Solid state relays???

Anyone knows how AC solid state relays get along with dc??

The load that is.

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Old 26th November 2004, 04:11 AM   #2
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Most of the AC solid state relays use a triac as a switch. Triacs only turn off when the current through them goes to zero, which generally happens twice in every AC cycle. So if you put DC through it you can turn it on, but not off!
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Old 26th November 2004, 04:25 AM   #3
Magura is offline Magura  Denmark
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So basicly a solid state relay is a thyristor with an optocoupler??

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Old 26th November 2004, 11:45 PM   #4
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by TwoSpoons
...So if you put DC through it you can turn it on, but not off!
Interesting point, if it switches on zero-crossing, would it turn on with DC?
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Old 28th November 2004, 08:41 PM   #5
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That depends on how fancy the relay is. Not all have zero-crossing turn-on circuits.
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Old 28th November 2004, 08:46 PM   #6
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Quote:
Originally posted by Magura
So basicly a solid state relay is a thyristor with an optocoupler??

Cheers

Magura
There are also other types with mosfet's and photocells but normally those relays are rather small. Mosfet's creates a almost pure resistive switch. May I ask your application?
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Old 29th November 2004, 01:50 PM   #7
Magura is offline Magura  Denmark
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I am thinking about using SS relays for switching between one secondary and both secondaries in series in a 20A 25/50V power amp.

Maybe the simple solution would be to do this before the rectifiers to avoid DC?? I just have doubt that the boards from the previous groupbut can take such current.

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Old 29th November 2004, 02:02 PM   #8
LBHajdu is offline LBHajdu  United States
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How are they for reliability? I’m thinking of using something like the pass labs A75 schematic to turn on and off an amp. It uses a triac. An other option is to use a real relay, they cost about the same. However I do done always trust semiconductors. Anybody got any opinions?
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Old 29th November 2004, 02:13 PM   #9
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Quote:
Originally posted by Magura
I just have doubt that the boards from the previous groupbut can take such current.

Magura
I'd say skip your relay thought unless you really have a problem. If it ain't broken, don't fix.

What diodes are you using and how much capacitance do you have and also what is your supply voltage?

Notice also that if you really have such problems a soft starter at the primary is better, smaller currents.
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Old 29th November 2004, 02:29 PM   #10
Magura is offline Magura  Denmark
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Hmm, better explanation of the application required.

The point is to be able to switch between 25 and 50V for a Zen V7.

RURG3020 diodes, 200.000uF, 1KVA trafo 2*18V.

The alternative is a hefty contactor, but those tend to wear out and they are huge compared to a SS relay.

Magura
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