| Magura |
Anyone knows how AC solid state relays get along with dc??
The load that is.
Cheers
Magura:) |
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| TwoSpoons |
| 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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| Magura |
So basicly a solid state relay is a thyristor with an optocoupler??
Cheers
Magura |
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| cpemma |
| 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? :confused: |
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| TwoSpoons |
| That depends on how fancy the relay is. Not all have zero-crossing turn-on circuits. |
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| peranders |
| 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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| Magura |
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.
Magura:) |
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| LBHajdu |
| 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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| peranders |
| 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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| Magura |
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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| peranders |
How often should you change the voltage and why? How much current does the amp consume?
Your diodes can take 325 A non-repetitive and 70 A repetitive. |
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| Magura |
It might happen pretty often. The reason to switch voltage is to save power for background listening at 20W music power, while having the option to get a lot of power for higher volume/ critical listening with 50W. This can be achieved by switching the voltage.
Magura:) |
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| peranders |
Can't you solve this with two transformers. Put the 230 V primary windings in series or in parallel. I assume that you don't need to switch when the power is switched on?
I think you can forget a solid state relay but I think it's quite possible to solve this with a couple of bauta mosfets. |
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| Magura |
Well, part of the trick would be the ability to switch while powered. The caps should be adequately big to supply 0.2sec. of power and let it run seamless.
I guess it would be worth mentioning that I use a PLC for the control of the amp.
The end solution just might be to build a more powerfull bridge and switch the secondaries before the rectifier.
I guess 2 bridges in parallel is a bad idea??
Magura:) |
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| peranders |
| That is a good idea and them switching between two transformers. |
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| Magura |
Thanks, problem solved.
Magura:) |
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