[Amps] SSR in amp PS not working as expected

[Manfred Mornhinweg]

Ulf,

> Most of the feedback I get is to avoid them
> and stay with traditional relays.

SSRs are great in many applications, but not in all. On the other hand, 
often you will hear advice to avoid them, given by people who had a bad 
experience because they selected the wrong kind of SSR.

 > But the delivery time from China is something
> like 60 days, so I might give because of that ☹.

That's typical for "economy shipping" from China. Most sellers also 
offer faster methods. I have lately ordered a lot of parts from 
Aliexpress using "standard shipping", which costs typically 3 dollars 
extra. The parts are sent by a fast freight forwarder to my country, and 
then distributed by the local postal service. That tends to take just 2 
to 3 weeks total.

> PS… the snubbing network didn’t work. Well, maybe a bit, I actually
> measured 103 volts on the 105 volt secondary.

Most SSRs actually have a snubber built in, so it was a far shot on my 
part to suggest external snubbering.

 > But still a very loud hum that I don’t like.

Do you have an oscilloscope, so you can look what's happening? And if 
not, maybe you could rig up a simple circuit that samples the voltage 
applied to the transformer, via a small transformer and resistor 
divider, and apply that to the computer's sound input. Then you can use 
any audio edition software to see the waveform.

In your further tests with SSRs in that application, I would suggest 
that you specifically pick SSRs that have antiparallel SCRs in the 
output, and which are instant-triggering, also called random triggering, 
rather than the zero-crossing triggering type. With some luck that will 
keep the SSR on over a larger part of the AC cycle, reducing the 
harmonics on the transformer.

Manfred


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