[Amps] SSR in amp PS not working as expected

[Manfred Mornhinweg]

Ulf,

there are two fundamentally different kinds of SSRs. One type uses 
thyristors (TRIACs or SCRs), the other uses back-to-back MOSFETs.

These two (or three!) types have very different behavior. Not only that 
MOSFET SSRs can handle AC and DC while the others are usable with AC 
only, but also in that their handling of inductive loads is very 
different. Generally TRIACs are poor with inductive loads, antiparallel 
SCRs are much better, and MOSFET SSRs might be best if rated for 
inductive loads, but that needs a special circuit inside them to avoid 
damage from inductive kick.

Thyristor SCRs will switch on either the instant you apply a control 
signal, or the next voltage zero crossing after that, depending on their 
type, and will always switch off at the current zero-crossing. With a 
purely inductive load (and the primary of a very lightly loaded 
transformer is very close to that), the current zero-crossing is 90 
degrees out of phase with the voltage zero-crossing. This leads to some 
head-scratching.

TRIAC SSRs will often trigger far more easily in one polarity than in 
the other. This is likely the cause for your transformer hum: It's 
getting only a semicycle instead of the full grid waveform, so there is 
a huge DC component present, and the transformer core gets hopelessly 
saturated, with the magnetic flux leaking out of the core.

Another possible reason for hum is that the SSR is switching on too much 
after the zero crossing, so that there is a hard voltage step inside 
each half cycle. The harmonics contained in such a step tend to be very 
loud.

You will need to look for SSRs rated for highly inductive loads, and 
preferably of the back-to-back MOSFET type. Or else use old-fashioned 
mechanical relays.

One last thing: In some cases you can cure such thyristor misbehavior by 
adding snubber networks. You might want to try this first. Simply add a 
100 ohm resistor in series with a 100nF capacitor, of adequate AC 
voltage rating, directly across the output side of each SSR, right at 
the SSR. With some luck that might tame them.

Manfred


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