[Amps] SSR in amp PS not working as expected

[David G4FTC]

Hi All,

I use SSRs as a mains input contactor in three HT PSUs. The SSRs are driven from a monitoring circuit which shuts down the PSU in the event of over drive, excessive currents, etc., being detected. The oldest PSU is now over 15 years old and I have experienced no problems with the SSRs.

I place a 1000pF capacitor and a reverse diode across the input to avoid issues from stray RF or reverse polarity. I don't know if either a strictly necessary but I added them as standard practice.

FYI in my W6PO HT PSU I use a Crydom D2450, from the datasheet the main control element comprises of dual back-to-back thyristors. The other PSUs also use Crydom SSRs which I haven't checked for part numbers but I believe are all back to back thyristor units - it was just easier to take the cover off the W6PO PSU to see what I had used!

Hope this helps

David G4FTC




________________________________
From: Amps <amps-bounces at contesting.com> on behalf of Manfred Mornhinweg <manfred at ludens.cl>
Sent: 06 January 2019 22:26
To: amps at contesting.com; ulf at sm0nor.com
Subject: Re: [Amps] SSR in amp PS not working as expected

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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