Ulf, I'm currently using SSRs to control the 240VAC primaries in two
large HV power supplies (one on each leg of the 240V mains) and one to
switch 120VAC to my operating table. I also use one as the main power
relay for an old Drake L4B (to take the strain off the power switch).
No special precautions on any of them, except I try to have the rated
voltage and load current rating to be roughly twice the needed voltage
and current load, and I use transient suppressors on the mains. I
also heat sink the relays to the chassis and use thermal grease. I've
never had any problems with any of them.
73,
Jim w8zr
Sent from my iPhone
> On Jan 7, 2019, at 11:31 AM, Ulf Tjerneld <ulf@sm0nor.com> wrote:
>
> Thanks a lot for all this information. I got some help from the local ham
> community here too. Most of the feedback I get is to avoid them and stay with
> traditional relays. I have learned a lot recently 😊.
> However, I don’t give up that easily so I will investigate this a bit
> further. If someone on the reflector has experience of SSR:s that actually do
> work, please advise! I know of at least one power supply project published in
> QEX where zero crossing SSR:s are used (W8ZR, QEX sometime back in 2013). At
> least the Chinese ones he proposes are very cheap, about 10 dollars, so I’m
> inclined to order a couple of them and try them out. But the delivery time
> from China is something like 60 days, so I might give because of that ☹.
>
> PS… the snubbing network didn’t work. Well, maybe a bit, I actually measured
> 103 volts on the 105 volt secondary. But still a very loud hum that I don’t
> like.
>
> 73’s Ulf
>
> Sent from Mail for Windows 10
>
> From: Manfred Mornhinweg
> Sent: den 6 januari 2019 23:26
> To: amps@contesting.com; ulf@sm0nor.com
> Subject: Re: 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
>
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
>
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