Hi George,
I think the main recommendation is to use random fire rather than
zero-crossing. But I’m new to the subject, so I’m open for arguments.
/Ulf
Sent from Mail for Windows 10
From: GEORGE WALLNER
Sent: den 9 januari 2019 14:25
To: Ulf Tjerneld; amps@contesting.com
Subject: Re: [Amps] SSR in amp PS not working as expected
Ulf,
I have used 100-s of SSR on various ship-board systems. They work great
provided they are used correctly and are of the right specifications, and a few
precautions are taken.
Generally speaking, there are two kinds: zero crossing and random switching.
For your application you should use zero crossing. Also, you should add 270 VAC
varistors (MOV) between "neutral" (non-switched leg) and both the input and
output. In an RF environment it also makes sense to add a 10nF capacitor
parallel with the control input and both outputs (from ground). Also, for 240 V
applications, I always use 480 V SSR-s. Just to have enough headroom: switching
inductive loads can create high voltage transients. An SSR witll have about 1.8
V of forward voltage drop when on. At 10 Amps that is 18 W of dissipated heat.
You want to mount it either on a large metal surface or a heat-sink.
It is better to find the right SSR for the application, rather than finding an
SSR and then trying to use it in an application. You need to know the
specifications, as there are many variations.
73,
George,
AA7JV/C6AGU
On Mon, 7 Jan 2019 19:31:21 +0100
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
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