Here are a couple of thoughts from experience doing this years ago with
commercial transmitters:
It saved money, but created headaches.
If installed in the primary AC circuitry of a radio transmitter, that has
other contactors on the same circuit (that spark like relays and switches), the
SSR can false itself on by the DV/DT on the line. To elminate this, RC snubbers
are applied across the contact of the suspect SSR. This is either for the SCR
type or the TRIAC or back to back SCR type.
It is good to put MOV or high quality transorbs across the contacts, to have it
survive lightning and line transients without falsing or latching on.
NEVER depend on an SSR to break the line and then work on the transmitter.
Always pull the main switch first. SSRs can close at the most unpredictable
time, when you have your hands on the HV terminals! SSRs may also ping on when
you first close the switch, due to the aforementioned DV/DT noise
susceptability. Newer devices may be better in this regard. My experience was
in the 1980-90s.
SSRs can be had with zero voltage crossing shutoff or not. Not sure whether
this makes a difference for inductive loads like transformers, where the
current and applied line voltage are not in phase anyhow.
SSRs have low leakage current, which, into a high Z, would show a low line
voltage on a high Z meter like a DMM. Because of this leakage, they are not
considered safe to depend on for zero voltage.
However, despite these caveats, they are used for this application, and they
can work, if you protect them and the related circuit. Going to a metallic
contactor gets around most of these problems, but still have maintenance and
lifetime issues.
73
John
K5PRO
> Message: 3
> Date: Sun, 3 Aug 2008 20:51:01 -0700
> From: "W7RY" <w7ry@arrl.net>
> Subject: [Amps] Solid State Relays in Plate Supply Primary
> Since there is nothing in the current archives... What are the pros and cons
> on using solid state relays for switching on the plate supply transformer of
> a 1500 watt tube type linear amplifier?
>
> What is the voltage drop across them? I'm going to use two of them, one for
> applying 240 volts and the other for shorting out the step start resistor. I
> see that in Jerry's K8RA project in the handbook, he uses four relays instead
> of two. Any idea why?
>
> I know that the relays are quiet and provide for immediate shutdown of the HV
> if an arc occurs but has anyone come into the shack and found the high
> voltage running on their amplifier because a relay shorted from of a voltage
> surge on the AC line?
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