Jim (W7RY),
If/when you have a spare SSR for replacement, is there any chance of
capturing a sample of the load side with a storage scope? I think it would
be interesting to look at the captures at random SSR activation times.
Lots of valid theories floating here, but a few measurements are worth a
thousand words. Although the measurement may not rule out some odd AC
transient, it would be good to know exactly what the SSR is switching.
Paul, W9AC
-----Original Message-----
From: Larry Benko
Sent: Tuesday, February 11, 2014 12:27 AM
To: amps@contesting.com
Subject: Re: [Amps] Solid State Relays Again
Unfortunately you don't get the choice of when to switch off the SSR
usually. Triacs and SCRs only switch off when the current falls below
some minimum hold current. There are ways to force a turnoff by
injecting reverse current etc. but this is not done in the SSRs we are
talking about.
As far as the "transformer" is concerned, the current/voltage seen is a
function of a lot more than just the transformer. The load, filter
capacitors, the small leakage inductance, etc. all affect the result.
Many switching power supplies make use of this to minimize the switch
size. I have never seen a transformer circuit where the current that
flowed after the input was opened exceeded the maximum current that
flowed while powered. However the voltage can rise to huge values.
Here would be my take on the demise of the SSR:
1.) A 40/50A SSR in a 20A circuit is not going to be damaged by current
if a step start circuit is in place and working. I also assume the SSR
was adequately connected to a heat sink that is appropriate for the SSR..
2.) If the SSR worked for many on/off cycles you would have seen all the
phase angles for turn on (assuming not a zero crossing device) and the
turn off occurs at the same current every time by the nature of the
triac/SSR.
3.) I am assuming that nothing in the amplifier was just recently
changed and the amplifier did not suffer a fault on the last operational
power cycle.
A reasonable guess would be a AC line transient caused the failure and
MOVs would be a good choice for future protection.
On a different note, the quescent power of a relay is about 1 to 2W for
the coil for a 20A relay independent of load current while an SSR is a
device that drops a voltage that is in the order of a volt or a little
more. At 20A this means the SSR dissipates over 20W which is why they
need to be heat sunk.
73,
Larry, W0QE
On 2/10/2014 8:50 PM, Roger (K8RI) wrote:
On 2/10/2014 2:22 PM, TexasRF@aol.com wrote:
Additionally, the step start R will alter the phase relationship as the
load is primarily resistive when it is in the circuit.
So, perhaps the more critical scenario is at switch off?
Having worked with SCRs and Triacs I'd much rather switch on at peak power
than switch off while drawing current. The Collapse of the magnetic field
can produce a tremendous voltage kick, or back EMF.
It's my understanding that even transformers that are low voltage, high
current, if turned off while drawing much current can produce many times
their regular input voltage.
73
Roger (K8RI)
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