See, I do need a schematic to be able to give you a precise answer!
If you are switching the Primary of a transformer and your fuses between the
SSR and the transformer are blowing, I will suspect quite
a HIGH Back EMF :) SSR are notoriously bad in switching INDUCTIVE loads :)
You might need to insert a capacitor of the proper value in PARALLEL with
the transformer. This capacitor will act as compensation for the inductive
reactance of the transformer presenting a conjugate match that will leave
you with the pure resistance only.
Much in the way the antenna tuner works :)
This is common occurrence in SMPS design :) In a sense you have already
provided this capacitor :) I think :) But is the value right ? or in the
best location ?
The simplest things are proven the most complex one :)
Alex, N2NNU
> -----Original Message-----
> From: Vic Rosenthal [mailto:rakefet@rakefet.com]
> Sent: Monday, June 17, 2002 4:34 PM
> To: Alex
> Subject: Re: [Amps] Solid state relay question++
>
>
> Alex wrote:
> >
> > Vic,
> > Your description leaves to mutch to my imaginations. For example,
> > what kind of 240VAC line are you switching? Is is a 2 wire 240VAC
> > with one Neutral and one Hot wire, or is it 3 wires, (2 times
> 120VC) with
> > one Neutral and two Hot wires ? If it is the later, as I suspect,is this
> > meaning that your SSR is Double Pole Single Throw?
>
> It's three wires. Only one of the hot wires is switched (there's a manual
> switch before the ss relay which is used to disconnect the
> supplies when needed;
> the relay is just used to turn them on and off during operation).
>
> > Also, i think that the SSR rating is to small, Voltage wise, as
> it is rated
> > for only 280V. As your main is 240AC your margin is only 40V.
>
> I think this relay is designed for 240v circuits. I will have to
> investigate
> this further.
>
> > When you key down, the power request becomes a pulse that can overshot
> > easily the 40V :( This can induce secondary break down in the
> internal SCRs
> > which will blow your fuses. You realy need to put an scope on
> the INPUTS to
> > the SSR and take a look at the voltage swings there.
>
> The fuses that are blowing are AFTER the scrs (these are the
> fuses in the power
> supplies). The breaker before the scrs is not blowing.
>
> > And a last suggestion. I have observed that SSR will produce
> enough EMI to
> > induce enough voltages into the Signal input (your 3-12VDC input)
> > This can produce brief oscillations that could blow fuses and
> the SCRs them
> > self.
>
> The facts that 1) the problem only happened when transmitting
> into an antenna
> and not into a dummy load, and 2) it went away after I bypassed
> the line for RF
> seems to indicate that it was caused by RF getting into the
> relay. The fact
> that the fuses in the supplies were blowing and not the breaker
> before the relay
> indicates that the problem was that the relay was introducing additional
> resistance into the circuit (and causing the supplies to draw
> excessive current
> to try to maintain the output voltage) rather tban breaking down.
>
> Vic K2VCO
> > Good luck,
> >
> > Alex
> >
> > > -----Original Message-----
> > > From: Vic Rosenthal [mailto:rakefet@rakefet.com]
> > > Sent: Monday, June 17, 2002 2:21 PM
> > > To: Alex
> > > Subject: Re: [Amps] Solid state relay question++
> > >
> > >
> > > Alex wrote:
> > > >
> > > > Vic,
> > > > Can you send me the schematic of your offending circuit for
> analysis ?
> > > > Also the exact condition(s) under which de fault occurs.
> > > > Address is: alex at sandlabs dot com.
> > >
> > > There's nothing to it: a 25a 280v solid-state relay (3-8v
> > > actuation) connected
> > > to a 5-volt source for actuation. The relay switched a 240v line
> > > which feeds a
> > > pair of 12v 70a regulated power supplies. The problem was that
> > > RF caused the
> > > relay to partially drop out, which caused the supplies to draw
> > > more current.
> > > Bypassing the actuation circuit had no effect on the problem, but
> > > bypassing the
> > > ac line solved it.
> > >
> > > Vic K2VCO
> > >
>
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