Re: Solid State Relays and EMI

From: Glenn Gundlach (stratus46_at_yahoo.com)
Date: 07/26/04 

Date: 26 Jul 2004 09:04:11 -0700

legg@magma.ca (R.Legg) wrote in message news:<e715b5cc.0407260139.6e4e6e38@posting.google.com>...
> dave.harper@gmail.com (David Harper) wrote in message news:<364fd697.0407240745.38d90c4c@posting.google.com>...
> > I'm using some solid state relays, and having some problems with (I
> > believe) EMI.
> >
> > http://www.cel.com/pdf/datasheets/ps7141a.pdf
> >
> > I'm using this part in the B and C configurations.
> >
> > These items are located near some RF sources operating at about
> > 148MHz. During transmit (.5W power), I'm getting some false "on"
> > states. I'm pretty sure this isn't due to the equipment on the load
> > side, as removal of the SSR's eliminates the effect.
> >
> > 1. Anyone know if these type of SSRs are subject to EMI that can
> > cause false on-states?
> > 2. Would induced AC voltages (from the RF source) in the load lines
> > (around 148MHZ) decrease the R-off resistance?
> > 3. If the problem is in the SSR's, anyone have any suggestions
> > regarding how to lessen/prevent this? (EMI shielding, other PN's that
> > are immune to this, inductors/caps that could be placed in the load
> > lines, etc).
>
> With the turn-on time being ten times the turn-off time, I don't
> expect that false turn-on will be a result of LED emitter modulation
> at the interfering frequency.
>
> Fet structures usually have a hefty input capacitance; the coupling
> capacitance to the gate is piddling by comparison.
>
> Biggest coupling capacitance that could produce false gate turn-on is
> from the two drains. Is there a fairly large amplitude of RF voltage
> present there? These thinga are really only isolators at low frequency
> or DC, unless a voltage close to at least 1/10 their rating exists
> when 'off'.
>
> RL

OK, but I've seen SSRs used at broadcast transmitter sites running
many kilowatts of RF with no problems. SSRs are like Rank telecines.
When you see it 'misbehave' don't assume it isn't being 'told' to do
it.
GG