Re: Relays - plea for help!

[Raymond <Reb.barnett@xxxxxxx>]

Cheers Nigel.

The SSR in question is from Maplin, page 624 of the catalogue, order
code MG35Q. The specs suggest 3-32v DC input, and a 24-280VAC output.
'Min current', which presumably means minimum sink from the mains side
for it to switch on properly, is 100mA. Max off state leakage is 15mA.

I'm pretty sure the input on the boiler draws little if no current, and
is used solely for switching a bigger relay somewhere else on. I thought
that the min current spec for the SSR might mean the SSR would never
switch on if the boiler draws less than 100mA from the input, but
presumably, by the same token, the boiler might have to draw 100mA for
the SSR to successfuly switch off?

Just to clarify, the SSR can switch the boiler on fine, but it can't
switch it off properly.


Ray.


> -----Original Message-----
> From: nigel.orr@xxxxxxx [mailto:nigel.orr@xxxxxxx]
> Sent: 12 October 1999 10:01
> To: REB.Barnett@xxxxxxx
> Subject: [ukha_d] Re: Relays - plea for help!
>
>
> At 09:27 12/10/99 +0100, you wrote:
> >The problem is as follows. With the SSR connected in series or in
> >parallel to the existing timer control, I can successfully switch
the
> >hot water/ CH on. However, when I turn the SSR off, I get a high
> >frequency clicking from one of the relays buried inside the
> boiler, that
> >does not sound healthy.
>
> It does sound like the boiler control isn't pulling enough
> current, so the
> resistance of the SSR is too high to let the relay fire or
> open the valve
> directly.  It would be worth measuring how much current the
> boiler actually
> draws when it fires.  The pump will probably be a couple of
> amps, but the
> boiler relay (or valve) might be a tiny amount.
>
> >volts output from the SSR. With the SSR disconnected, the existing
> >control timer seems to hold the boiler input at approx. 60v, but
this
> >seems to drop to zero if it's left off for long enough.
>
> That might be a 'phantom' reading, just mains induced on the
> cables.  It
> does sound quite high, usually you get something around 10V.  Try
> connecting a mains light bulb across it and see what the
> voltage is now, or
> if the bulb lights- I would guess the voltage will drop and
> the bulb will
> stay off, but I might be missing something.
>
> >My only guess is that the SSR never completely switches off,
> or pulses
> >voltage, but I'd of thought the multimeter would have spotted
that.
>
> It's definitely spec'ed for AC?  If it was 1/2 wave
> rectifying the power,
> the relay or valve would buzz at 50Hz.  I presume you don't have easy
> access to an oscilloscope ;-(
>
> >Consequently I can think of two solutions: to wire a load
> (low wattage
> >lamp) in series with the output from from the SSR, to
> determine if it's
> >because the boiler represents too low a load to cause the
> SSR to switch
> >off (because nothing to sink current from the SSR?).
>
> If it was never switching off, I would expect it to work fine, but the
> boiler wouldn't switch off.  Try connecting a bulb in
> parallel with the
> boiler and see if that solves the problem, by increasing the
> minimum load
> seen by the SSR.  What is the part number of the SSR, and
> where did you get
> it from?  I can skim through all the specs and see if
> anything leaps out
> (though SSRs _aren't_ my speciality!).  Could it be that the
> boiler valve
> is too inductive for the SSR to cope with?  That may or may not mean
> nothing to you, but it might jog Keith D's memory as I'm sure
> he'll have
> some ideas too ;-)
>
> >more logically bin the SSR and simply use a relay between Comfort
and
> >the boiler.
>
> Well that should work, if all else fails!
>
> You could also ask on uk.d-i-y if you have usenet access (or via
> www.deja.com).  Someone there might have more experience of
> what you're
> attempting!
>
> Nigel
>
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