Re: Relays - plea for help!

[cytech@xxxxxxx]

The problem may be the 15 mA leakage current in the off state - it may
be enough to partially operate the boiler input, if it requires very
little current

Chiu

raymond <reb.barnet-@xxxxxxx> wrote:
original article:http://www.egroups.com/group/ukha_d/?start=1065
> 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
> >


------------------------------------------------------------------------

eGroups.com home: http://www.egroups.com/group/ukha_d
http://www.egroups.com - Simplifying
group communications