Re: Relays - plea for help!

["Dr John Tankard" <jtankard@xxxxxxx>]

Nigel,

The off state current is a bit high, I have the Crydom data sheets, they do
parts with off state draw of only 0.1mA.

Can you bring the tails out to a convenient spot which will allow you some
time to sort this problem ? also remember that the boiler should be
protected with a fuse, SSR can deliver for a short period of time
considerable amounts of current, you also need some form of isolator should
any work need to be done on it you have to remember about the leakage
current. My lighting gadget has a mechanical relay in series which opens
after a short period without a triac firing this automatically protects the
circuit, it also means that should the triac fail (they usually go closed
circuit) that I can still switch it of by stopping firing the triac.


John
jtankard@xxxxxxx

> -----Original Message-----
> From: Raymond [mailto:Reb.barnett@xxxxxxx]
> Sent: 12 October 1999 10:17
> To: 'ukha_d@xxxxxxx'
> Subject: [ukha_d] Re: Relays - plea for help!
>
>
> 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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>
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