Re: Relays - plea for help!

[Nigel Orr <nigel.orr@xxxxxxx>]

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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