Re: Relays - plea for help!

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi Ray,

Its probably the SSR not switching off completely due to the inductive
nature of the load (relay).
This causes a small residual current to flow at the zero crossing of the
mains thus keeping the SSR conducting. As the mains voltage rises it
conducts further. the relay chatter is due to the current dropping below
the holding current for the relay at the zero crossing but then the relay
reoperates as the voltage/current increases for the next cycle of the
mains.

Look in the Maplin catalogue at  YR90X R-C Network (p548) @ ?2.99 each.
This is just a resistor and mains rated capacitor in series with each other
which is connected across the output of the SSR as a snubber network. This
absorbs the back EMF from the relay allowing the SSR to switch off fully.
You can use discrete components if you wish although the snubber is a
neater solution.

You could temporarily also try connecting a small (15W) light bulb across
the supply to the relay to see if that will allow the SSR to switch off.

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation

-----Original Message-----
From:	Raymond [SMTP:Reb.barnett@xxxxxxx]
Sent:	Tuesday, October 12, 1999 9:28 AM
To:	'ukha_d@xxxxxxx'
Subject:	[ukha_d] Relays - plea for help!

I've ordered three solid state relays and have spent the weekend playing
with trying to get them to operate the boiler. I'm having some
difficulties though, and I've got a hard deadline to get it working...
there are 2 days before the new kitchen is fitted, and access to the
boiler becomes possible but a lot more awkward. Any help would be
extremely appreciated!

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. In the off state, a multimeter records zero
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. If I disconnect
all inputs to the boiler (thus mimicking the zero volt input from the
SSR), the boiler remains off (as expected) and does not produce any
clicking.

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.
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?). Second solution is
to wire a relay either between the output of the SSR and the boiler, or
more logically bin the SSR and simply use a relay between Comfort and
the boiler.

Any comments, or preferably solutions?

p.s. I forgot to mention, I stuck a multimeter across the inputs to the
SSR when Comfort had it switched off, and it recorded 0v, so the SSR
*should* have been completely switched off.

Cheers,

Ray Barnett.

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

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





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

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