Re: Relay

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi Ray,

Dont know where you got the rating of 5W from.
Looking in the Farnell catalogue breifly most solid state relays need
around 15mA trigger current. From a 12 volt supply this would dissipate
0.8W.  The conventional relays with a 12V coil all seem to have a coil
resistance of above 200 ohms giving a power consumption of 0.72W or less.

The 5W could be the loss on the mains side of the solid State Relay at
MAXIMUM current. I would imagine that your heating system would use very
little in the way of current via the control input.

Also remember that the power would only be used whilst the boiler was
running and the fact that your HA system is saving fuel by not running
unneccessarily, it will save you money in the long run.

Also at approx 7p for a unit of electricity you can lose 5W in your relay
for 2857 hours before it has cost you the 40 quid you would have spent on
the X10 module. The X10 module probably uses that much power anyway !!!

:-))

Keith

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

Subject:	Relay

I've successfully trialled connecting Comfort to my central heating
system.

My boiler has three terminals of interest: connecting the first through
to mains live switches the hot water on; connecting the second through
to mains live allows the central heating to run; the third input is
effectively the thermostat input, and connecting this to mains live
commands the boiler to fire up when the central heating is running.

For my trial, I used an X10 appliance module. I wired the live output
from the X10 module through to the second input. Thus I could command
the central heating to switch on and off by transmitting an X10 signal.

For the real thing, I don't want to spend ?40 on each X10 appliance
module if I can help it. Consequently I wondered if anyone on this group
can explain the difference (practically) between a solid state relay and
a 'real' relay? So far as I can gather, a real 12v input, mains rated
relay, can be connected directly to an output from Comfort. However they
draw a fair bit of current so I'm concerned that I may need a auxillary
12v supply. Solid state relays appear to do the same job, but draw less
current from the 12v supply - the downside being the actual switch is
not perfectly efficient, and wastes 5w of mains supply from what I could
tell... Any recommendations on this would be appreciated.

Ray Barnett


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