Re: Relay

[Raymond <Reb.barnett@xxxxxxx>]

Hi Keith,

Thanks for the reply: I was trying to work out which was best solution
out of a solid state relay versus a 'real' relay.

I was using the Maplin catalogue specs for their solid state relays to
get the 5W power loss, but I may well have got my maths wrong, and will
certainly have calculated it for the worst case if the heating system
drew 5A from the relay - I don't know how much current will be drawn so
that's a project for tonight with the multimeter... But anyway, the
thing is, if the solid state version wastes 5W, that's a point to the
'real' relay.

However, if the 'real' relay requires an auxilliary 12V power supply for
it to work without overloading Comfort, then a second transformer will
probably waster 5W, so we're back to evens.

The solid state relay has no moving parts, so presumably will last
longer than a real one; the solid state relay moves ahead.

I understand how a real relay works, and am a touch concerned about a
chip that has mains voltage on one pin and 12v on another - I don't want
to start a fire or blow up Comfort. So the real relay is back on evens.
The Maplin catalogue details two types of solid state relay (zero volts
and something different - I don't have the catalogue at work) and I
haven't a clue what the difference is and whether it matters...

So which one should I use? I think that if you reckon a solid state
relay will work, I'll order a couple of these and play with them. In the
mean time, I'll check the current drawn by the boiler and check what
spare capacity I've got on Comfort's 12V supply - I've added more
detectors since last time I checked.

Ray.

> -----Original Message-----
> From: keith.doxey@xxxxxxx [mailto:keith.doxey@xxxxxxx]
> Sent: 30 September 1999 16:02
> To: REB.Barnett@xxxxxxx
> Subject: [ukha_d] Re: Relay
>
>
> 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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