Re: Relay

["Keith Doxey" <keith.doxey@xxxxxxx>]

Hi Raymond,

Go for the SSR Zero Volt Switching.
No moving parts and no RF Interference.
SSR's (instant or zerovoltage) all switch off at mains zero unlike a
conventional relay which could release at peak mains with some arcing at
the
contacts.
Normal relays require a diode in inverse parallel with the coil to protect
the driver circuitry from the back EMF when the relay is de-energised. (a
12V relay can easily give a back EMF or 300V or more)
No clunks from the cupboard when the boiler gets turned on.

Some decidedly dodgy maths coming up....

...looking at the figures in the Maplin catalogue the SSR drops 1.6V @ 5A
which works out to be 8W.
BUT....to draw 5A at 240V would require a resistive load of 240/5 = 48
ohms.
If the SSR was dropping the 1.6V then the supply to the load would only be
238.4V
Small difference but....238.4/48 = 4.97A
Power =Current(squared)*Resistance = 4.97 * 4.97 * 48 = 1186W

Power used by the load plus the power lost in the SSR
1186 + 8 = 1194W

THATS 6 WATTS LESS THAN CONNECTING IT DIRECTLY TO THE MAINS!!!
You will SAVE money :-))

It wont work out EXACTLY like that as the load will not be purely resistive
but the point is that any power lost in control circuitry will reduce the
available voltage to the load by a small percentage thus lessening its
power
consumption slightly.

Dont worry about the power consumption, its so low as to be non existant.
Make sure you put a Hi-speed ceramic fuse (of the lowest possible rating)
in
series with the SSR to protect it in the event of a fault.

As to your point about mixing mains with low voltage...no problem. The SSR
is designed to withstand a minimum of 2500V without the isolation breaking
down. Use heatshrink sleeving over the connections if you are soldering
wiring direct to the SSR. If you are making a printed circuit board to
mount
them make sure you have maximim separation between the tracks by keeping
all
the mains stuff to one side and the low voltage to the other. Also put an
earthed track between the low and high voltage sides of the PCB keeping it
away from the mains side to avoid any possibility of a short circuit.

Dont forget to take a few pictures of the finished project for us all to
see. :-))

Keith

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


> -----Original Message-----
> From: Raymond [mailto:Reb.barnett@xxxxxxx]
> Sent: 30 September 1999 17:13
> To: 'ukha_d@xxxxxxx'
> Subject: [ukha_d] Re: Relay
>
>
> 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.
>


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