piclist 2001\01\30\211433a >
Thread: 2.5KW Load
face BY : Roman Black email (remove spam text)

Peter L. Peres wrote:
> Look, the relay heats up because it and the surrounding wiring will be
> passing 12A at about 0.5 ohms total resistance (including board, screw
> terminals, and some wire). That's about 6W dissipated.

Man you are so far off base! I have a good milliohm meter
and I am quite aware of the resistive losses in a
relay passing 12 amps! You figures are about 20 to 30 times
out of whack. Expect a few milliohms (<20) for any decent
quality 20A relay contacts. Get a copy of Ultracad's
"pcbtemp.exe" which will help you calculate PCB track
resistance for say a 1/4 inch track with solder stripe.
Again a few milliohms.

Also I would not use screwdown tewrminals I would solder
the wires into the board or ont PCB pins. But if you were
to include screw down terminals you would also have to
allow for them in your thyristor solution so it's not
fair to include them as part of the relay losses.

> Relays have no
> heatsinks

Of course not, a 20A relay runs quite cool at 12A and
doesn't need a heatsink. I work with relays every
week, and they have their place. I think it is bad
to turn someone off a relay solution based on false
evidence! :o)


PS. Find a relay rated for more than 12A that
dissipates 6W and you have found your "relay with a
heatsink" ha ha!

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