piclist 2001\01\29\111013a >
Thread: 2.5KW Load
face BY : Thomas McGahee email (remove spam text)

> I also like this very much!!!
> What is a save time delay to wait for the relay contacts to fully open or
> close?

That depends on the relay used. It varies from a few milliseconds to several
tens of milliseconds. If you are not sure, try something like 20 ms as a
starting point.

Besides regular relays, I have also built units that used small
"contactors". These are more heavy duty than relays, and are
often used to control equipment drawing 20 amps or more.

> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
> (with a small heatsink). Will this be OK?

Yes. One gentleman that I showed the method to decided to not use
any heat sink at all on his triac. His unit has been operating
without any problem for about 15 years now. Personally I don't
like to take too many unnecessary chances. I would rather pay
a buck or two more, throw on a decent heat sink, and KNOW that
my unit will not overheat.

> According to Peter L Peres this relay will also be hot after 30 minutes...Is
> it a better option to look at bidirectional power MOSFET modules then?

Well, there are two sources of heat involved. One is the heating of
the current-carrying contacts. You can reduce this somewhat by
wiring multiple contact sets in parallel. In other words, use a
DPST or DPDT relay and wire the poles in parallel, and the N.O. contacts
in parallel.

The other source of relay heat is the relay coil. You can alleviate
this source of heat by using PWM. BEGIN by applying 100% PWM for the
first 50-100 ms, and then back off to about 35% PWM for holding the
relay closed. To open the relay, apply 0% PWM. Use a PIC, of course!

> I must also say that the ambient temperature is sometimes relatively hot
> (30 - 50 deg C) and can affect the efficiency of the heatsink.

In that case your heatsink will be less effective, as the heat
transfer is based on the difference between the heatsink temperature
and the temperature of the air flowing over the surface of the
heatsink. A cooling fan helps.

> Space is not the main consern but the thing must be very reliable and it
> must last for years...I also don't want the project to consist of 99%
> heatsink if I can prevent it and the cost must be preferrably as low as
> possible.
> Francois

If you use a Triac rated at from 15 to 20 amps continuous duty, then
you can get by with minimal heatsinking if you use the Triac/Relay
method mentioned in my prior post. I often use triacs that are
insulated from their case, and use the metal enclosure as my
heatsink. All you really have to do is make sure the triac is able
to survive the relatively short turn-on and turn-off sequence
when it is operating alone (relay off). Even without any heatsink
at all, most triacs can handle this with ease. But I prefer
to be a bit conservative about such things. I want my stuff
to run for many many years with NO problems.

Fr. Tom McGahee

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