protecting relay contacts

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The.Central.Scru...@invalid.pobox.com

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Jun 11, 2001, 7:33:45 PM6/11/01
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I'm repairing a swamp-cooler controller. It has two relays, one to control the
water pump and the second to control a 3/4HP blower motor.

Last year during a blackout, the power oscillated 5 times a second for a few
seconds and wiped out the contacts on the fan relay. It isn't really pitted
enough to limit travel (seen those, huh?) but the plating is gone and it's a
brownish burnt mess.

Right now the plan is to swap the relays. The injured one should have no
problem with the 30W water pump.

Question: is there something I can do to reduce the arcing on the relay
carrying the blower's current? Caps/inductors/?

If necessary, I'll install an electrical box in the attic with a "real" relay, an
honest to god 50A relay instead of the whimpy relay in the controller. They say
it'll handle a 1HP blower, but it doesn't look like much more than a 3A relay.

Dave M

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Jun 11, 2001, 8:11:35 PM6/11/01
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You might think about protecting your relay contacts with a snubber circuit.
In case you aren't familiar with them, snubbers are just a series RC circuit
that limit dv/dt effects across opening/closing contacts. There's a
nomograph at http://www.reedrelay.com/relayb1.html that will help calculate
the values. The nomograph is for reed relays, but should work well with
standard relay contacts too.
--
Dave M

Before you criticize someone, walk a mile in his shoes. That way if he gets
angry, he'll be a mile away... and barefoot.
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Trevor Wilson

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Jun 11, 2001, 8:28:23 PM6/11/01
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<The.Central.Scru...@invalid.pobox.com> wrote in message
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**For inductive loads, those solid state relays are pretty hard to beat.
I've installed them on various motor and transformer applications (after
relays were destroyed) and enjoyed many years of reliable service. 2.5kVA
was the largest inductive load, I've used them on. Check the manufacturer's
specs, just to be on the safe side, though.


--
Trevor Wilson
http://www.rageaudio.com.au


Warren Weber

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Jun 12, 2001, 10:14:12 AM6/12/01
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I had installed contact type relays on my swamp cooler so I could thermostat
control it. Relay arcing caused much noise on starting and TV interference.
Changed to solid state relay and has been working great for over 5 years.
Warren


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The.Central.Scru...@invalid.pobox.com

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Jun 12, 2001, 12:49:22 PM6/12/01
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On Tue, 12 Jun 2001 08:14:12 -0600, Warren Weber <hi...@ris.net> wrote:
>I had installed contact type relays on my swamp cooler so I could thermostat
>control it. Relay arcing caused much noise on starting and TV interference.
>Changed to solid state relay and has been working great for over 5 years.

I may do the same thing. One bit of wierdness is the signal going to the coils
of the low-speed and high-speed relays. On a dvm I get 70VDC/40VAC. On the
scope it (open circuit) is mostly clipped at an upper supply rail with about
30% of the signal dipping to 50% in an inverted halfwave sinewave. Wierd. The
relay is marked for 110VDC; I assume that's the coil.

I didn't spend much time looking at before deciding that I'd just put a 120vac
pushbutton switch in place of the thermostat in the meantime.
(http://members.home.com/adamf123/hack.jpg)

They maker says they're good for 20 amps, but they're dreaming. I've seen more
robust contacts on a 3A relay.

The plan was to install an electrical box in the attic between thermostat and
swamp cooler and put a 30A AC relay in line. Now I think I'll go with a solid
state relay. I have no problem throwing together a triac w/ an optoisolated
diac trigger (3010?). I should be able to put the triac circuit in the same
place that the relays occupied.

The.Central.Scru...@invalid.pobox.com

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Jun 12, 2001, 1:50:35 PM6/12/01
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If I decide on a solid state solution -- what do I need to do to protect it
against a 3/4HP inductive load with the power jumping on and off 4 times a
second?

When I use a triac driver, I just use a diac optoisolator (MC3010?) and a triac
with no protection whatsoever. Works for having a preamp control a couple of
100W/ch stereo amps, but a 3/4HP blower is a whole 'nother story.

I'd like to keep the relay's replacement circuit in the same space as the relay.
Got about 1"^2 to play with.

Spehro Pefhany

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Jun 12, 2001, 2:19:19 PM6/12/01
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The renowned The.Central.Scru...@invalid.pobox.com wrote:

> When I use a triac driver, I just use a diac optoisolator (MC3010?) and a triac
> with no protection whatsoever. Works for having a preamp control a couple of
> 100W/ch stereo amps, but a 3/4HP blower is a whole 'nother story.

Use an SSR that is RATED FOR INDUCTIVE MOTOR LOADS with an appropriate
heatsink (figure on roughly 1W/Ampere, see data sheet for exact numbers).

> I'd like to keep the relay's replacement circuit in the same space as the relay.
> Got about 1"^2 to play with.

F'geddabowdit (as they say in NJ), your heatsink will be much bigger than
that.

Best regards,
--
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Spehro Pefhany --"it's the network..." "The Journey is the reward"
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leeppp

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Jun 12, 2001, 2:24:05 PM6/12/01
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"Warren Weber" <hi...@ris.net> wrote in message news:<tic91qh...@corp.supernews.com>...


Heavy duty "SSR`s" are definitely the way to go for heavy load
situations. We`ve used `em for years in the lab on the heavy equip`t
stuff. Very few go bad.
LEE

The.Central.Scru...@invalid.pobox.com

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Jun 12, 2001, 4:06:38 PM6/12/01
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On Tue, 12 Jun 2001 18:19:19 GMT, Spehro Pefhany <sp...@interlog.com> wrote:
>The renowned The.Central.Scru...@invalid.pobox.com wrote:
>
>> When I use a triac driver, I just use a diac optoisolator (MC3010?) and a triac
>> with no protection whatsoever. Works for having a preamp control a couple of
>> 100W/ch stereo amps, but a 3/4HP blower is a whole 'nother story.
>
>Use an SSR that is RATED FOR INDUCTIVE MOTOR LOADS with an appropriate
>heatsink (figure on roughly 1W/Ampere, see data sheet for exact numbers).

I don't have room for those solutions... My latest plan is just to use an
optoisolator 'n' a 35A 600V triac. If it blows up I may desice to go your
way w/ another electrical box somewhere in the attic.

Trevor Wilson

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Jun 12, 2001, 5:36:19 PM6/12/01
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<The.Central.Scru...@invalid.pobox.com> wrote in message
news:slrn9ictr4.15n.The.Cent...@C1459607-A.arvada1.co.h
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**You'll need a fair chunk of heatsink, or maybe a PentiumT heatsink and
fan, to keep it cool.

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