24VAC Relay troubles

[kingery]

One year ago, I installed a DPDT relay in my furnace to allow it to use an
antique thermostat that I really like. The old thermostat has two contacts
that are on the same bimetallic coil that tend to "bounce" and allow the
furnace to cycle too frequently. I adjusted the thermostat so that as the
temperature drops, first contact "A" closes, then at a lower temperature,
contact "B" closes. Conversely, as the house warms, first Contact "B"
opens, then Contact "A" opens.

I then installed a Idec RR2P 24VAC DPDT relay in the furnace and wired it so
that the relay closes when both "A" and "B" are closed, and the relay won't
open until both "A" and "B" are open. This was accomplished by placing "A"
and "B" in series with the relay coil and utilizing one of the two relay
"contact" circuits to bypass "B".

The system has worked fine for 1-year, but the relay began to intermittently
fail to close this past year. I have tried replacing the relay, but the new
relay behaves the same. I have also tried bypassing the thermostat and the
thermostat wiring when the relay is in the malfunctioning state, but still
the relay will not close. I next suspected that I had a poor connection, so
I measured the current flowing into the relay in the malfunctioning state,
and reset all of the wiring connections. The measured current through the
relay coil was 110 ma (the relay is rated to use 105 ma, with a 182 ma
inrush current. This all seemed fine, but the relay is still not closing,
and is getting noticeably warm. My next suspicion was the power
transformer.

The power transformer is a 40 VA, 24-VAC transformer that is currently
outputting 28.48 VAC when the furnace is off (the furnace circuit board uses
126 ma in this state). It outputs up to 840 ma at 26.2 VAC when the furnace
is running and the relay is closed. The transformer is outputing 310 ma
when the relay is in the malfunctioning state. I am stumped. It seems that
the transformer has plenty of power and the relay coil has the proper amount
of current flowing through it. The system cannot be made to start by
tapping the relay, but it will always start if the system is "reset" by
turning off the thermostat (or the furnace power), then turning the
thermostat (or furnace power) back on. It seems unlikely that I have two
bad relays (they look like high quality units and work most of the time).
The wiring all seems good, and the transformer seems to be working. Any
ideas on why my relay isn't closing?

 

[Sir Charles W. Shults III]

Try the relay on a separate power supply that meets its needs. If it fails,
then you know it is the relay. If it does not, then you have some other more
subtle problem.
In some cases, you can have problems with the switching conditions that
require adding a capacitor to the coil of the relay- this will momentarily
extend its switching time. Unfortunately, this trick will only work with a DC
system.
I suspect the contacts are at fault somehow here. Try cleaning and
adjusting them.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

[John Popelish]

kingery wrote:
(snip)

I measured the current flowing into the relay in the malfunctioning state,
and reset all of the wiring connections. The measured current through the
relay coil was 110 ma (the relay is rated to use 105 ma, with a 182 ma
inrush current. This all seemed fine, but the relay is still not closing,
and is getting noticeably warm. My next suspicion was the power
transformer.

If your thermostat wiring has gotten some additional resistance in it
that it didn't have when the relay worked, it might explain the
failure to pull in. It normally takes the higher current to get the
relay pulled in, and then, the increase in the coil inductance drops
the current ot the holding value. If your wiring cannot deliver the
182 ma with the armature held open, you have some resistance limiting
the current to deal with.

AC relays generally run a little warmer then the equivalent DC relay.
--
John Popelish

 

[als]

In article <HApAb.1278$2S4.1068@fe10>, kingery@ixi.net says...

One year ago, I installed a DPDT relay in my furnace to allow it to use an
antique thermostat that I really like. The old thermostat has two contacts
that are on the same bimetallic coil that tend to "bounce" and allow the
furnace to cycle too frequently. I adjusted the thermostat so that as the
temperature drops, first contact "A" closes, then at a lower temperature,
contact "B" closes. Conversely, as the house warms, first Contact "B"
opens, then Contact "A" opens.

I then installed a Idec RR2P 24VAC DPDT relay in the furnace and wired it so
that the relay closes when both "A" and "B" are closed, and the relay won't
open until both "A" and "B" are open. This was accomplished by placing "A"
and "B" in series with the relay coil and utilizing one of the two relay
"contact" circuits to bypass "B".

The system has worked fine for 1-year, but the relay began to intermittently
fail to close this past year. I have tried replacing the relay, but the new
relay behaves the same. I have also tried bypassing the thermostat and the
thermostat wiring when the relay is in the malfunctioning state, but still
the relay will not close. I next suspected that I had a poor connection, so
I measured the current flowing into the relay in the malfunctioning state,
and reset all of the wiring connections. The measured current through the
relay coil was 110 ma (the relay is rated to use 105 ma, with a 182 ma
inrush current. This all seemed fine, but the relay is still not closing,
and is getting noticeably warm. My next suspicion was the power
transformer.

The power transformer is a 40 VA, 24-VAC transformer that is currently
outputting 28.48 VAC when the furnace is off (the furnace circuit board uses
126 ma in this state). It outputs up to 840 ma at 26.2 VAC when the furnace
is running and the relay is closed. The transformer is outputing 310 ma
when the relay is in the malfunctioning state. I am stumped. It seems that
the transformer has plenty of power and the relay coil has the proper amount
of current flowing through it. The system cannot be made to start by
tapping the relay, but it will always start if the system is "reset" by
turning off the thermostat (or the furnace power), then turning the
thermostat (or furnace power) back on. It seems unlikely that I have two
bad relays (they look like high quality units and work most of the time).
The wiring all seems good, and the transformer seems to be working. Any
ideas on why my relay isn't closing?


A 40VA transformer should be good for a couple of definite-purpose type

relays, or half a dozen control-type relays. Probably not your problem.
What are you measuring current with? What is the "furnace circuit board"
and what is it doing?

Things to check:
1) voltage at the contactor coil when operated
2) mounting of relay. I have seen some that absolutely did not like
being mounted horizontal (i.e., to a vertical panel)
3) resistance across both switches (mercury?). Remove any connections.
Should be very low. Check at terminal points (relay/transformer).

<als> alsAThalDASHpcDOTorg