How does a "current sensing relay" work?
David Gersic
Backround: I discovered that the fool that installed the humidifier on my
furnace (before I bought the house) wired the 110V/24V transformer for the
control circuit to the furnace motor supply. Worked ok when the furnace was
on "heat", as the furnace motor gets 110V then. Blew the transformer when
the furnace was on "A/C" or "fan" settings, though, because then the motor
gets 170V to run faster. So, I did some research and found that the proper
way to set this up is to wire the 110V/24V transformer to a 110V supply
external to the furnace, then use a "current sensing relay" to switch the
24V control circuit on/off based on when the furnace motor is running.
I got one of these devices, essentially just a little black (sealed)
plastic box, with a wrap to go around a wire, and a screw to hold the wrap
in place. A two-wire tail comes out the side to switch the 24V circuit.
I have it wired in per the directions, so that the relay, the humidistat,
and the 24V side of the transformer, are all in series, providing power to
the relay in the humidifier that opens the water valve when both the relay
and the humidistat are closed. It seems to be working, but there's
something weird.
When I connect up the circuit, I can hear the humidifier relay pulse. It's
very short, and it doesn't pull in and hold, it just gives a quick "click".
So, I put my meter on the circuit to see what was up. I found that the
humidifier relay is getting voltage at all times.
Measuring with the meter, I found that with the furnace off (and the
"current sensing relay" ostensibly "off") that there is about 2.5V / 8mA
at the humidifier. With the furnace on, there is 18V / .4A at the
humidifier.
So, with that rather long-winded explanation, the questions are:
1) How does one of these "current sensing relays" work? I'm guessing it's a
solid state device, not something with moving parts.
2) Should it be turning all the way off and blocking all voltage/current
from the circuit? Is this one defective, or is it working correctly
and I just shouldn't worry about it?
Any comments, opinions, suggestions, or pointers to information welcome.
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|David Gersic dgersic_@_niu.edu |
|Systems Programmer Northern Illinois University |
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| EMACS: Equine Mammals Are Considerably Smaller |
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|I'm tired of receiving crap in my mailbox, so the E-mail address has been|
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Ian Field
voltage operated relay - except the coil consist of only a few turns of
thick wire - thick enough to carry the highest anticipated load current, and
just enough turns to be sensitive enough to pull-in at the lowest
anticipated on state current. The best example I can think of is in the old
type electro-mech' car dynamo regulators, (I think this worked against the
voltage sensing coil that was trying to open the field winding contacts as
the voltage rises - so that a leavy load supplied via the current winding to
close the field contact, and keep the dynamo output "on the up"). In
theory - you could modify a regular voltage operated relay by splitting the
windings with a sharp knife, so you can peel them off and fill the bobbin
with a couple of dozen or so turns of heavy guage enamelled wire - but with
most relay designs, this isn't easy without damaging the contact's assembly!
This would almost certainly be easier with a sealed reed-relay, single pole
changeover reeds do exist - so if you need break instead of make - that's
not a problem.
The modern electronic way, can be implemented with any OP-AMP that has a
"rail to rail common mode range" - you simply insert a very low value power
resistor in series with the load, and use the OP-AMP to sense whether any
current is flowing to develop a volt drop. If the current is high enough and
you can run sensing leads to a pair of terminal securing points a foot or
two apart - the volt drop along the length of cable might be high enough to
sense! However great care is needed to design the OP-AMP input circuitry to
avoid surges/spikes blowing the chip - and the whole circuit has to "float"
at the same potential as the monitored line, which can make servicing very
hazardous! Such a "bare minimum" circuit would usually only be used for low
voltage equipment!
An easy way to provide mains isolation, is to wind a sensing coil on a
toroid core - and simply pass the monitored line through the hole in the
middle. This solves a lot of problems and the sensing circuit can be
simpler. Another option is to see if it is possible to simply add a "dropper
resistor" in series with the transformer primary. Set things up so the
applied voltage is at it's lowest, then introduce a series resistance and
increase it in small increments, up to the point that reliable operation is
just lost - than back up to the point that there is just enough current.
Next reset the equipment so the voltage is at it's highest and chech that
the primary voltage has been reduced to an acceptable level. Even better -
you can now get PTC thermistors specifically designed for thermal protection
of transformers. In any event, such a device should protect the transformer
from burnout - that's what it's designed to do! The problem might arise that
it switches off because the transformer is being driven with too high
voltage, the solution would be to use the PTC for a much bigger transformer
so the resistance didn't rise so fast approaching the critical temperature.
If the biggest PTC you can get hold of has a slightly lower critical
temperature than the correct type - you can have the best of both worlds,
the bigger PTC would start to increase resistance first and hopefully make
the transformer self compensating - if that works out OK, theres no
problem - but if you also add the correct PTC as well, it makes the
transformer completely failsafe.
"David Gersic" <dgersic_@_niu.edu> wrote in message
news:a16avl$ij8$8...@husk.cso.niu.edu...
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