DaveM wrote:
wrote in message
ups.com...

I have an AC circuit problem I need help with. At my mother's
boyfriend's house he has a furnace with air conditioner/heat pump
attached. When he bought the house natural gas prices were lower then
electricity and he had the heat pump disconnected but there still is a
flow shutoff valve attached to the air conditioner/heat pump that is
operated by a solenoid. The solenoid is normally energized but the
voltage suppied to it exceeds the normal recommended operating voltage
and gets very hot. The unit is an ALCO Controls, Coil Type DMG, 24
volts, 7 watts, 50 - 60 Hz. The computer in the furnace is supplying 26
to 28 volts.

What I would like to do is attach a power resistor to reduce the
voltage. The assumption I would like to make is the highest voltage to
the solenoid would be 30 volts. The solenoid will activate at as low of
a voltage as 17 volts. I would like to add a resistor that will take
the 30 volts and reduce the voltage to 24 volts.

How do I go about computing the value for the resistor? Is there a
better way of going about this than using a resistor?

FYI: The reason the voltage is so high is the line voltage is 133 to
138 volts. It is then stepped down by a transformer. The secondary
output of the transformer is the passed through a relay, control by a
computer, to the solenoid.

Thanks In Adavnce,
Derek



You find the value of the resistor by first calculating the current drawn by the
solenoid.
I = P/V
I = 7W/24V = .3A

You can also calculate the impedance of the solenoid's coil.
Z = V/I
Z = 24V/.3A = 80 ohms

Now, calculate the voltage that the resistor needs to drop.
V = Vsupply - Vsolenoid
V = 28 - 24 = 4V

Now, using these two values, calculate the resistor value.
R = V/I
R = 4V/.3A = 13.33 Ohms (a close standard value would be 12 ohms)

Calculate the power that the resistor will dissipate.
P = VI
P = 4V * .3A = 1.2 Watts
Always add a safety factor of at least 2 on the power rating, giving us a
resistor power rating of at least 2.4 Watts. A standard wattage rating for
power resistors is 3W or 5W.

Putting all this together results in a 12 Ohm/5 Watt resistor.

You can deviate from these values a bit if you need... i.e., rework the figures
to further reduce the solenoid voltage.

Cheers!!!


Excellent reply, Dave. Very instructive and clearly written. You must be a
teacher.

I only want to jump in here to stress that Tam's and Jim's replies should be
considered first as the high line voltage has effects on other household items.

I have a mobile home that survived for about 15 years on about 135 line
volts. We lost a lot of light bulbs and a ventilator fan over that time. We
allowed the high line voltage due to the fact that the home has about 600
feet of #4 AWG between it and the pole. It was thought that the A/C unit
needed the higher voltage to start. About 5 or so years ago we had a
problem, the nature of which I no longer remember, and the power company
came out and lowered the voltage. I was afraid the A/C would not start
properly, but it turned out to not be a problem after all.

In any case, I would just like to say that it is better to cure the problem,
if possible, rather than make modifications. Some day the solenoid may need
replacing and the replacement part may not be the same. There are times,
though, when modifications are called for.

Cheers to all.

John