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!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

They call it PMS because Mad Cow Disease was already taken.