AC Circuit Problem (voltage - resistor)

snipped-for-privacy@gmail.com wrote in news:1152996421.209694.220710@

35g2000cwc.googlegroups.com:

That line voltage is WAY too high. The life of lightbulbs and appliances will be reduced, in addition to the solenoid you mention.

The real solution is to contact the company that furnishes the electricity and get them to come out and lower the voltage coming into the house.

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.

An electric stove or dryer is not going to like 276 V for very long. Also, check if the line voltage is high at all outlets. If some are high and some are low, you have an open ground, which the power Co needs to fix ASAP. In my case, they arrived within the hour.

Tam

the

figures

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

Thank you for all your replies. The high voltage only to seemed to happen for a while but we are going to call the electric company, National Grid, and have them put a recorder on the line.

When I took the reading of 133 volts it was from the hardwired terminals inside the furnace. I then tried a couple of other outlets and got similar readings. I then went looking for another meter to confirm these readings but when I finally got another meter and tried it the voltage was at 119.

I'm still going to try applying the resistor to the solenoid because even with the line voltage at 119 and the furnace in an idle state, I think the only load on the 24 volt transformer is the solenoid and it is still getting hot.

Thanks again everyone, Derek

You mean an open neutral.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Just wire it up without a resistor, 28 volts is fine for a 24 volt nominal solenoid.

The solenoid coil is not a resistance, it is an impedance composed of orthogonal resistive and reactive parts, and this impedance is a strong function of whether the solenoid is in or out, varying over a 2-3 range. Placing a resistor in series may very well blow the transformer because the resistor presents too much drop at "inrush" and the solenoid hangs at a current loading in excess of the transformer VA.

Your line readings are suspect and unreliable. You don't make it clear about the solenoid being permanently on or not, , thinking it more important to tell us totally irrelevant things like it is in a house belonging to your mother's boyfriend. If this is the kind of thing you focus on, then stay away from the furnace.

by the

for

figures

If this were DC, that would be the right way to calculate this, however, he is dealing with AC. The problem here is that the impedance of the coil is largely resistive *and* inductive. To make things worse, the inductance will change, likely a lot when the plunger is in vs. out. Since when the plunger is out, the magnetic path would have a much larger air gap, and thus making the resistance of the coil more dominant, and when the plunger is in, the air path for the magnetic field is much less, and therefore it's inductance is much higher. The problem comes when the plunger needs to pull in, the impedance of the coil is at a minimum, causing a higher dissipation in the resistor, and a much weaker then expected magnetic field in the coil. When the plunger is in, the impedance increases, dropping the current, and increasing the voltage. Simply said, adding a series resistor can cause the plunger to not pull in, and/or keep the power dissipation in the coil high. I quickly figured out this a long time ago when I wired two contactors with

120V coils in series on a 240V supply. Only 1 contactor would reliably pull in!

Derek-

The solenoid appears to be functioning correctly, from what you've said. Yes, it will be hot since it is burning up 7 watts of power.

The real problem is the apparently high line voltage. Unless you are connected to a small town power source that is not connected to the national grid, it is unlikely that your house is receiving 276 Volts AC.

What is more likely is that the neutral connection between the house and the power company's transformer has come loose. The two 120 Volt AC halves do not carry the same current, so voltage divides between them in relation to the ratio of loads. If this is the case, you may measure voltage at several places in the house, and find two values: 138 and 102, for a total of 240 Volts.

The loose connection can be either at the power pole or at your power meter. The power company should be able to isolate it, but the owner will have to pay an electrician to fix a problem if it is on the house side of the power meter.

Fred

If you are going to put a resistor in series, you need an on delay relay.

24VAC ---o \\ S1 o o--+---[R]---+------+ | | | +----- | | N/C ^----+ [Solenoid] _ | | | | | | +--|/|----+ | | |_|

The solenoid is normally on. I think it is part of a panic circuit that is left over from the heat pump that we don't using any more. So far, the best we have been able to figure out, the only time it is off is when the thermostat is turned off. If the furnace is sitting idle it is still active. From the wiring diagram, very crude and high level, it shows that its operation is influenced by a flash detector, temperature sensor (for over heating) and auxiliary input. Haven't determined if the the aux is hooked up to anything.

What we have observed since I oroginally posted is this:

If the furnace, the circulator is running, the voltage at the solenoid is about 24 volts but when it shuts down, it goes up to 28 volts. So something else is putting a load on the same circuit to cause it to drop.

Our concern is for the solenoid being hot to the touch. Should we be trying to change the voltage or the amperage to solenoid?

Derek

---

From your original post and your latest, you have a circuit that looks like this: (View in Courier)

ACIN>----+-------+---E1 | | | O-->| | |S1 [SOL] +---O | | | | | [Rl] | | ACIN>----+-------+---0V

Where S1 is normally closed and E1 is normally 24V, but when S1 opens E1 goes up to 28V and the solenoid runs hot.

From the solenoid's spec's it dissipates 7 watts with 24VRMS across it, so its resistance is:

E² 24² R = --- = ----- ~ 82 ohms P 7W

and the current through it must then be:

E 24V I = --- = ----- ~ 0.293 amperes R 82R

When the switch opens and the voltage changes to 28V, the current rises to:

28V I = ----- ~ 0.341 amperes 82R

and the power dissipation goes to:

P = IE = 0.341A * 28V ~ 9.55 watts

An easy way to fix the problem would be to keep a constant load on the transformer, and you can do that by shunting the solenoid with a pair of 24 Zener diodes in series opposition:

ACIN>----+-------+-------+---E1 | | | | [1N5359B] O-->| | |K |S1 [SOL] | +---O | | | | |K | | [1N5359B] [Rl] | | | ACIN>----+-------+-------+---0V

That way, with S1 closed and 24V across the diodes they'll be effectively out of the circuit, but when S1 opens the current which formerly went to Rl will be diverted through the Zeners and the voltage on the solenoid clamped to 24V + the Vf of the other diode, about 0.7V.

As it turns out, the current into the switched load is 48mA and the Zener test current is 50mA, so that's a pretty good match!

-- John Fields Professional Circuit Designer

I found the DMG coil the OP specified, but it does not show it to be 7 Watts.

You're in upstate NY so a hydronic system would be typical.

Right- this is normal.

A few volts less is not going to make the solenoid run cool, it will still be hot to the touch at its nominal rated voltage, it was designed to run that way and the heat is normal unless the solenoid gets so hot that it fumes and/or discolors- then it is defective in some way and should be replaced. You could make an amperage check and verify that the AC current through the coil is in the range of 0.75 +/-20% amperes just to be sure. You say this is a dual cooling/heat system, so I assume the solenoid valve is off when the thermostat is in COOL mode, meaning you can't just remove it from the system.

You're about as useful to this thread as t*ts on a boar hog, why don't you take hike.

Sorry to upset you, Fred. I just thought a link to the coil specs might be helpful.

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Israel IS at peace with Egypt(officially) and Jordan,and WAS at peace with Lebanon,until Leb. allowed terrorists to remain armed and initiate attacks against Israel,acts of WAR.

You should read what the Islamics goals are for Europe and the rest of the world. It's either Islam,or enslavement.(per the Koran)

--
Jim Yanik
jyanik
at
kua.net

--
Oh well... The circuit stays the same but the numbers change a
little.