On Tue, 18 Jul 2006 01:17:39 GMT, John - KD5YI
wrote:

John Fields wrote:
On 17 Jul 2006 08:42:19 -0700, wrote:


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?


---

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!



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

http://www.radiofrigor.com.br/download/coils.pdf

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


--
John Fields
Professional Circuit Designer