Why is a 150 ohm resistor required in this HVAC control circuit ?

[bdn2004]

We are wiring in some additional discrete inputs on the HVAC system. Why does it require a 150 ohm resistor in the circuit?

 

[davidbeach]

Current limitation?

 

[itsmoked]

Good question! Not explainable given the little shown about the inside circuitry and the fact that you remove a short to install the resistor and additional dry contacts.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Three states - Open = off, shorted = Ignore, and 150 Ohms = On?

 

[Palletjack]

Without that resistor when closing the other low voltage controls the voltage on the PTL would go to 0, causing it to stop working.

The resistor keeps a very small amount of current going to the PTL.

 

[Mbrooke]

Best guess: To limit short circuit- and in the event the fuse gets bypassed. My own furnace has that same thing in series with the 5 amp automotive fuse.

Edit: never mind- specs say only 1/8 of a watt. Ohms law says 3.84 watts at 24 volts.

The resistor in my furnace is 510 ohms rated 5 watts, so that 1.129 watts. A dead short would protect the trafo and stop anything from catching fire.

 

[waross]

Why does it require a 150 ohm resistor in the circuit?

A schematic diagram of the PTL Percent Timer will probably explain that.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Palletjack; I think you might be on to something.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

MBrooke:
I suspect that your thermostat has a small heating coil.
The pre-act coil is energized when the thermostat is on and acts to reduce the overshoot of the thermostat.
The resistor will limit the current through the pre-act element.
Many generic replacement thermostats have an adjustment to match the setting of the heater to the current of the furnace control circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

]

The circular line on the cream coloured disk is the heater element.
The wiper is used to adjust the amount of overshoot of the thermostat.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@Waross: Its an electronic thermostat, but this furnace model would also go into older homes with mercury thermostats like yours. I had the round version years back. I'd say you're right. Anticipator btw?

 

[waross]

Anticipator

That's the word that I was looking for.
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Any time!