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BS2 24VAC Propane Furnace Hour Meter — Parallax Forums

BS2 24VAC Propane Furnace Hour Meter

mtbphil1mtbphil1 Posts: 6
edited 2014-01-20 19:32 in Accessories
I had the DS1620 near a heat register in my house and I was using a BS2 with USB datalogger to track the temperature over time. I'd like to increase the accuracy of the measurement to determine when and how many hours my furnace is on to monitor my energy usage and compare to the number of gallons I get on propane refills. I wired up a circuit to tap into the 24VAC thermostat wire going to the furnace and to convert the voltage to <5VDC so I can precisely record when the flame is burning. The circuit diagram is attached. I used an adjustable pot to trim the voltage to just below 5 since my 24VAC was not exactly 24VAC where I tapped in. I've tested this circuit with the 24VAC supply and a voltmeter and it does produce 5VDC at the output pin. Before I hook everything up to the furnace and the stamp, I'd like a second opinion on the circuit to make sure I'm sending the stamp a safe current and voltage to read while at the same time causing no disruption to the furnace circuitry. I believe the 24VAC line primarily feeds the relay switches on the furnace control board. I'm treating the thermostat wire as a voltage supply from which I'm pulling next to no load with the 100k resistors. Any feedback would be greatly appreciated!
pdf
pdf
24VAC to 5VDC.pdf
171K

Comments

  • kwinnkwinn Posts: 8,697
    edited 2014-01-18 15:43
    That should be ok. The 100K series resistor will limit the current to the pin protection diodes even if the 24V is a bit higher.
  • mtbphil1mtbphil1 Posts: 6
    edited 2014-01-19 13:23
    Thanks. Any issue tapping into 24VAC line on the furnace side? I don't have much information on how the signal gets used on the furnace, but my plan was to just monitor voltage and not pull any current to prevent any issues with the furnace board. Just trying to double check my work so I don't make an expensive mistake. Thanks!
  • Prophead100Prophead100 Posts: 192
    edited 2014-01-19 18:52
    You could also use an optcoupler (e.g. Vishay SFH615A) to monitor the 24 VAC line. This method could give you a greater degree of protection from over-voltage based upon its isolation from the microcontroller. With up to 5Kv isolation even if the ~$2 optocoupler burns out the rest is ok. It could also reduce the number of components with just a few resistors. Your 24 v would power the led on the coupler through a current limting resistor and the on the output side you could use the 5 v from the Stamp with a current limiting resistor and pull down. Most data sheets have good example circuits.
  • mtbphil1mtbphil1 Posts: 6
    edited 2014-01-20 05:06
    Thanks! I like that idea a lot. It will definitely give me more piece of mind. If I current limit the 24VAC line to the 10mA the spec calls for, do you expect the positive portion of the AC signal to cause the optocoupler to cycle? Would I need a capacitor as well?
  • kwinnkwinn Posts: 8,697
    edited 2014-01-20 19:32
    An optocoupler is a good idea. You could count the 1/2 cycles the AC inputs to produce a very accurate on time.
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