Detecting 24 volts
JerryB
Posts: 10
Hi all,
Ive just had·a propane furnace installed and I want to estimate when the propane tank is near empty by measuring the amount of furnace run time.· One method I thought might work is to detect the 24 volts ac used to activate the gas valve. How can I detect the 24vac with my bs2?
Jerry
Ive just had·a propane furnace installed and I want to estimate when the propane tank is near empty by measuring the amount of furnace run time.· One method I thought might work is to detect the 24 volts ac used to activate the gas valve. How can I detect the 24vac with my bs2?
Jerry
Comments
One method would be to connect the 24 VAC to a full wave bridge rectifier, and connect a large electrolytic capacitor on the output of the bridge recitifier.
This will give you about 23 volts DC. The 23 volts DC·could then be used as the input to a 7805 voltage regulator, and the 7805 regulator has its normal caps on the input and output side. Lastly, put some load on the regulator output, like 4.9 k ohms or so.
Now when the 24 vac is present, the 7805 regulator will output 5 volts, and when there·is no 24 VAC the 7805 output will be 0 volts, two values the stamp can easily deal with.
A second thought is to feed the 23 volts DC into an opto isolator, and the transistor side of the opto to the stamp input pin with a pull-up resistor.
As for a solution, if it were me, I'd go with the optoisolator. Cheap, easy, three parts, no worries. You could probably fit the whole thing in a film can if you wire carefully.
-dave
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
This is not a sig. This is a duck. Quack.
I like the optoisolator route. Simple things appeal to me. Is this optoisolator something Radioshack would carry? If not maybe from jameco. Would you have a part No?
Jerry
The optoisolator route is the easiesy way to go. A good old 4N25 type shoulod do you nicely. Everyone sells them (Although I do not know if they are stocked at 'The Shack')
Use a 1.1k to 1.5k series reaistor on the input side of the opto and you will be able to sense the on and off cycles to the solenoid. Put a diode reversed across the input pins of the opto to bypass any spikes the solenoid may have when turning off. A 1N4007 or any high PIV diode will work.
You will also need a pull up resistor on the stamp input pin. I usually use a 100k resistor.
Good Luck.
Alan Bradford
Plasma Technologies
I'll take a look today, if I can find anyone open, for a 4N25. Thanks for the help.
Jerry
http://www.aaroncake.net/circuits/supply5.htm
And yes there will be a lot of people saying not to do it
but you are working with 24 volts not 120 / 240 for which
it will work also.
CHECK FOR LEAKS FIRST
If you run 23VAC thru a 5.6K resistor then a 1K across that will give you about 3.5 VAC then thru a diode(3.5*1.415-0.7) and cap you should get about 4.25V.
··················· 5.6K······914 DIODE
AC In
\/\/\/\
|>
To Stamp Pin (input)
······························ |········· |························ |
····························· \·········· |························ \
····························· /········
·· 1UF CAP···· /··· 4.7K
··························1K \·········/---\···················· \
······························/·········· |························ /
······························ |········ ·|························ |
AC In
Stamp GND
Bean.
Post Edited (Bean) : 11/25/2004 4:33:40 PM GMT
I guess all of this could be done at the thermostat inside the house
Larry Gaminde
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage
Knight Designs
324 West Main Street
P.O. Box 97
Montour Falls, NY 14865
(607) 535-6777
Business Page:·· http://www.knightdesigns.com
Personal Page:··· http://www.lightlink.com/dream/chris
Designs Page:··· http://www.lightlink.com/dream/designs
·
This is my safety 100% proof taught.
Ion
The furnace is a 2 stage unit with 4 dampers and 3 thermostats and a control box. I'll be getting my signal from the control box. The installer showed me the terminals I could use to pick up the signal for each stage (24vac). I couldn't get an opto-isolator yesterday(closed) but I'll check on it today. If not then I'll try the voltage divider circuit on a breadboard to see how it works.
Bean, how do I select the resistors for the divider? Could I use higher value resistors as long as the ratio is approx the same, ie 10k and a 2k?
The installer will be back on monday to fire up the furnace and tweak the system and I plan on having my circuit and display ready then.
The propane tank is a 100# unit and I think I will get about 35 +/- hours of run time from it.
Jerry
The voltage divider method has its drawbacks.· You are trying to reference an AC voltage to a regulated 5 volt DC circuit. It is easy to get ground loop problems. This can cause erratic operation of the Stamp and possible damage to the circuit.
Isolation is the safest way to avoid the problems. The relay method will work as well as the optoisolator. Radio Shack does sell 24 VDC relays. If you put a diode in series with the coil it will work on AC just fine. (THis is how they make AC relays, they hide a diode in the coil.)
Anotherr problem with viltage dividers is if one resistor opens or you have a bad connection, not only will the circuit not work, but you will put the full 24 VAC on the input pin of the stamp and damage it. (Been there, done that, and got the T-Shirt).
I use voltage dividers for sensing 100-500 VDC in my products, and it takes lots of filtering and attention to components and construction to avoid problems.
Good Luck,
Alan Bradford
Plasma Technoloogies
·
Harry