Counting AC frequency with a BS2
Here we go, I'm looking to get the AC frequency off one of the legs of a 3 phase wind turbine alternator. The goal is to get the RPM of the wind turbine.
I have seen solutions using the LM2917 that end up returning a voltage that I could then pass to an ADC.
http://www.scoraigwind.com/circuits/index.htm
Are there simpler ways to do this using the COUNT/PULSEIN functions
Any Circuit diagrams greatly appreciated
Parameters
Voltage AC on a leg 0 to 25 Volts, current 0 to 60 amps (this is a 12 volt system) Ac is rectifed and then fed to·12 volt batteries. Approximate frequency 0 to 100 Hz
·
I have seen solutions using the LM2917 that end up returning a voltage that I could then pass to an ADC.
http://www.scoraigwind.com/circuits/index.htm
Are there simpler ways to do this using the COUNT/PULSEIN functions
Any Circuit diagrams greatly appreciated
Parameters
Voltage AC on a leg 0 to 25 Volts, current 0 to 60 amps (this is a 12 volt system) Ac is rectifed and then fed to·12 volt batteries. Approximate frequency 0 to 100 Hz
·
Comments
Just a resistor and a zener diode is all you should need.
And COUNT will get you the frequency.
·
Bean.
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There is a fine line between arrogance and confidence. Make sure you don't cross it...
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· Or you could you a regular diode to the BS2 Vdd (+5V) pin.
Bean.
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There is a fine line between arrogance and confidence. Make sure you don't cross it...
Post Edited (Bean (Hitt Consulting)) : 3/3/2009 11:36:05 PM GMT
And you might want to start with a larger resistor value, maybe 100K or 47K and see if that works okay. Use the largest value that works. The 22K will limit the current to 1mA when the voltage is 22V.
Also PULSIN will give you better resolution with less time, but you'll need to measure the LOW time and HIGH time with seperate PULSIN commands and add the result together to get a period measurement.
Bean.
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There is a fine line between arrogance and confidence. Make sure you don't cross it...
Post Edited (Bean (Hitt Consulting)) : 3/3/2009 11:42:24 PM GMT
The input resistor limits the zener current and should be selected to match the zener's specs. The zener and the second resistor limit the LED forward current, and the zener limits the LED reverse voltage.
-Phil
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· -- Carl, nn5i@arrl.net
If it was 120V I'd definately use an opto. But for 25VAC max... I would chance it.
Bean.
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There is a fine line between arrogance and confidence. Make sure you don't cross it...
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On the other hand, I also like the direct measuremnt approach as opposed to the proxied measurement through an opto.
-Phil
-Phil
You said... "I'm looking to get the AC frequency off one of the legs of a 3 phase wind turbine alternator." ... could you use two legs?
If so, then this circuit with an ordinary 741 Op-Amp would provide a 5V square wave output that would be proportional to the turbine speed.· Then you could read the 5V square wave·directly into a Stamp using the COUNT or PULSIN command.
Note: ANY two phases will work.
Edit: The Diodes from the Op-Amp inputs to +5V are optional, the built-in ESD diodes should function here, but for added·robustness you can leave them in the circuit.· With 50V output from the·wind·turbine, these diodes would only see about 450 micro Amps.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 3/4/2009 4:27:45 AM GMT
My father was a career Chief Radioman in the U S Navy.· Once, when he was stationed on USS Saint Paul, a heavy cruiser (temporarily home from the bombardment line in Korea), he took me on a tour.· I was 12 at the time.· He showed me a pair of footprints in a steel deck.· "What's that, Dad?"· He explained that an electrician was drilling through the bulkhead (the hole was still there, but welded up)·and drilled into a large copper bus bar that was carrying 24 volts DC, and that his steel-shank safety shoes had welded themselves to the deck, which was very wet with seawater at the time.· He was dead, a Crispy Critter.
I've remembered this always.· It's not voltage that kills, it's current.· Isolation is good.
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· -- Carl, nn5i@arrl.net
I'm quite partial to the H11N1 optocoupler. (It uses a powered detector, so it's schematic is a little different from the one in the circuit above.) It has a built-in Schmitt trigger and will respond to an LED current as little as 3.2mA, or possibly less.
-Phil
You said ... "What type an size for the two diodes?" - Since the current is so low, just about any germanium diode will work. I'd probably use something like a 1N34A.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.