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Measuring voltage and current of rough DC

  1. Oct 31, 2012 #1
    Hey, I am working on a science fair project and I have an electrolytic cell that will produce hydrogen. My goal is to measure the energy input and output to calculate the efficiency of the cell. Right now I am trying to find the best way to calculate the input energy. I am in the process of building my own power supply for the cell that I have already built and need an accurate way to measure voltage and current. Right now I have an old microwave oven transformer that I rewound for about 40 volts that gets rectified. Now I need to know what is the best way for me to measure the power and eventually energy that I am putting into the cell. I think that I can smooth the rectified signal and measure it with normal moving coil meters but I am not sure if that will throw off my calculations. Ive been looking into peak detectors and other circuits that may be useful to detect voltage and current if I leave the signal rough (The electrolytic cell doesn't need smooth dc). Any help would be much appreciated. Oh and the if you are curious to measure energy input and output i will measure the average power over a certain amount of time as well as the amount of gas produced in the same time span. I believe this is the correct way to measure it.
     
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  3. Oct 31, 2012 #2

    The Electrician

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    Is this a high school science project? What instruments do you have available, such as voltmeters, ammeters, oscilloscopes? Do you have meters that can measure true RMS values? Will you be using instruments that your school provides?

    What will be the approximate voltage applied to your cell, and the current passing through it?

    In another thread of yours from a year or so ago, you mentioned using a router controller. Will you still be using that?

    To give you useful advice, we need to know details about what you are doing, what instruments you have, if you have a budget to buy things you don't already have, and how much you can spend, etc.
     
  4. Nov 1, 2012 #3
    Hey, Thanks for responding. This is a high school science fair project but our school doesn't have much electronics equipment. I have a basic multimeter and plan on buying a used oscilloscope and lab supply soon. So I don't have a meter that will calculate true RMS but if I need to I can probably find some money to buy one. (My price range stops around $300 USD)

    The cell will have approximately 40 volts applied to it and will pull between 10 and 20 amps depending on how much electrolyte is in it. But from previous experimentation I know that it can vary slightly as it runs.

    I need be able to calculate not only the voltage but the current too in order to find the power I input over a period of time to find energy.

    Thanks for your help
     
  5. Nov 1, 2012 #4

    The Electrician

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    You've got a couple of ways to go. If you don't smooth the output of your rectifier, the current from the rectifier will be relatively narrow pulses of current. When the current is not pure DC, but rather pulses, you can't measure power by just measuring the voltage and current separately and multiplying their values; you would need a wattmeter. See these references to help you understand:

    http://www.brultech.com/home/files/Difference Between Power and VA.pdf

    http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

    http://en.wikipedia.org/wiki/Wattmeter

    Wattmeters are usually rather expensive, although sometimes you can find one for a bargain price on eBay, such as this one:

    http://www.ebay.com/itm/Meter-wattm...846?pt=LH_DefaultDomain_0&hash=item232164ddb6

    A problem can be that the voltage and current ranges of a particular wattmeter may not match your requirements.

    If, however, you smooth the output of your rectifier with a big electrolytic capacitor, then you can measure the voltage and current separately and multiply their values. For this to give accurate results, the voltage and current must be (nearly) pure DC.

    You didn't give much detail on your multimeter--whether it is digital or analog, its accuracy level, etc. Most multimeters can't measure currents over 10 or so amps continuously. You could get a higher current ammeter, but you would probably be better off to get a shunt and use your multimeter to measure the shunt voltage. Here are a couple of reasonably priced shunts:

    http://www.ebay.com/itm/50A-50MV-A-...590?pt=LH_DefaultDomain_0&hash=item1c05453b86

    http://www.ebay.com/itm/50A-50MV-50...564?pt=LH_DefaultDomain_0&hash=item588685d7f4

    To use them, your multimeter needs to measure millivolts. Is your multimeter a digital meter with a millivolt scale?
     
  6. Nov 1, 2012 #5
    OK, well my I have a digital meter that can measure millivolts up to 0.1 millivolt. So I guess that if I were to use the shunt that I would have the ability to measure up to .1 amp. If I want more accuracy and range I am willing to spend some money on a quality shunt and a good accurate meter. I don't know if you have any recommendations.

    I think that I will smooth the rectified DC and then use the shunt to measure current and then measure the voltage drop of the cell.

    I did recently see a formula that could calculate the value of the smoothing capacitor I believe it is C=(I×T)/V

    So I believe that I = 20 amps T=.008333 seconds (1/120Hz) V= 4 volt ripple


    C= (20×.008333)/4 C=.041666 Farads ≈ 42000μF
     
  7. Nov 1, 2012 #6

    The Electrician

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    If you get a 50 millivolt, 50 amp shunt, then as explained on the eBay page, your meter will read the millivolts across the shunt and that will be the amp reading. That's the most convenient way to measure high currents.

    You should be able to find some suitable electrolytic caps on surplus sites; for example:

    http://www.allelectronics.com/make-a-store/item/EC-335/33-000UF-50V-COMPUTER-GRADE-CAPACITOR/1.html

    Don't forget that if the output of your transformer is 40 volts AC, after you rectify it, the filter caps will charge up to nearly the peak voltage of the transformer output, which is 1.414 time the RMS AC voltage. If you can adjust the transformer output, the 50 volt capacitors from the link above will be ok; just check the DC voltage on the capacitors to be sure it doesn't go over 50 volts. You might also be able to find some 63 volt rated capacitors.
     
  8. Nov 2, 2012 #7
    Ideally you would want to measure the Energy - not just the power and multiply over time - easy to do on the AC side. You can buy Watt-Hour meters - both for general home use ( ~$25) or even a surplus / used KiloWatt Hour (KWH) Meter like is on your house for about the same - these would be revenue grade - and will probably still work at 40V on the AC side( but the scaling may be too low to register - until your experiment runs for a considerable amount of time. Unfortunatly there are not too many ready made DC Wattmeters or watt hour meters - if you can find a 2 channel ""pocket" oscilloscope - you can use the multiply function to get a power reading

    Also - if you wish to include the efficiency of the total system - not just the cell you can measure at the high voltage side, but please be carefull.
     
  9. Nov 2, 2012 #8

    The Electrician

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    I think you are mistaken when you say "Unfortunatly there are not too many ready made DC Wattmeters". All mechanical (electrodynamic) wattmeters (such as the one I linked to on eBay) measure DC as well as AC power, and many of the electronic ones do as well.

    Watt-hour meters are another matter. The typical mechanical KWH meter on the outside of your house won't respond to pure DC, but electronic ones might. The lowly Kill-A-Watt responds to a DC current in the load (I didn't know this to be true until a few minutes ago when I tested mine), but it requires a voltage higher than 40 volts to operate.

    What would be perfect for the OP is this meter:

    http://www.gossenmetrawatt.com/english/produkte/metrahitenergy.htm

    It can measure power, and can also accumulate power to give watt-hours, and it responds to DC as well as AC. But, at around $1000, it's out of his budget range.

    If the OP filters the output of his rectifier, then with essentially pure DC to his cell he doesn't need an oscilloscope's multiply function to get power; he can just measure DC voltage and current separately and multiply with a calculator.

    I had considered as an option that the OP might use a low cost watt-hour meter, the Kill-A-Watt, in the primary of his transformer and measure the total system efficiency. The ability of the Kill-A-Watt to accumulate watt-hours would be handy, and this is still an option for him.

    I am impressed by the OP's level of knowledge. He plainly understands that without a watt-hour meter to accumulate power over time, he will have to measure power at intervals of time and do the accumulation himself.
     
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