How do you measure pulsed amps?

Discussion in 'General Electronics Chat' started by H2OWiz, Apr 22, 2008.

  1. H2OWiz

    Thread Starter New Member

    Dec 12, 2007
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    My power supply is evolving and now I need to figure out how to measure the power it is delivering. The supply puts out about 10 amps at 12 volts but it is pulsed by a 555 timer chip. I have an amp meter on the front side of the power supply but I think this is measuring average current. I have a scope and can measure the voltage and pulse width so this is easy, but how do I calculate the amps so I can measure total power delivered?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Is the duty cycle of the pulses the 555 outputs 50% ?

    But what you drive with a 555 and draws 10 amps. what other components the circuit is made of?
     
  3. Caveman

    Active Member

    Apr 15, 2008
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    If no power is being delivered to the load between pulses, you just need to measure your peak current. If you know your load resistance, you can use that. Or you could use a small resistance in series with the load and measure its voltage, then calculate its current using ohms law. Then the pulsed current * the voltage = the power. However, this power is only during the on periods, not the average power delivered.
    For that you need to multiply by the duty cycle that you are on.

    For example, if you find that you are putting out 15Amps at 12V with a 50% duty cycle, your average power is 15A*12V*0.5 = 90W.
     
  4. H2OWiz

    Thread Starter New Member

    Dec 12, 2007
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    I was never able to measure the load resistance. The funny thing with water is if you measure the resistance of a large volume of water in an electrolyzer, the water charges with the power from the ohm meter and the resistance climbs slowly. Also the next time you measure the resistance has changed. Load resistance unknown.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    This is really good and useful advice applicable in many situations.

    The resistance takes the full 10 amps so needs to be quite small, say 0.01 ohms so the power is only 1 watt and the voltage drop 120 millivolts. You can readily obtain such current shunts or make one with 10 parallel 0.1 ohm resistors.
     
  6. Caveman

    Active Member

    Apr 15, 2008
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    That is because water is basically non-conductive. It's the stuff in it that makes it conduct. I've done a conductivity sensor (professionally) for water purity measurements before. You drive it with AC and measure the current before all of the ions migrate to the electrode. Touchy little bugger though.
     
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