Scope current measurement

Discussion in 'General Electronics Chat' started by Porkchop, Aug 12, 2007.

  1. Porkchop

    Thread Starter Member

    Feb 5, 2007
    This is probably plainly easy! But if someone could explain what im missing i would be greatful !

    I found a step down transformer 230/24 So thats 9.6:1 ratio.

    I wanted to measure the average current to a load via this. I'd use it as a current transformer.

    I put a 1Ohm resistor across the primary. And put the secondary winding in series with the load. I measured across the resistor.

    But now i'm unsure of what to do.

    I have the transformer ratio of 9.6:1 and the 1ohm resistor

    From the scope i can see the current waveform, i can see the Peak voltage. This is what i want to use i think?

    Iv used loads that i "know the current through" adn have checked with a meter but i just don't know how to arrive at it via the scope info, transformer ratio and resistor value.

    Also when i get another probe so i can use to channels :rolleyes:

    If i overlay the voltage waveform on top of the current waveform will this represent which is leading or lagging? If so how can i find by what angle?

    Cheers :)
  2. techroomt

    Senior Member

    May 19, 2004
    your set-up is a little confusing to me
    does that mean in parallel with the primary? realize that any reading on the primary side will include the transformers power consumption and losses in addition to the actual loads values.

    if the 1 ohm is a current shunt, it would be better suited in series with the secondary/load. average current (I avg) can be calculated by dividing the (1 ohm) resistor voltage drop (peak) by 1 ohm (unity i guess) x 0.637.

    i'm pretty sure you can dislplay any lead or lag without a second probe by setting your scope trigger source to "line".
  3. Porkchop

    Thread Starter Member

    Feb 5, 2007
    I had to put the 1ohm across the primary winding ( shorting out the start and end winds) because when passing voltage through the secondary it induced a voltage on the primary which caused massive internal arching and a nasty burning smell :eek:

    So then i thought about the resistor across to stop it.Then i noticed for a change in load current a different voltage appeared across the resistor.

    Then i spent ages trying to figure out how i could use this to calculate the current :confused:

    I realize the iron core transformer im trying to use to make measurements is not ideal.As you said it has losses of its own. Is there a better way? DIY onyl though please i can not afford a scope current probe! lol
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    Try putting your 1 Ohm recistor in series w/ your load, and omit the xfmr. Voltage measured across the 1 Ohm resistor will be the same number as current through it.
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    If it's low voltage, try .1 ohm.