Connect with us

120V accurate current source - how?

Discussion in 'Electronic Basics' started by Shawn, Sep 6, 2005.

Scroll to continue with content
  1. Shawn

    Shawn Guest

    I'm seeking opinions on the best way to generate an accurate current
    at 120V AC. I need to calibrate a power meter design, and it calls
    for the ability to accurately generate specific currents between 1A
    and 5A. I've considered the use of power resistors, but 5A would
    require a 600W resistor, which, while not impossible to find, is a bit
    drastic. I also considered six 100W bulbs, but since it's a power
    meter I need a degree of accuracy greater than the tolerance of a
    light bulb.

    Any suggestions on other possibilities I might explore, or old
    equipment that I might be able to pick up from eBay?

    Thanks all!
  2. Dan Hollands

    Dan Hollands Guest

    Power meters generally have 2 inputs, one to measure voltage and one to
    measure current.
    Calibrating them is usually done with a variable 120V voltage into the high
    impedance voltage input and a variable current input at a low AC voltage
    into the low impedance - close to zero - current input. It is not necessary
    to have the current input come from a 120 volt source. Commericial current
    sources suitable for calibration are available and are expensive. Ideally
    the phase angle between the voltage and current sources is also adjustable
    to test the effect of inductive or capacitive loads.

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  3. Chris

    Chris Guest

    Hi, Shawn. If you need 5A at 120V, you're going to have to dissipate
    600 watts. No way around it.

    Current calibrators are drastically expensive -- that shouldn't be
    considered unless you have a really hefty budget for this job.

    Light bulbs are a no-go here. At rated voltage, their wattage can vary
    by more than 20%. Also, their resistance is non-linear -- as the
    applied voltage decreases, the temperature of the filament decreases
    and the resistance decreases.

    You might as well use power resistors. This is what they're made to
    do. And they needn't be expensive, either. Nebraska Sales and Surplus
    has 120 ohm, 120 watt adjustable power resistors available in quantity
    for $9.00 USD each.

    String 5 of these in parallel, give each one it's own series junkbox
    switch, and voilla! You have a 1-2-3-4-5 amp 120VAC load. You won't
    find an AC current calibrator at ebay for $45 bucks. I would guess
    shipping alone would probably be more. ;-)

    If you want more accuracy (line voltage typically varies a bit, and
    power wirewound resistors are +/- 10% unless otherwise specified), you
    can still use these resistors. Just borrow a big Variac, and get a
    resistance meter that can measure accurately. Adjust each resistor to
    115 ohms +/- 1%, and then apply a measured 115VAC through the Variac.
    This will give you what you need, with good accuracy.

    Before you start, though, find out if you can do calibration with only
    resistive loads.

    Good luck
  4. john jardine

    john jardine Guest

    I needed to do something similar without spending any money. Used the
    workshop Variac (every workshop should have a Variac) and 2kW electric
    fanheater (every workshop should ... ).
    Monitored the current with a DVM.
  5. mike

    mike Guest

    I hate to rain on your parade, but that's what I do best.

    1:What's your definition of "accurate"???? Just how accurate?

    2:What's your definition of "power"??? If you expect to measure RMS
    power but calibrate it using some kind of (non-true RMS) voltage
    measurement and a resistor...see #1.

    3: If you expect to get a pure sine wave out of the wall plug...see #1.

    4: Do you expect your measurement bo be accurate with reactive
    loads...over what PF range? see #1.

    5: How about non-sinusoidal currents? Range of peak to average ratio?
    See #1.

    The theory is trivial. Acutally (accurately) measuring power over
    a range of non-ideal conditions is not so easy.


    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted 12" LCD for Compaq Armada 7770MT.
    Bunch of stuff For Sale and Wanted at the link below.
  6. ehsjr

    ehsjr Guest

    Incorporate a 10:1 shunt in your power meter design.
    Calibrate the circuit at at 12 volts and 1/2 watts.
    For 120 volt measurements, use the shunt.

  7. Bob Masta

    Bob Masta Guest

    Ecellent idea. If the OP doesn't have a DVM for the rated
    current, he can always use a known small resistance
    shunt and measure the drop to determine current.

    Also, note that even without the variac there are
    devious ways to handle high power in lower-power
    resistors. First off, you can apply the power just
    for the second it takes to get a reading, then give
    them a long cool-down. Better yet, immerse the
    resistors in a paint can full of oil to dissipate the
    heat. This trick was used back in the '60s by
    hams needing dummy loads for transmitters.
    There are of course limits to just how far you can
    go with this... don't use a 1/4 watt resistor and
    expect it to work. But if you can get something
    in the 10s of watts, I think you will be fine.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day