DVM can't measure high frequency AC?

Discussion in 'General Electronics Chat' started by Billy Mayo, Apr 9, 2013.

  1. Billy Mayo

    Thread Starter New Member

    Mar 24, 2013
    The power supply is a PWM power supply or a high frequency pulse power supply

    The power supply uses a UC3844N chip to Pulse a high frequency AC signals to an AC to AC transformer.

    UC3844N datasheet:

    I'm using a Fluke 87 meter, I was trying to measure the AC voltages on the transformers primary and secondary windings

    There was no AC voltage on the secondary windings

    My Manager said it's because the Fluke DVM meter can't measure High Frequency AC , only very low AC frequency's to measure an AC voltage

    The Fluke DVM meter only measure AC voltages at low frequency

    Is this true and what kind of DVM meter measures High frequency AC voltages?
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    Why don't you go look up the specifications for various DVMs and see what their frequency limit is?

    My Fluke 189 can measure up to 100 kHz in spec. Keep in mind that the upper frequency limit will be for sine waves. Other waveforms, such as you might find in a switcher will have harmonics at frequencies above the fundamental of the waveform and a particular DVM may not have specified response at the frequencies of the harmonics, leading to erroneous measurement of a non-sinusoidal waveform.

    If you don't require high accuracy, you can usually make an adequate measurement with a modern digital oscilloscope.
    #12 likes this.
  3. vk6zgo

    Active Member

    Jul 21, 2012
    Or an ancient analog oscilloscope!:D
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    Unless you have one of those fancy DVMs with true RMS measurements capability. I will think your DVM will only be good for good sine wave measurements up to say around 250Hz give and take. http://en.wikipedia.org/wiki/True_RMS_converter
  5. Billy Mayo

    Thread Starter New Member

    Mar 24, 2013
    I was using the DVM meter to measure the AC ripple of a switching powers supply? and also the High frequency pulse of a switching power supply

    The switching power supplies ripple frequency is from 10K to 50K?

    A Switching Power supply adjusts the Pulse width to do output voltage regulation? how does this work?

    I'm not sure how the Pulse Width works to adjust the output voltage regulation in a switch power supply
  6. Billy Mayo

    Thread Starter New Member

    Mar 24, 2013
    To get the turn ratio of a power supply

    The formula is the transformers secondary voltage and divided by the primary's voltage = turn ratio

    Secondary voltage is 25 volts
    Primary voltage is 150 volts

    25 divided by 150 = 6

    Turn ratio is = 1 to 6?
  7. Billy Mayo

    Thread Starter New Member

    Mar 24, 2013
    A switching power supply has a stage where is converts DC into an High frequency pulse

    Is this Stage/section that does this called an inverter or switcher?

    How does it Convert DC into a high frequency Pulse?

    The high frequency pulse is at what frequency range mostly? and does it have a Ripple frequency also riding on top of the pulse?