Actually, most meters average the waveform, then apply a rote formula
converting that average to an rms reading which is more likely to err on the
side of too low a reading.
For a square wave it will read too high by 6% (actually worked this out, but
forgot the actual number, but you have the idea) Then assume that the
meter is reading a pulse with twice the amplitude and half the width. The
averaging meter will show the same value, but a true rms will show the
correct higher value. Then go to eight times higher amplitude with 1/8
pulse width - averaging meter will still read the same, but the true rms
will show the correct even higher value.
So you see, an averaging meter which has its output slightly adjusted higher
to compensate for the difference between a sine wave average and its rms
value will usually err on the low side.
However, if you check the discrepancy between different shapes one normally
encounters, like square, sine, triangle; the error is not that big. The
error only really gets big for "spike" waveforms.
- Robert -
-----Original Message-----
From: Rich Nute <ri...@sdd.hp.com>
To: matsu...@curtisinst.com <matsu...@curtisinst.com>
Cc: emc-p...@majordomo.ieee.org <emc-p...@majordomo.ieee.org>
List-Post: emc-pstc@listserv.ieee.org
Date: Tuesday, October 03, 2000 3:04 PM
Subject: Re: Leakage Current Measurements
>
>
>
>
>Hi Ken:
>
>
>> I found something interesting today. While measuring leakage current
with a
>> digital multimeter, I noticed a high amount of leakage. The unit
>> incorporates a switching power supply, with some EMI/EMC circuitry.
Anyhow,
>> I read somewhere that DMMs are some times inaccurate with high
frequency
>> line conducted emissions. Took a old Simpson analog meter (took
forever to
>> figure it out, since I've never touched a analog meter hehehe) and
found
>> measurements that were rather low. The questions is, I'm wondering if
UL or
>> for that matter, any other testing agency actually know about this
fact, or
>> merely fail their clients with incorrect data.
>
>*
>The following is Copyrighted 2000 Hewlett-Packard
>Company.
>*
>
>When measuring leakage current, and, if the current
>waveform is non-sinusoidal, then the measurement
>MUST be made with a true rms meter (whether analog
>or digital). (The Simpson leakage current meters
>are true rms.)
>
>If a voltmeter is not true rms, it measures the peak
>of the waveform, and then reads 0.707 of the number.
>So, for any non-sinusoidal waveform, the voltmeter
>reading will generally be higher than the rms value,
>and will not represent the rms value of the current.
>
>UL and most other testing houses do understand this
>requirement, and ALWAYS use true rms meters.
>
>For switching-mode power supplies, the leakage
>current waveform can be sinusoidal or it can be non-
>sinusoidal. The waveshape is dependent on how the
>EMC filtering is done. The non-sinusoidal portion
>of the waveform is the leakage from the primary
>circuits ON THE LOAD SIDE of the rectifier. And,
>conversely, the sinusoidal portion of the waveform
>is leakage from the SUPPLY SIDE of the rectifier.
>
>I believe that the non-sinusoidal leakage current
>waveform does not exceed the bandwidth of most
>voltmeters. The frequencies contained in the non-
>linear leakage current waveform do not have
>significant current magnitudes above the 40th
>harmonic. If we consider all frequencies up to the
>40th harmonic of 60 Hz, we have 2400 Hz. Most ac
>voltmeters have a bandwidth of at least 10 kHz,
>well above 2400 Hz.
>
>The body network is likewise not particularly
>significant to the leakage current measurement.
>This is because leakage current is derived from a
>source that approaches a current source. For the
>sake of this discussion, a current source is a
>source that provides a constant current regardless
>of load.
>
>Consider that the source resistance for 0.5 mA
>leakage current is 120 volts divided by 0.5 mA, or
>240 kohms. Adding 1.5 kohms (the body impedance
>model) to 240 kohms gives 0.4969 mA. So, any
>reading without the body impedance network is a
>very, very slightly pessimistic reading.
>
>Likewise, the 1.5 kohm resistance and 0.15 uF
>capacitance have a pole at about 1 kHz. So, the
>capacitor has no effect for frequencies below 1 kHz.
>
>So, if the non-linear current waveform has lots of
>harmonics, then the network will give a lower
>reading for those frequencies above 1 kHz. Again,
>any reading without the body impedance network is a
>very, very slightly pessimistic reading.
>
>My advice: re-equip your lab with only true rms
>meters.
>
>
>Best regards,
>Rich
>
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