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Old 25th April 2012, 07:52 PM   #1
akis is offline akis  United Kingdom
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Default Measuring voltage/current at high frequency

How can I measure RMS voltage and current at 200 KHz? My bench multimeter displays wrong results - most likely 200KHz is out of its range.

I currently use the oscilloscope, but it is awkward to read and use.
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Old 25th April 2012, 08:24 PM   #2
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A thermocouple ammeter is the old accepted method, but becoming scarce.
Nowadays use a current transformer and digital scope with averaging.

you could build a rms-dc converter, or simple diode+cap for dc meter, but no good for small signals.
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Old 25th April 2012, 08:48 PM   #3
Simon B is offline Simon B  United Kingdom
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What sorts of levels of currents & voltages are we talking about here? What's the application? Switching converter, class D Amp? Measurement types - single ended voltage, ie wrt ground, or differential? If you're wanting RMS value of a varying DC current, then a current transformer is no good, a hall effect probe is more like it. The more info you give, the easier it is to answer.....

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Old 25th April 2012, 09:59 PM   #4
akis is offline akis  United Kingdom
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It is around 200KHz sinusoidal, 50 V peak, 100mA.

I am thinking I can simply rectify it and then measure the rectified voltage which should be very close to the peak voltage. If I use low forward voltage drop diodes then the difference will be in the order of 200mV. Am I right ?
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Old 25th April 2012, 11:25 PM   #5
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linear technologies LTC6968 == the late Jim Williams wrote an application note on it: http://cds.linear.com/docs/Applicati...ote/an106f.pdf I compared these to the HP3403C I own and they are very good indeed. You can buy the dev board from LTC and it will save you many hours of burning a pcb yourself.

Analog devices also has a number of RMS converter chips useable to a few hundred kHz, and a slew of logarithmic converters useable to several hundred MHz.

Stateside you can occasionally get a Hewlett Packard 3403C or Fluke 8620 calorimeter meters at attractive prices.
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Old 25th April 2012, 11:26 PM   #6
Simon B is offline Simon B  United Kingdom
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If you know it's sinusoidal, yes, rectify, measure, scale for RMS, bit higher than 200mV for schottky diodes I think. Feed the rectifier into a cap, say 1u, measure that, otherwise the 200kHz will still freak your meter.

Jim Wlliams is/was a fascinating writer, very addictive, beware!
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Last edited by Simon B; 25th April 2012 at 11:28 PM.
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Old 26th April 2012, 08:17 AM   #7
akis is offline akis  United Kingdom
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Many thanks. I think for the time being and since this is for specific measurements only, and as Jim Williams has written in his opening statement, a simple rectifier will do.

I tried using germanium diodes and then schottkies (BAT48) but at +/- 40 Volts they are behaving very badly, so I am using 1N4148. It is not perfect, at low voltages there is a small error (say 300mV) which then increases for higher voltages.

And it shows peak voltage not RMS. Is there an easy way to convert peak voltage to RMS and display on a meter ?
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Old 26th April 2012, 01:22 PM   #8
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by akis View Post
......I tried using germanium diodes and then schottkies (BAT48) but at +/- 40 Volts they are behaving very badly, so I am using 1N4148. It is not perfect, at low voltages there is a small error (say 300mV) which then increases for higher voltages.

And it shows peak voltage not RMS. Is there an easy way to convert peak voltage to RMS and display on a meter ?

With a 50v-peak signal, the diode will have to withstand 100v during negative half cycles, due to the +50v still being on the cap, hence your disappointment with the BAT48, only rated at 40v piv repetitive. Even 1N4148 is right on its limit here at 100v piv repetitive.

The diode across the cap isn't really doing anything.

Unless the error needs to be less, don't worry about it, or allow for it in your readings, it's quite predictable. As for RMS from peak, as it's a sinusoidal waveform, just divide by square root of 2, ie 1.41 etc. If you have to have it on the meter, you could build a voltage doubler, followed by a suitably scaled voltage divider of two resistors, (don't forget to allow for your meter resistance) but frankly, why bother, just use your calculator as you work, much easier.

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Old 26th April 2012, 03:12 PM   #9
macboy is offline macboy  Canada
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Quote:
As for RMS from peak, as it's a sinusoidal waveform, just divide by square root of 2, ie 1.41 etc. If you have to have it on the meter, you could build a voltage doubler, followed by a suitably scaled voltage divider
No need for a doubler, the RMS we want is lower than the peak we have, so just a simple divider will work.
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Old 26th April 2012, 03:18 PM   #10
akis is offline akis  United Kingdom
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Quote:
Originally Posted by macboy View Post
No need for a doubler, the RMS we want is lower than the peak we have, so just a simple divider will work.
any examples of dividers so as to take a look please?
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