Author Topic: Measuring voltage from flyback  (Read 533 times)

Offline abstruse1

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Measuring voltage from flyback
« on: April 11, 2022, 08:26:27 PM »
I need to measure breakdown voltage of various liquids, so I need to be able to get a stable, repeatable voltage measurement for the output of my flyback (home made).  I've been using a 1,000:1 probe and Fluke multimeter, but the results are disappointing.

The displayed voltage jumps around by 20% or so, and often the meter shows the OL reading.  When I do get what may be accurate readings, they show as much as 50kV, so I shouldn't be overdriving the meter.

I'm measuring on the AC scale.  Using the DC scale gives no reading.

What am I doing wrong?

Also, I can't plug the probe directly into an analog AC meter.  When I do, the meter reads nothing.  It'd be very convenient to be able to do this.  Why doesn't that work?

Thanx, guys.
Abstruse1
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Offline Mads Barnkob

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Re: Measuring voltage from flyback
« Reply #1 on: April 11, 2022, 09:15:40 PM »
You can only use a multimeter with 1:1000 divider on a stable HV DC output. The multimeter ADC frontend for AC does not work well, if at all, beyond some 400 Hz.

When trying to connect a analog meter, be sure to check "how it works", does it have an internal resistor or does it need a external one etc.

You should post some pictures of your setup or a schematic with markings of probe placements, to make it easier for us to help you. I literally got no idea how you are trying to do what you describe, but I ASSUME you do it correct :)
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Offline klugesmith

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Re: Measuring voltage from flyback
« Reply #2 on: April 12, 2022, 01:22:00 AM »
As Mads said, the HV probe is meant for DC.  Can you rectify your flyback output, and charge a HV capacitor to have a DC potential?  Cap could be a ceramic doorknob type, or Leyden jar.  Or, of course, a television CRT.   

Unrectfied flyback secondary voltage always has an average value of zero. (Average _current_ is nonzero in the nominal application.)
When attenuating probe is connected to analog meter in AC mode, as Mads said you need to know what's inside the analog multimeter.
One clue is the meter indication in AC Volts mode, if you apply positive or negative DC.

For your application, dielectric breakdown test of materials, DC or line frequency AC are more customary than high frequency spikes from a flyback converter.   
Ever think about what determines the peak voltage of a flyback-converter secondary that's not connected to anything?
« Last Edit: April 12, 2022, 01:29:49 AM by klugesmith »

Offline abstruse1

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Re: Measuring voltage from flyback
« Reply #3 on: April 12, 2022, 02:11:11 AM »
Very helpful replies, guys. Thanx.
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Offline MRMILSTAR

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Re: Measuring voltage from flyback
« Reply #4 on: April 12, 2022, 05:19:44 AM »
I needed to measure the peak output voltage of the AC flybacks that power my voltage multiplier at a charging frequency of 28 Khz. I constructed a special HV full-wave bridge rectifier using fast diodes (100 ns recovery). I added a HV filter capacitor to smooth the ripples of the output waveform. I then use a commercial HV DC probe with integrated analog 40 KV voltmeter to measure the output voltage of the rectiifier. The measurement then is the peak voltage of the AC waveform coming from the flybacks. The measurement is performed with voltage multiplier in operation so as to provide a proper load. I have attached a picture of the apparatus.
« Last Edit: April 12, 2022, 05:24:36 AM by MRMILSTAR »
Steve White
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Offline erlendse

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Re: Measuring voltage from flyback
« Reply #5 on: June 09, 2022, 07:00:43 PM »
A alternartive, possibly less accurate:

Have a sense turn around the core connected to a peak hold circuit. The winding ratio is known, so it should be possible to log highest seen voltage before breakdown.

Peak hold can be done with a diode and capacitor if there is no desire for a opamp circuit, and the sense winding can have more turns if more voltage is desired (like make the diode drop less significant).

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Re: Measuring voltage from flyback
« Reply #5 on: June 09, 2022, 07:00:43 PM »

 


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