# Measuring "high" voltage

#### Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

•  Subject
• Author
• Posted on

Any idea on how I can measure 1200 Vrms accurately? The multiimeters I
have here at the office seem to be giving wrong results!

-Henk

Re: Measuring "high" voltage

**  That all depends on:

1.  The frequency.

2.  The waveform.

3.  The source impedance.

Care to elaborate ??

......   Phil

Re: Measuring "high" voltage

Phil Allison wrote...

I could tell him how to accurately measure 10kV at 600kHz,
that's high voltage.  But he's probably not interested...

--
Thanks,
- Win

Re: Measuring "high" voltage

Phil,

Yes - the groper would love to elaborate the best that he can!! :)

I'm an using the CXA-M10A-L inverter from TDK to power the backlight of
my CSTN panel. The open voltage should be 1200 Vrms. Below is the short
datasheet:

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

1.  The frequency   = ~28 kHz

2.  The waveform    =  sine wave?

3.  The source impedance = The only resistance/impedance I see in the
datasheet is for the load for their testing.

Thank you for yor time and any information!

-Henk

Phil Allison wrote:

Re: Measuring "high" voltage

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

An "unloaded" CFL inverter puts out a sine wave where the rest of the
input square wave power gets lost in the core.
But a CFL is a non-linear load; crudely speaking a voltage regulating
arc - so i doubt the voltage waveform is a sinewave.
Most handhelds measure peak and read RMS of an assumed sine wave; one
potential cause of error.
Then there is a frequency response, which should be translated to
accuracy (or error) VS frequency.
So FFT the waveform and "overlay" an error correction curve for that
part.
Then inverse FFT back to a "corrected" waveform and mathematically
apply a peak detector, and then "convert" to an RMS "reading".
Ain't going to be much better than 5% methinks..

Re: Measuring "high" voltage

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

Looking at the specs, this device is rated at 6 W, 400 VRMS, and the 1200
volts open circuit is measured directly at the transformer secondary (see
note 1-6). They specify a 1000:1 probe, and a true-RMS meter. If you are
measuring this directly with an ordinary DMM, you are very likely exceeding
its 1000 VDC and 600 VAC input limitations, and unless it is a high quality
true RMS meter rated up to 30 kHz, you will get errors because the waveform
is almost certainly not sinusoidal. The high voltage is required for
initial "strike" voltage of the CCFL. If you use a high voltage probe or a
resistive divider, you must make sure it is properly compensated, because
on AC, there is a large capacitive component that can cause errors as well.
You could add a high voltage diode and capacitor, and measure the peak DC
output with a simple uncompensated divider.

Paul

Re: Measuring "high" voltage

Paul!  Best post I have ever seen!  Thank you! :)

What type of true RMS meter would you recommend for purchase?

Thanks!

-Henk

Paul E. Schoen wrote:

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

Re: Measuring "high" voltage

On 30 Jul 2006 19:49:46 -0700, " snipped-for-privacy@hotmail.com"

One that has a max input of 1500 V AC.

Stop top posting, or be known as a Usenet retard.

Re: Measuring "high" voltage

I recommend a Fluke 45 (which I bought about 15 years ago as my lab
standard). It has AC+DC True RMS to 100 kHz.
http://meter-man.com.au/pdf/FLK_45.PDF

It would require a voltage divider. I found one that looks interesting:
http://www.emcohighvoltage.com/V1Gseries.PDF

Re: Measuring "high" voltage

On Mon, 31 Jul 2006 02:27:04 -0400, "Paul E. Schoen"

Nice little device.  Probably hard potted in "Stycast". Those little
shells are cool high dielectric strength suckers too. Nice design. The
probe lead negates the need for a long probe tip.  Needs to be
attached before energization though.

One MUST make sure that pin one is tied to ground (read: HV supply
return or "low" side in the case of a positive supply) as well as the
will then be at LV potential.  Pin 3 should also be already hooked up
to the HV out of your circuit device under test (DUT).

Remeber also that a negative output supply will have its positive,
"high side" lead as its "grounded" side.  Make sure you know which you
are testing and get your returns hooked up, or you can blow the meter
if it "floats high" (or "low" for that matter).

Re: Measuring "high" voltage

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

** A ordinary DMM  is not suitable at all - 28 kHz is way outside the AC
range's capability.

What you need is a bench scope fitted with a 100:1 divider probe rated at
1500 volts or more.

If the wave you see is close to square shaped ( which is as I expect)  the *
rms * voltage is  1/2  the peak to peak value as seen on the scope
multiplied by 100.

.......   Phil

Re: Measuring "high" voltage

snipped-for-privacy@hotmail.com wrote...

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

This is proper territory for a scope-probe waveform examination.

1500V Tektronix 100:1 probes are available from the factory, or
are often seen on eBay, but you can easily make a home-made probe
by ignoring the dc resistive-divider aspect, and making an ac-only
probe with a capacitive divider.  For example, if a scope's 1M
input impedance is paralleled with 200pF of capacitance, the ac
"probe" will have a high-pass response down to ~800Hz.  If the
input capacitance is 2pF, then the division ratio will be 100:1

Instead of struggling to achieve exactly 2.02pF, or some other
capacitance value, a twisted-wire "gimmick" can be used to make

.         100:1 ac probe with response down to 800Hz.
.  2pF    ______ _
. --||---)______|_|--scope
.           \        1M 25pF
.            \
.             '--- cable, 175pF = 6 feet 50-ohm coax

--
Thanks,
- Win

Re: Measuring "high" voltage

On 30 Jul 2006 19:54:04 -0700, Winfield Hill

HV coax has a much higher capacitance per foot, and will allow you to
safely arrive at the desired capacitance value using much less cable
length. One must be careful to properly terminate the tail with HV
heat shrink.

Re: Measuring "high" voltage

Phat Bytestard wrote...

What are some HV coax types (p/n) you like to use?

BTW, it should be pointed out that the coax in the home-made
probe above does not see HV, only the 2pF capacitor does.  It
should also be pointed out that stray capacitance conducting
any signal current to the coax center wire, besides the 2pF,
will reduce the desired 100:1 division ratio.  This means
the 2pF and its LV output wire must be shielded.

The issues and techniques involving this shield and the method
of creating the 2pF and the heat shrink to hold it all together
and the nice but safe finger-hold and the sharp probe tip and
the various schemes to adjust or calibrate the probe (take a
breath) presents considerable opportunities to a clever mind.

One more comment.  That 2pF had better never break down.

--
Thanks,
- Win

Re: Measuring "high" voltage

[...]

At the cost of some more capacitance, you can add a Sidac or other over
voltage clamp.  Lets say that the scope's input is rated at 500V.  You can
use a clamp rated for, lets say half that and then make sure you are
making your measurment with an input much less than that.

--
--
snipped-for-privacy@rahul.net   forging knowledge

Re: Measuring "high" voltage

On Mon, 31 Jul 2006 13:55:50 +0000 (UTC), snipped-for-privacy@green.rahul.net
(Ken Smith) Gave us:

We made "ripple checkers" that had 90V neon lamps as perfect little
clamps in them.  Want more? Series two together. Much better than a
spark gap as they are "soft" clampers.

So any high transients that get through to the input get shunted in
the ripple checker, not your scope probe or scope.

Re: Measuring "high" voltage

can

Yeah. Now try a 1kV 1ns rise time step across your neon and look.

--
Thanks,
Fred.

Re: Measuring "high" voltage

On 31 Jul 2006 05:57:08 -0700, Winfield Hill

We used a military rooted, PTFE dielectric (IIRC), teflon jacketed
SPC multistranded core with a full, tightly bound SPC shield.  It was
a little over an 8th inch in outside diameter. I think it was 30kV
form factor, but we used it for ultra low noise supplies.  You be
amazed (or not) how much noise the supply injects into itself from the
multiplier array.  I developed arrays that were shielded all around,
and the output node was a big round blob of solder.  It helps when you
are trying to get a 4kV supply down to 4mV ripple.  :-] The whole
thing was in a can, but the multiplier array was itself shielded from
the feedback network and output lead and RC filtering.  It does make a
difference at those levels.  I could more than double the output
capacity by not incorporating the RC filter, and the supply would push
it, no problem.  We needed noiselessness, not brute power, however.

I can't recall exactly how many pF per foot we got from the coax,
But we used them as HV loads, and various other uses, like for
characterizing a feed forward needed in an HV feedback loop.  Most
were between 5 and 20pF across several hundred MegOhms or more of
feedback resistor. Some supplies worked well with a little feed
forward on the feedback... some needed none.

Yes...  noise is easy to inject in small signal electronics in an
unshielded environ.

Our stuff was DC, and the ripple was so small as to really be
insignificant.  We ran at frequencies anywhere from 17kHz to a couple
hundred on our HV switcher driven supplies. We also made supplies for
transmitters. The England facility made up to 400kV supplies. We still
shielded things.

We used the left overs from potting sessions placed in a dixie cup.
It makes great stand offs for HV.  One could have a one inch or a six
inch plug (drinking glass shaped) holding a circuit element... several
actually.  One can even cut V shaped grooves in it to help "hold" the
wire or object in question,

One should always have a coax rated several times greater than the
voltage being tested. Corona can poke holes right through teflon.
Good thing coax doesn't have air gaps.  Another reason why the HV
shrink is needed over the tail.

The coax you suggested has a 2kV breakdown strength on the core
insulator, IIRC.

Have to watch out for poke throughs on that stuff too.  More even
than my "Made for the purpose" coax.
It's like RG-174 on steroids.  :-]

Re: Measuring "high" voltage

"Winfield Hill"

http://power.tdk.com/dcac/brochure/pdf/CXA-M10A-L%20 (CTR-0742-A)%20PRODUCT%20DRAWING.pdf

** As I posted already, hours ago.

**  So are much cheaper ones that are just as good for occasional HV tests.

........   Phil

Re: Measuring "high" voltage

On Mon, 31 Jul 2006 15:23:46 +1000, "Phil Allison"

They also typically top out at 1000V.