# Measuring "high" voltage - Page 2

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Re: Measuring "high" voltage

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

Get a high voltage differential probe for your oscilloscope, that is
the best and safest way to measure it.
Probably cheaper than finding a DMM that can do 28KHz True RMS too.

Dave :)

Re: Measuring "high" voltage

snipped-for-privacy@hotmail.com wrote:

Just putt a massive resistance across the voltage, and use as a voltage
divider...  Maybe, it all depends on frequency, source impedence and
what you mean by accurate.

Re: Measuring "high" voltage

Thanks for the response.  I'm taking a stab at this here guys so please
correct me if I am wrong:

My multimeter does have an ~AC mode but I am not sure on how it is
calcuating the AC voltage. ( I need to dig up the manual hope I can
find it! )  If it is just taking the peak value instead of the rms
value it will always be wrong because the majority of the time the sine
wave is not at its peak value.  Does that sound about right? I want to
measure 1200 Vrms.

NuclearFirestorm - do you mind saying a few words on how the frequency,
source impedence or whatever else will effect my measurement?

Thanks.

Re: Measuring "high" voltage

Err..you have an incorrect idea as to what is happening.
It does not matter about "the majority of the time".
Peak value is peak value whether it is a pulse or triangle or square
or sine wave.
If the waveform is not ("close" to) sine, then the RMS value reported
by the meter will be incorrect, assuming the majrity of the waveform
frequencies are inside the bandwith of the measuring circuitry.
If you really want true RMS, then get a meter that actually measures
true RMS.
If you have a "high frequency" square wave, or pulses, or "RF" then
you should consider using thermal bolometers or the like.

Re: Measuring "high" voltage

Thanks for the response.  I'm taking a stab at this here guys so please
correct me if I am wrong:

My multimeter does have an ~AC mode but I am not sure on how it is
calcuating the AC voltage. ( I need to dig up the manual hope I can
find it! )  If it is just taking the peak value instead of the rms
value it will always be wrong because the majority of the time the sine
wave is not at its peak value.  Does that sound about right? I want to
measure 1200 Vrms.

NuclearFirestorm - do you mind saying a few words on how the frequency,
source impedence or whatever else will effect my measurement?

Thanks.

Re: Measuring "high" voltage

On 29 Jul 2006 20:27:13 -0700, " snipped-for-privacy@hotmail.com"

You have to use an HV probe.  You have to make sure that your ground
points are in the right place.

Re: Measuring "high" voltage

A voltmeter always loads the voltage to be measured and the voltage

The multimeters in the office are probably all giving the correct
answers  - that is they all correctly indicate the voltage which
appears across their input terminals.

If the high voltage to be measured has a high internal resistance then
the resistance of the meter causes the high voltage to fall somwhat.

What you could use is a voltage divider consisting of two resistors in
the ratio of the order of 100 or 1000 to 1.

The high value resistor should have a value say 10 times the source
resistance of the high voltage to be measured.  The low value resistor
should have a value 1/10th of the meter used.

You will need Ohm's Law, a knowledge of the meter resistance, and a
bit of arithmetic to work it out.

As I say, your multimeters are probably giving you the correct
answers.  But you need to know what the internal resistances are of
the meter and of the voltage to be measured in order to calculate what
the open=circuit voltage of the source is.
----
Reg

Re: Measuring "high" voltage

On Sun, 30 Jul 2006 20:28:06 +0100, "Reg Edwards"

Not if a proper high voltage probe is utilized.  In such a case, the

Re: Measuring "high" voltage

On Sun, 30 Jul 2006 20:28:06 +0100, "Reg Edwards"

Not if an HV probe is used.  In such a case, the probe resistance is
the value which needs to be known.

Re: Measuring "high" voltage

What I have said is perfectly correct.

But you seem to know enough about it to sort out the errors without
bothering newsgroups with vague questions. Swat up on Ohm's Law and do
a little arithmetic.
----
Reg.

.

is

Re: Measuring "high" voltage

On Sun, 30 Jul 2006 22:32:41 +0100, "Reg Edwards"

Nope.  Also, not quoting what you are claiming is rather stupid as
well.

Bugger off, bother boy.

Dumbass.  That's what an HV probe does.  It is a very high resistance
presented to the load so that the meter's internal resistance does not
present a load to the supply being probed.

It is Ohms law, and you have failed the test.  If you knew anything
at all about HV probes you would never have come back to post this
utter crap.

You are the one that needs to BONE UP, and do some math.

Questions:

What loading does a 10Meg Ohm meter present to a 1200 Volt supply?

What loading does a 10Gig Ohm HV Probe monitored by a 10MegOhm meter
present to the same source?

Are those too "vague" for you to grasp?

Ooops... You lose.

Re: Measuring "high" voltage

A nym shifting troll, last known here as Roy L. Fuchs,
currently on a binge in a thread crossposted to
(alt.engineering.electrical).

Roy L. Fuchs <roylfuchs urfargingicehole.org>

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Subject: Re: Measuring "high" voltage
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On Sun, 30 Jul 2006 22:32:41 +0100, "Reg Edwards"
<g4fgq.regp ZZZbtinternet.com> Gave us:

Nope.  Also, not quoting what you are claiming is rather stupid as
well.

Bugger off, bother boy.

Dumbass.  That's what an HV probe does.  It is a very high resistance
presented to the load so that the meter's internal resistance does not
present a load to the supply being probed.

It is Ohms law, and you have failed the test.  If you knew anything
at all about HV probes you would never have come back to post this
utter crap.

You are the one that needs to BONE UP, and do some math.

Questions:

What loading does a 10Meg Ohm meter present to a 1200 Volt supply?

What loading does a 10Gig Ohm HV Probe monitored by a 10MegOhm meter
present to the same source?

Are those too "vague" for you to grasp?

Ooops... You lose.

Re: Measuring "high" voltage

snipped-for-privacy@hotmail.com wrote:

As long as the tube lights, the voltage is kinda unimportant, compared
to the CURRENT.

These CCFL tubes have a rather variable resistance, varies during each
cycle, and varies as the tube warms up.

You'll probably find it much easier and helpful to measure the tube
current.

Just insert a 1K resistor in one of the HV leads, put a good AC RMS
meter across it, and go to it.   Note that you'll have to suspend the
meter in the air if you can't ground the HV lead going to the meter.

As others have noted, the voltage and current wavefornms are likely to
be rather peaked, so even a "true RMS" meter of the simpler variety is
likely to be a bit off.  Check the meter's "Crest Factor" rathing.  If
it's less than 6:1 or not mentioned, that meter is unlikely to give you
giood results.    Get a real, true, RMS meter, such as the HP 3403C.
They're not too pricey on eBay, just make sure you get a working one.

Re: Measuring "high" voltage

Gave us:

Very few do give crest factor data.

giood post.  :-]

Re: Measuring "high" voltage

[....]

They light up purple.

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

Re: Measuring "high" voltage

Ken Smith wrote...

They may, but we don't care because the scope input is toast.
We need a TVS, transient voltage suppressor, AKA zener diode,
with its massive sub-ns response to overvoltage.  They do have
a capacitance penalty, but in this case we can live with it.

--
Thanks,
- Win

Re: Measuring "high" voltage

On 1 Aug 2006 18:36:55 -0700, Winfield Hill

One trick I've used to mitigate Zener capacitance  is to use a regular
small-signal diode in series with the Zener (with the appropriate
polarity).  Typically the forward capacitance of the small-signal
diode is much lower, so you get two capacitors in series.  Very handy
for a rude-and-crude clamp at an opamp's output, but I've never
tried it in a transient-suppression application.

Steve

Re: Measuring "high" voltage

Steve, neat trick. Wouldn't you have to pre-bias the zener to get it near
the operating voltage?

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:

Re: Measuring "high" voltage

We used this as a slow protection clamp on an op-amp output to protect
some downstream stuff.  Something like an amplifier with +-15V driving
some other bits with +-5V supplies and feeble ESD diodes.  So we didn't
care a lot about accuracy, and we could wait a few us for it to turn on.
The primary concern was to make sure the op-amp didn't oscillate, which
it would have done had we just used the Zeners.

Had we wanted stiffer clamps we would have prebiased the Zener at a
cost of more power and complexity, but this wasn't needed.

BTW, I've just changed my posting address (getting the hang of this
Forte Agent thing).  To reply directly you'll need to remove the food

Steve

Re: Measuring "high" voltage

[....]

You have to provide biasing.

Fortunately, the diodes fail shorted, usually.

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