Measuring EHT

C

Cursitor Doom

Guest
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
 
On Thursday, March 26, 2015 at 7:21:56 PM UTC-4, Cursitor Doom wrote:
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

Scope tubes are very high impedance. You would need to know exactly how much current the tube pulls in order to calculate what the voltage drop should be across any resistor. Although there is but a small voltage drop across the resistor, there is still a big voltage potential from what you're measuring and any ground. You also can't put the resistors from the HV lead to ground for the same reason (high impedance, low current HV trans). Maybe you can borrow a HV probe from a TV repair shop. I have three of them and rarely use them anymore.
 
"Cursitor Doom" <curd@notformail.com> wrote in message
news:mf248u$fgh$2@dont-email.me...
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

Those voltage levels are intimidating. You don't really want to use just
series resistors at tha voltage level. Most of the 1/2 to 1 watt resistors
are only rated for about 300 to 500 volts across them. Even with 20 in
series you are pushing it over the limit. They do make high voltage probes
for the meters. As most digital meters are around 10 megohm input, they are
probably universal probes for the high voltage.
 
On 03/26/2015 5:08 PM, John-Del wrote:
On Thursday, March 26, 2015 at 7:21:56 PM UTC-4, Cursitor Doom wrote:
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.

Scope tubes are very high impedance. You would need to know exactly how much current the tube pulls in order to calculate what the voltage drop should be across any resistor. Although there is but a small voltage drop across the resistor, there is still a big voltage potential from what you're measuring and any ground. You also can't put the resistors from the HV lead to ground for the same reason (high impedance, low current HV trans). Maybe you can borrow a HV probe from a TV repair shop. I have three of them and rarely use them anymore.
Click to expand...

I picked up HV probes from eBay over the years. They are usually pretty
cheap...and handy for our shop. Put then we still service picture tube
based monitors for arcade games...

$35 and up on eBay right now:

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR2.TRC1.A0.H0.Xhv+probe.TRS0&_nkw=hv+probe&_sacat=0

And Tinyurl version:

http://tinyurl.com/qdaq77t

John :-#)#

--
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
 
On Thu, 26 Mar 2015 23:21:02 +0000, Cursitor Doom wrote:

Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

Your query had me rushing off to see if I still had the nylon rod strung
with multiple large resistors that I used to use as a probe. But I don't
think I was measuring quite so high EHTs.

And as others have said, an EHT supply may not deliver 1mA. It only has to
charge the tube's capacitance and deliver leakage current. It may supply
enough current to measure with a sensitive ammeter and give voltage by
calculation. And multiple measurements with different loads should give an
indication of the regulation.

But as other have also said, don't exceed the individual resistor voltage
limit. So quite a lot of resistors may be called for!

Mike.
 
On Thursday, March 26, 2015 at 4:21:56 PM UTC-7, Cursitor Doom wrote:


On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
Click to expand...

It's usual to use a HV probe that contains series resistors in the
tip area, and a safe-to-hold ground shielded area for the handle, and
the input resistance of the meter to complete a voltage-divider.
So, aside from some calibration and packaging, you've got the right
idea. Always attach the ground wire from the probe first...
 
On Friday, March 27, 2015 at 1:34:37 PM UTC-4, whit3rd wrote:

It's usual to use a HV probe that contains series resistors in the
tip area, and a safe-to-hold ground shielded area for the handle, and
the input resistance of the meter to complete a voltage-divider.
So, aside from some calibration and packaging, you've got the right
idea.
Click to expand...

In order for him to do that, he'll need about 600 meg worth of resistors to keep from swamping the HV supply. But rigging series resistors to ground wasn't what he was suggesting. Reread his original post.
 
"Cursitor Doom" <curd@notformail.com> wrote in message
news:mf248u$fgh$2@dont-email.me...
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

You are unlikely to be able to get a sensible result with just series
resistors if you are using a digital meter. The probes of yesteryear that we
all used to use, were made from a string of series connected high value high
voltage - and as others have said, that is an important consideration -
resistors, but they were intended to work into a 'standard' analogue meter
having an input resistance of 20k ohms / volt. In the uk, that would have
been typically an AVO 8 in most TV workshops. The meter was switched to a
fairly low range - I seem to remember the 25v DC range - so the input
resistance of the meter would be 500k ohms full scale. The probe contained
resistors adding up to 500 meg ohms, so with it plugged into the normal DC
"+" socket of the meter, it and the input resistance of the meter to ground
formed a 1000:1 potential divider with the 'tap' being the meter socket. So
on the 25 volt range, the meter would read directly 25 kV full scale.

With a digital meter having a very high input resistance, the value of the
external series string to form a similar 1000:1 potential divider, would be
so high as to be impractical to implement. You could, however, make your own
potential divider if you wanted. If you could lay hands on a bunch of high
value high voltage resistors - say 10 M apiece, you could string 20 of them
in series to get 200M and then another couple to get 20 M and hang the whole
shebang across the output from the EHT multiplier. That would get you a
100:1 divider and the 14 kV would measure on a digital meter as around 140
volts give or take.

Personally, at this stage, I wouldn't be too bothered about it though. If it
comes on, and you get a nice bright trace, sharply defined over the full
width of the tube face, and the size doesn't alter much (any) as you wind
the brightness up and down, then the chances are that the multiplier is
working as it should, and the EHT is ballpark correct. As the input to the
multiplier is derived from a winding on the same transformer as the rest of
the supply, if the other low voltage rails are correct, chances are that the
schematic-declared input voltage of 1.5 kV to the multiplier is also
correct. Generally, EHT multipliers are fairly reliable, and if they do
fail, it is usually a pretty straightforward diagnosis procedure.

Arfa
 
Arfa Daily <arfa.daily@ntlworld.com> wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mf248u$fgh$2@dont-email.me...
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.


You are unlikely to be able to get a sensible result with just series
resistors if you are using a digital meter. The probes of yesteryear that we
all used to use, were made from a string of series connected high value high
voltage - and as others have said, that is an important consideration -
resistors, but they were intended to work into a 'standard' analogue meter
having an input resistance of 20k ohms / volt. In the uk, that would have
been typically an AVO 8 in most TV workshops. The meter was switched to a
fairly low range - I seem to remember the 25v DC range - so the input
resistance of the meter would be 500k ohms full scale. The probe contained
resistors adding up to 500 meg ohms, so with it plugged into the normal DC
"+" socket of the meter, it and the input resistance of the meter to ground
formed a 1000:1 potential divider with the 'tap' being the meter socket. So
on the 25 volt range, the meter would read directly 25 kV full scale.

With a digital meter having a very high input resistance, the value of the
external series string to form a similar 1000:1 potential divider, would be
so high as to be impractical to implement.
Click to expand...

Use the meter to read microamps.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
 
In article <v5GdnYGi4cyqA4nInZ2dnUU7-eudnZ2d@earthlink.com>,
"Ralph Mowery" <rmowery28146@earthlink.net> wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mf248u$fgh$2@dont-email.me...
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.

Those voltage levels are intimidating. You don't really want to use just
series resistors at tha voltage level. Most of the 1/2 to 1 watt resistors
are only rated for about 300 to 500 volts across them. Even with 20 in
series you are pushing it over the limit. They do make high voltage probes
for the meters. As most digital meters are around 10 megohm input, they are
probably universal probes for the high voltage.
Click to expand...

Many people seem to overlook the voltage rating of a resistor because
power dissipation is not being exceeded. For example a 1/2 watt 10
MegOhm resistor would dissipate 1/2 watt at 2240 volts, but might have a
dielectric breakdown at 350 Volts.

As John Robertson suggested, look for used high voltage probes on E-Bay.
RCA, Eico, Heathkit and others made them. Be sure you pick one that is
rated for your voltage.

Fred
 
..
Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

Check the schematic carefully.
TEK scopes have a built-in test point on the internal HV regulator.
May already have someplace to measure.

As others have stated, 20 resistors in series may not have sufficient
breakdown voltage to measure 14KV. May also load the scope
too heavily. My HV probe is over 1,000M Ohms.
 
On Friday, March 27, 2015 at 1:20:42 PM UTC-7, Adrian Tuddenham wrote:
Arfa Daily <arfa.daily@ntlworld.com> wrote:

With a digital meter having a very high input resistance, the value of the
external series string to form a similar 1000:1 potential divider, would be
so high as to be impractical to implement.

Use the meter to read microamps.
Click to expand...

The 'microamps' function might fail open (there's usually a fuse),
which would be unfortunate. A 100:1 or 30:1 divider is easily arranged,
using the voltmeter's V scale resistance, and will take my Fluke to 100 kV (if
I were to require that); 1000:1 is excessive.

Good HV resistors are fat things, a centimeter or so diameter is recommended.
 
On Thu, 26 Mar 2015 23:21:02 +0000, Cursitor Doom wrote:

Hi all,

On this scope I've been working on there are voltages present (according
to the schematic Arfa kindly provided me with) of up to 14kV. Does one
need a special voltmeter to measure such levels or can I use a work-
around of cobbling together say 20 x 1Meg resistors in series and
measuring the voltage across one of them with a regular DVM?
My (hobby) background has been in messing around with typically <12V so
I'm finding these EHT levels a bit intimidating to say the least.
Thanks.
Click to expand...

Remembering that at college (in the 1960s) we had an electrostatic
voltmeter (which may have even been an entirely passive device like a
calibrated electroscope) I had a google for that title.

I see that it is possible to buy a non-contacting voltmeter measuring up
to 20kV. I didn't look for the price but I guess it is not in the hobby
range...

Mike.
 
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