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EHT
STICK:
an Extra High Voltage
for Digital Multimeter
Do you need to measure the EHT voltage in a CRT-based scope,
computer monitor or TV receiver, or perhaps in a photocopier, laser
printer or microwave oven? You’ll need an EHT probe to suit your digital
multimeter (DMM) to do this and you’ll find they are pretty pricey.
Not to worry though, because here’s one you can build for less than $40.
M
easuring really high voltages
is not something you can
normally do easily or safely.
So if you want to measure the EHT of
CRT-based TV receivers or the corona
voltages in photocopiers or laser printers, what do you do?
They are up around 22kV or more
– far out of the range of a DMM. And
if you want to measure the high volt58 Silicon Chip
age in a microwave oven – about 3kV
or so – that’s also way out of range of
a DMM.
You can’t make this kind of measurement just with a normal multimeter
or DMM, because in most cases they
have a maximum input voltage rating
of 1000VDC or 750VAC.
The only way this type of meter
can be used to make measurements
on higher voltages is to connect a specially designed EHT divider probe between its input sockets and the source
of high voltage. The probe divides
down the voltage to be measured by a
known factor (usually either 100:1 or
1000:1), to bring it within the voltage
range which can be handled safely by
the meter itself.
This type of EHT divider probe has
siliconchip.com.au
SAFETY WARNING
In order to use EHT divider probes like the one described in this article
safely, please note carefully the following points:
1. The probe’s ground return must always be connected securely to
the ‘earthy’ side of the EHT circuit in which you are making the measurement – BEFORE you connect the probe’s measuring tip to the ‘hot’ side
of the circuit. This is most important because if the probe tip is connected
first, all of the probe’s internal circuitry AND YOUR DMM will be ‘floating’
at the full EHT voltage and thus represent a very serious safety risk.
2. The probe’s ground return lead and its connection clip must be
regarded as a vitally important part of the probe itself. It is crucial to
achieving correct probe operation, because it provides the only connection between the bottom end of the probe’s voltage divider and the EHT
circuit in which you are making the measurement.
Probe
rs
By JIM ROWE
been available commercially for many
years, and they’re still available if you
hunt them down.
They’ve never been particularly
cheap though and if you want to buy
a brand-new probe nowadays you’ll
find they’re priced from around $100
and upwards – not easy to justify if you
only want to measure EHT voltages
every now and again.
siliconchip.com.au
3. NEVER connect the probe’s ground return lead to the ‘hot’ side of
the high voltage circuit, as this will also cause your DMM to be floating
at the full EHT voltage. If you need to measure an EHT voltage that happens to be negative with respect to ground, simply reverse the polarity of
the probe lead connections to the DMM input jacks. The probe’s ground
return lead should ALWAYS be connected to the ‘cold’ or earthy side of
the EHT circuit.
4. If at all possible, turn off the power to the EHT circuit before you
connect the probe’s ground return lead and input measuring tip. Only
turn the power back on when both connections are secure and your
hands are safely withdrawn. This will help ensure that you don’t receive
a shock when the probe tip comes into contact with the ‘hot’ side of the
EHT circuit, and also that a ‘flashover’ arc cannot develop.
5. Turn off the power to the EHT circuit again after you have made the
measurement, and before you remove the tip and ground return connections (in that order).
6. If it is not feasible to turn off the power to the EHT circuit before
making the probe connections and you have to hold the probe body in
your hand while making the measurement, make sure you hold it down
at the output lead end. Do not risk a flashover or punch-through by holding it closer to the tip end.
7. Do not attempt to use this type of probe to make measurements
in high voltage power distribution systems. These can supply a huge
amount of energy/power and in most cases cannot be turned off in order
to make the probe connections. The risk of serious injury or death is
therefore extremely high.
April 2010 59
Test setup using the EHT Stick and a digital multimeter.
Always ensure that the green grounding lead is firmly
attached to a suitable ground point in the circuit under test
BEFORE probing the EHT.
Our probe, which we’ve dubbed
the “EHT Stick”, has been designed to
allow you to measure DC voltages up
to around 23-25kV, using any standard digital multimeter (DMM) which
has an input resistance of 10MΩ. It
provides a division ratio of 1000:1, so
kilovolts at the input are read simply
as volts on the DMM.
Like many commercial EHT probes,
it provides an input resistance of just
over 800MΩ. So when it’s connected
across a circuit with a voltage of say
20kV (20,000V), the probe will draw a
modest ‘loading’ current of only 25A.
In their divider’s crucial input leg,
commercial EHT probes have always
used special very high value ‘long
spiral’ resistors rated to withstand
very high voltages but these haven’t
been readily available for some time.
So instead, we have used 80 (yes,
eighty!) high voltage (1.6kV) 10MΩ
0.5W metal film resistors in series to
produce the 800MΩ input leg. The
Farnell type number for the 10MΩ is
110-0295.
Because of the large number of
60 Silicon Chip
resistors in series, the voltage drop
per resistor is kept well within their
maximum voltage rating.
Even when the EHT Stick is measuring a voltage of 25kV for example,
the voltage across each resistor in the
input leg is only 313V. The power dissipation per resistor will also be less
than 10mW.
By the way, don’t be tempted to
substitute standard 0.25W or 0.5W
resistors for the high voltage types
specified. Most 0.25W and 0.5W resistors have a voltage rating of only
200-250V or so – certainly not enough!
Now before we move on to look at
the probe’s circuit and how it’s built
and used, please read the text in the
safety warning box carefully.
Making measurements in EHT
circuits inevitably presents an increased safety risk, because even in
a CRT-based TV set or a microwave
oven the EHT circuitry can provide a
lethal shock.
So it’s important – in fact, vital – that
you not only build the probe exactly
as we describe and that you follow
the correct procedures when making
a measurement. If you are careless,
the measurement may be the last you
ever make!
Circuit description
As you can see from the circuit of
Fig.1, the probe is just a resistive voltage divider, with an input leg formed
by the 80 10MΩ resistors in series.
The lower leg is formed by the
820kΩ and 30k resistors in series
with trimpot VR1, with the 10MΩ
input resistance of the DMM itself in
parallel.
When the value of this composite
lower leg is adjusted using VR1 to
have a resistance of 1/999 of the input leg (ie, nominally 800MΩ/999, or
800.801kΩ), the divider provides an
exact division ratio of 1000:1.
Trimpot VR1 allows you to compensate for the within-tolerance variations
in all of the other resistors, to give the
probe maximum accuracy.
So while the circuit of the probe
is very straightforward, the physical
construction presented us with quite
siliconchip.com.au
Resistors (0.5W metal film high voltage – Farnell MH25 series)
80 10MΩ (1.6kV rating – Farnell 110-0295)
1 820kΩ
1 50k 25-turn vertical trimpot (VR1)
siliconchip.com.au
1 30kΩ
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1 PC board, code 04104101, 228 x 37mm
1 250mm length of 43mm OD/39mm ID PVC-U DWV conduit
2 43mm ID PVC pipe cap to suit conduit
1 230mm length of 30mm diameter heatshrink
1 4mm banana socket, red with matching double-adaptor banana plug
1 3.5-6mm cable gland
2 1.2m long 600V-rated test leads (one red, one black) with shrouded
banana plugs
1 1m length of mains-rated flexible earth lead, with green insulation
1 32mm (medium) alligator clip, with black or green insulating shroud
4 1mm diameter PC board terminal pins
1 Nylon cable tie, 4mm wide
1 short (~50mm) length brass rod, around 2-3mm diameter (for tip)
10M 1%
Parts List – EHT Stick
10M 1%
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The approach we came up with was
to fit all of the resistors and the trimpot
onto a long narrow PC board, measuring 228 x 37mm and coded 04104101.
The 80 resistors in the divider’s input
leg are laid out in a long ‘zig-zag’ pattern over most of the board’s length,
to provide the necessary spacing in a
reasonably compact area.
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Construction
The PC board, sleeved in 30mm
heatshrink tubing, is designed to
fit inside a 250mm length of 43mm
OD/39mm ID PVC-U conduit, with a
43mm ID PVC pipe cap at each end to
complete the safety isolation.
The PVC-U conduit (the -U standing
for unplasticised) is type DWV (stands
for drain, waste & vent) and it and
the matching end caps are made by
companies such as Vinidex and Iplex.
The conduit and the end caps can be
obtained from hardware stores and
plumbing supply outlets. Most will
supply minimum lengths of 1 metre
– a bit of a waste, if you’ll pardon the
pun – but our 1m length only cost us
a couple of dollars. One of these days
10M 1%
10M 1%
TO
DMM
10M 1%
a challenge, in order to meet the
somewhat conflicting needs of fitting
no fewer than 82 0.5W resistors plus
a trimpot into a case that would be
compact enough to be hand-held, yet
provide a suitably high level of electrical isolation and safety.
10M 1%
Fig.1 (above): the circuit is simply a voltage divider giving a suitable output
to measure on a digital multimeter.
Fig. 2 (right) shows the PC board component overlay. It’s not difficult to
build but it is quite tedious fitting and soldering 82 half-watt resistors. Note:
do not substitute other resistors as their voltage rating may be insufficient.
10M 1%
EHT STICK (1000:1 HIGH VOLTAGE DIVIDER FOR DMMS)
10M 1%
ALWAYS CONNECT GROUND RETURN CLIP TO
THE HIGH VOLTAGE CIRCUIT'S GROUND SECURELY,
BEFORE CONNECTING THE PROBE TIP TO THE HV SOURCE
10M 1%
SC
2010
* MOST IMPORTANT FOR YOUR SAFETY:
10M 1%
GROUND
RETURN CLIP*
CALIBRATE
XA M Vk 4 2
820k
CALIBRATE
10M 1%
VR1
50k
25T
VR1 50k
10140140
30k 1%
THIS SECTION ALL WITHIN
PROTECTIVE PVC CYLINDER
REDIVID 1: 0 0 0 1
(Rin = 10M )
30k 1%
820k 1%
GND
OUTPUT
TO DMM
A
80 x 10M 1% = 800M +/-1%
HV
PROBE
TIP
10M 1%
GROUND
HV PROBE TIP SOCKET
CLIP
April 2010 61
Here’s a shot of the completed PC board,
immediately prior to fitting it to the input socket on the
front end cap, soldering on the DMM connecting leads and
ground clip lead and finally covering it with heatshrink.
we’re sure to come up with a use for
the rest!
Or you might also try your friendly
local plumber for an offcut. The plumber might also be good for a short length
of 4mm brass rod (eg, brazing rod) to
fashion a probe tip from.
The end caps are a (tight!) friction
fit onto the conduit. This provides
adequate physical security while maintaining good electrical isolation. We
suggest you don’t try to check the fit out
before final assembly, because once on,
they’re not easy to get off again!
A 4mm banana socket is mounted in
the centre of one end cap to provide the
probe’s ‘hot’ input, the idea being that
whichever probe tip (or very short clip
lead) you use plugs into the socket via
a standard 4mm banana plug.
As we mentioned earlier, a short
length of brass rod makes an excellent
probe tip – we made ours from a piece of
brazing rod about 50mm long (certainly
not critical) with a point filed one end
and soldered to a banana plug to mate
with the banana socket.
A cable gland is mounted in the
centre of the other end cap to provide
an exit for the probe’s output leads and
its ground return input lead.
Wiring up the probe board is not
difficult but is a little tedious because
of the large number of resistors to be
fitted. The easiest part is fitting the four
PC board terminal pins used to make
the off-board connections – one at the
input end to mate with the solder lug
at the rear of the banana socket, and the
other three at the output end to provide
the cable connections.
Note that the single pin at the input
end should be fitted from the copper
side, with its ‘top end’ cut off flush
when you have soldered it to the pad
underneath.
Once the pins have been fitted, you
can proceed with installing the fixed
resistors. They’re fitted in the standard way, with the leads bent down at
90° quite close to the resistor body so
that when they pass through the board
holes, the resistor is lying flat on the
top of the board.
The leads are then soldered carefully to the pads underneath, with just
enough solder used to produce a nicely
rounded joint. The excess leads are
then cut off with sharp side cutters as
close as possible to the joints, so that
no sharp wire ends or ‘points’ are left.
This is quite important, because any
sharp points on conductors carrying
high voltage tend to concentrate the
surrounding electric field and cause
ionisation of the air – producing a
‘corona’ discharge.
The only other thing to watch when
you’re fitting the resistors is to fit the
820k and 30kresistors down at the
output end of the board, as shown in
the overlay diagram.
You might want to fit these first, to
make sure they’re in the correct positions. Then you can fit the remaining 80
resistors, happy in the knowledge that
they are all of the same value.
With all of the fixed resistors installed, the only remaining step is to
fit trimpot VR1 and your probe’s PC
board assembly will be complete. It can
then be put aside while you prepare the
probe’s tube and end caps and assemble
the whole thing.
Final assembly
Final assembly also involves calibration. This could be done now that the
PC board is complete but it’s better to
wait until the unit is partly assembled
(and therefore partly insulated) as it
involves high voltages.
First cut your length of 43mm OD
PVC-U DWV conduit to 250mm long.
If necessary, square off each end with
a flat file, using it to remove any burrs
as well.
Next, drill the holes in the centre of
Here we have removed the end cap
for clarity but normally the cap (with input socket)
would be in place before the heatshrink is applied. With the
heatshrink, the completed PC board is a snug fit inside the DWV conduit.
Inset top left is the “probe” in its banana plug, here with optional crocodile clip connector.
62 Silicon Chip
siliconchip.com.au
INPUT
PROBE TIP
PLUGS
INTO
SOCKET
LEADS TO I
NPUT
OF DMM
43mm ID PVC
END CAP
43mm ID PVC
END CAP
3.5-6.0mm
CABLE GLAND
250mm LENGTH OF
43mm OD, 39mm ID
PVC CONDUIT
4mm BANANA
SOCKET
DIVIDER PC BOARD
ROUNDED SOLDER JOINTS WITH
WIRE ENDS TRIMMED OFF
(NO SHARP POINTS)
NYLON CABLE
TIE BINDING
ALL THREE LEADS
GROUND RETURN
ALLIGATOR CLIP
GROUND RETURN LEAD
Fig.3: this shows how the completed project goes together. The only
thing not shown here (again, for clarity) is the heatshrink tubing over
the PC board. This provides extra electrical insulation.
each end cap to receive the ‘hot input’
banana socket and the output cable
gland. These both need round holes
of around 9mm diameter but the exact
diameter will depend on the particular
components you use – and the holes
shouldn’t be any larger than is necessary to receive them.
So it’s probably best to drill a ~5mm
hole in each cap first and then use a
tapered reamer to enlarge it carefully
until the socket or gland will just pass
through. Then remove any burrs as
before.
Mount the input banana socket
securely in its end cap, using one of
the two nuts supplied to fasten it in
position. Next, fit the solder lug and
the second nut, tightening this up so
that the lug is securely attached to the
back of the socket. Then bend the lug
over against the side of the second nut.
This will bring it into position where
its end hole will be as near as possible
to the input terminal pin on the end
of the divider probe’s PC board, when
assembled. The bent lug will also help
to hold the nut in position.
Now slide the PC board into the
end cap so that the solder lug on the
banana socket and the PC board input
pin can mate. This is a little tricky but
if you keep the solder lug and PC board
parallel to each other, you should have
success.
Once the pin does pass through the
hole in the solder lug, you can solder
the two together carefully to make the
connection permanent.
Make sure that you apply enough
solder to form a strong and nicely
rounded joint – also take care not
to burn the side of the PVC end cap
with the barrel of the soldering iron.
Your end cap and PC board assembly
should now look very much like the
photo below right.
Putting it together
Loosely fit the cable gland to the
other end cap and pass the bare ends
of the three exterior wires (ie, the two
leads which go to the DMM and the
ground lead) through the gland from
outside to inside. Pull these three wires
through as far as they will go so that the
DMM plugs and ground clip lead are
against the cable gland.
If necessary, cut the 30mm diameter
heatshrink to length (~230mm, give or
take) and either cut or drill a pot access
hole. We placed a scrap of timber inside
the heatshrink and drilled a 6mm hole,
right in the centre and 10mm down
from the end.
Pass the three external wires right
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through the heatshrink, from the pot
access-hole end, all the way along the
wires (you want to keep the heatshrink
away from the heat of soldering in the
next step).
Similarly, pass the three external
wires through the PVC-U pipe and slide
the pipe up the wire. Don’t push the
end cap onto the pipe, at least not yet!
Now solder the three external wires
to their appropriate positions on the PC
board, as shown on the overlay. Fit a
small Nylon cable tie around the three
wires to keep them together but not
so close to the PC board that it causes
undue strain on the wires.
The final step before calibration
is to slide the heatshrink back down
the three wires and all the way onto
PC board, locating the pot access hole
over the pot and then shrinking the
heatshrink onto the PC board. A hot air
gun is best but a hair drier on a high
heat setting will work – it just takes a
bit longer.
Calibration
Before completing the Probe, now
is the time to adjust trimpot VR1 for
a division ratio of exactly 1000:1 – in
other words, calibration.
Ideally, you’ll need a convenient
source of stable medium-high voltage
to do this (say 750-950V DC).
If you don’t have such a source your
best plan would be to simply set VR1
to around the middle of its range, using
one of your DMM’s resistance ranges
to do this. Simply connect the DMM
leads directly across VR1, and turn its
adjustment screw with a small screwdriver until you get a reading of close
to 25k. This should give your Probe
a division ratio within about 3% of the
correct figure.
If you do have a source of stable high
voltage, calibrating the Probe is quite
simple. You just need to be fairly careful, because high voltage can “bite”!
Having a banana socket with removable tip also makes it easier (and
safer) to connect your high voltage
to, as exposed metal is kept to a
minimum.
You’ll also need to connect
the board’s ground return lead
pin (at the output end of the
board) to the negative side of
your high voltage source securely, before you start.
Measure and note down the voltage using your DMM directly, set to
its top DC voltage range. Remove the
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DMM leads from the DC voltage source
and connect instead the output leads
from the Probe board.
Then connect the Probe’s input
socket to the positive side of the high
voltage source, and you should be able
to read the Probe’s output voltage on
the DMM. It should be very close to
1/1000th of the first reading and all
you now have to do is adjust VR1 with
a small screwdriver until it becomes as
close as possible.
To end this procedure disconnect the
probe tip from the positive side of the
high voltage source, then disconnect
the temporary ground return lead from
the negative side and finally disconnect
it from the ground lead pin in the rear
of the Probe PC board. Your EHT Stick
should now be calibrated, and ready
for final assembly.
Give everything the once-over again,
just in case – remember that once the
end caps go on, they’re rather difficult
to get off again!
In fact, it’s a good idea to loosely
place the end caps as you follow the
next steps and then make some trial
measurements, just to make sure everything is still working.
It’s complete!
Slide the pipe back down the wires
and over the heatshrink-covered PC
board. It’s a snug fit but it does go in.
Place the front end cap onto the pipe
now and slide the other end cap right
back down the wires to the cable tie.
Leaving a small amount of slack inside the pipe, tighten the cable gland
and then push the rear end cap loosely
onto the pipe.
If your test measurements look satisfactory, push both end caps hard onto
the pipe. No screws (or glue) are necessary to hold the caps in place – they
SC
won’t come off by themselves!
Looking into the end
cap, showing how
the input socket
solder lug
connects
to the PC
board
pin.
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April 2010 65
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