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Basics of High Voltage Probe Design Basics of High Voltage Probe Design
Version 1.27
Copyright © 1994-2004
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page
.

Reproduction of this document in whole or in part is permitted if both of the following
conditions are satisfied:
1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.

Table of Contents
Preface

Author and Copyright

DISCLAIMER

Introduction

Scope and Purpose of This Document

SAFETY

High Voltage Probe Design

Basic Considerations

Frequency Response

Simple High Voltage Probe Design Examples
DMM
50,000 V Maximum Using 10Mohm Z-in DMM

50,000 V Maximum Using 30kohm/V VOM

Sample Circuit

Calibration

More Information on High Voltage Probes

Construction of High Voltage Probes

High Voltage Probe Frequency Response

High Voltage Probe for AC Measurements

More on Measuring High Frequency High Voltage

Commercial High Voltage Probes


Page 1 of 12
Basics of High Voltage Probe Design
8/10/2004
http://repairfaq.ece.drexel.edu/sam/hvprobe.htm Back to
HV Probe Design Table of Contents
.
Preface
Author and Copyright
Author: Samuel M. Goldwasser
For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page
.
Copyright © 1994-2004
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are
satisfied:
1.This notice is included in its entirety at the beginning.
2.There is no charge except to cover the costs of copying.
DISCLAIMER
The devices, equipment, circuits, and other gadgets described in this document may be dangerous. Much
of it deals with potentially lethal voltages. Getting electrocuted could ruin your whole day. Using an
inadequate or improperly designed or fabricated high voltage probe to measure high voltage can be
equally dangerous.
We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages,
spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that may
result from the use of this material.
For really high voltage equipment, also see:
Tesla Coils Safety Information
.
WARNING: The microwave oven is perhaps the most dangerous equipment you are likely to encounter
around the house. The high voltage (up to 5,000 V) along with the high current (1 A or more)
availability make this an instantly lethal combination. It is highly recommended that NO measurements
be made on a powered microwave oven. Only after the plug has been pulled and its high voltage
capacitor has been safely discharged should you even think about touching or probing anything. Most
troubleshooting can be done with at most an ohmmeter. See the document:
Notes on the
Troubleshooting and Repair of Microwave Ovens
for more information. By comparison, TVs, monitors,
and even large helium-neon lasers, are tame. While still very dangerous, they don't have quite the deadly
quality of the microwave oven!
Back to
HV Probe Design Table of Contents
.
Introduction
Scope and Purpose of This Document
Page 2 of 12
Basics of High Voltage Probe Design
8/10/2004
http://repairfaq.ece.drexel.edu/sam/hvprobe.htm There are all sorts of times when being able to determine the value of a high voltage DC source is
desirable. Most multimeters have a maximum range of 750 or 1,000 V. (One exception is the workhorse
Simpson 260 which has a 5,000 V range). Whether testing a TV with a dim picture, a helium-neon laser
power supply that does work quite right, or troubleshooting some home-built high voltage project, the
ability to measure 10, 20, 30, or more kV can come in handy.
This document provides information on constructing very basic high voltage probes suitable for
measuring the high voltages found in consumer electronic equipment like TVs, monitors, and
microwave ovens (though the latter is not recommended for safety reasons).
These simple approaches will work for DC and low frequency AC voltages but no effort is made to
compensate for stray capacitance - which will seriously limit high frequency response. However, some
of the issues are discussed.
If you will be making HV measurements regularly, by all means invest in a real HV probe for your
multimeter. A commercial HV probe will still be a far better long term investment than some cobbled-
together unit. However, for occasional HV testing, what is described below can be built and used safely
but probably won't have the accuracy, consistency, or frequency response of a good commercial probe.
Aside from purchasing a HV probe new, these do show up surplus as well as on eBay, possibly at
greatly reduced prices. Even if a model isn't available for your particular multimeter (which is likely), it
should be possible to adapt almost any commercial probe to work with it, requiring at most a scaling
factor when taking a reading.
SAFETY
Read the associated document:
Safety Guidelines for High Voltage and/or Line Powered Equipment

before attempting to work with high voltage systems. High voltage can jump amazing distances when
you least expect it. The direct or indirect consequences of this can ruin your entire day or a whole lot
more.
Back to
HV Probe Design Table of Contents
.
High Voltage Probe Design
Basic Considerations
CAUTION: DMMs may not be particularly forgiving of voltages on their inputs exceeding their
specifications. Autoranging DMMs may be even more likely to blowout as they are selecting the correct
range - if there even is one. Depending on your electrical and mechanical components, the chance of
excess voltage due to arc-over, leakage, or component breakdown may be a major consideration. My
analog VOM has survived many close encounters with HV. You should not assume the same for the
typical low cost or even expensive DMM. There is a reason for the high cost of commercial HV probes -
these kinds of factors are incorporated (hopefully) in their design.
A simple high voltage probe for a DMM or VOM may be constructed from a pair of resistors. This is
suitable for DC measurements but without compensation, will have a unknown AC response due to the
very high impedance and stray capacitance forming a filter - low pass or high pass depending on the
amount of stray capacitance and input capacitance of your meter or scope. However, this simple design
Page 3 of 12
Basics of High Voltage Probe Design
8/10/2004
http://repairfaq.ece.drexel.edu/sam/hvprobe.htm is sufficient for the majority of consumer electronics work which are mostly DC measurements. I have
not characterized the AC response of this probe design. However, if there is AC riding on your high
voltage, it may mess up your readings if there is no compensation provided as it may act as a high pass
filter.
To design the voltage divider, the input impedance of the meter must be taken into account. There is a
minor but significant difference between DMMs and VOMs.
DMM: Z-in is usually constant, often 10M ohms.
VOM: Z-in is the voltage range (full scale) times the ohms/volt rating of the meter.
Here is the basic circuit:
High Voltage <------/\/\/\/\/\---------+-------------> + to DMM/VOM
R1 | |
\ \
R2 / R3 /
\ \
/ /
| |
Ground Clip <-------------------------+-------------> - to DMM/VOM

R1 together with R2||R3 form a voltage divider where R3 is the internal resistance of the DMM or VOM
on the scale for which the probe is designed.
While R2 is not strictly needed, it is recommended that it be included and approximately equal to the Z-
in of the meter on the scale you will be using. The reason to include R2 is to insure that high voltage
never can reach the meter. The ground clip should be securely connected to the metal chassis of the
device being tested - the frame of a microwave oven or CRT grounding/mounting strap of a TV or
monitor - before it is powered up. Both R1 and R2 should be located in the probe head.
The only difficult part is locating a suitable resistor for R1 that has high enough resistance and
physically is long enough such that arc-over is avoided. The only difficult part is locating a suitable
resistor for R1 that has high enough resistance and physically is long enough such that arc-over is
avoided.
Caddock
,
OhmCraft
,
Victoreen
, and
Vishay
are among the major companies that manufacture
suitable resstors. But don't expect them to pay much attention to you for an order of 5 resistors!
However, it may be possible obtain free samples if you explain what you're doing - and their lawyers
don't get involved! If this doesn't work out, electronics surplus outfits occasionally come up with odd
lots of strange components such as these and they even show up on eBay from time-to-time.
The high value high voltage resistor can also be constructed from several equal lower value resistors in
series if they are all approximately the same size. Another possibility is salvaging the focus divider
networks from dead flybacks or TV/monitor voltage multiplier assemblies. Even if the unit was
discarded as being faulty, where there are no internal shorts in the HV rectifier or resistive network
itself, the entire unit can be used int