Updated: 7 November 2004
An adjustable, DC, current-limited, breakdown
voltage tester [BVT] -- also known as a high-potential
tester, or "hi-pot" -- can be used for non-3destructively measuring
the voltage withstanding ability of relays, capacitors, transistors
and silicon rectifiers. A BVT can be used to test amplifier-tubes for
the presence of gas, and oxide-cathode type [8877, et cetera]
amplifier-tubes for the presence of sputtered gold. A BVT can help
you identify a tube that could seriously damage your amplifier if you
inadvertently plugged it in.
For most amateur radio applications, a maximum voltage capability of
15kV is adequate. For testing commercial components, 40kV or more is
usually needed.
Troubleshooting an RF power amplifier without a BVT is difficult and
problematic. Building an RF power amplifier without a BVT is
risky.
An adjustable transformer, T1, delivers
approximately 0-132vac to the primary of the HV transformer, T2. A
120v, incandescent lamp is used to limit the primary current in T2. A
full-wave voltage-doubler rectifier with capacitor filters is used to
produce the DC. R8 protects the device under test [DUT] from
excessive current during testing. The typical current during a test
is 2 to 100 uA.
- A voltage-quadrupler could be used instead of the voltage-doubler.
Quadruplers have one advantage and two trade-offs: Only half as much
secondary voltage is needed from T2. And four filter capacitors are
required instead of two. If a voltage-quadrupler is shorted, two of
the four filter capacitors discharge through two of the rectifiers
and the transformer secondary winding. This can damage rectifiers
and/or the transformer. - If a quadrupler is used, it is appropriate
to use long leakage path current limiting resistors in series with
all of the capacitors and a robust MOV across the transformer
primary.
Sources of T2 : A 50 - 60 Hz HV transformer from a: junked
copy-machine, a neon sign, or an old oscilloscope should work
acceptably.
Construction Notes - Due to the high
voltage levels, the ideal material for constructing a BVT is not
metal. Polyurethane varnished plywood, Lexan®
[polycarbonate], acrylic, or fiberglass are satisfactory
choices. A metal cabinet with a non-conductive front-panel and
chassis can also be used.
It is not good engineering practice to pack the components in a BVT
too tightly. In dry air, at sea-level, 1kV will jump about 1mm. In
moist air and at elevations greater than sea level, less voltage is
needed to jump the same distance.
To minimize corona current-leakage into the air, solder connections
that are at high-potential should be rounded "blobs." Avoid using
sharp points on conductors. Corona-effect is easy to check for: In a
darkened room, turn the voltage up to maximum and watch for blue
lights accompanied by a hissing sound and the odor of ozone.
[O3]. If needed, apply more solder and/or smooth trouble
spots with a file. Mounting components too close together can also
cause corona problems.
Current Limiter/Detector--R8/M2 is at high potential--so these
components should not be placed close to M1, which is at
ground/common potential. M2 and R8 should be mounted on an insulated
panel with no low potential conductors nearby. The current
limiter/detector is wired to banana-jacks so that it can be connected
to the positive or to the negative output terminal as needed.
D1 and D2 can be made from a series of individual 1kV diodes or a
series of multi-diode HV low-current rectifier units [a.k.a.
focus rectifiers]. So-called "equalizing-resistors" and
"equalizing-capacitors" should NOT be connected across modern
rectifier diodes. Use diodes with similar ratings and they will
equalize themselves.
The basic technique behind non-destructive voltage
testing is: slowly increase the applied voltage across the DUT until
a small current flows through the device. Record the V and stop the
test. If the voltage were allowed to increase beyond this critical
point, the component could be damaged. The applied voltage usually
does not need to remain on the DUT for any longer than it takes to
read the voltmeter, M1, and the current-meter, M2. For testing
devices with lower breakdown voltages, only half of the doubler
should be used. One end of the DUT is connected to Common instead of
to one of the HV OUT terminals. Thus, the voltage readings on M2 must
be divided in half. If the breakdown voltage of the DUT is expected
to be less than 1000v, you may want to connect a DMM from the output
side of R8 to Common so that the voltage can be read more accurately.
Be careful. Most DMMs can not safely tolerate more than 1000v without
using a voltage multiplier probe.
Vacuum-capacitors and vacuum-relays should be tested for gas before
installation. I have seen new, unused vacuum-devices that were
defective due to very slow air leaks. Such leaks typically show up
several years after manufacture. In use, a gassy glass-envelope
vacuum-relay can often be identified because the ionized air inside
emits a blue light around the open contacts. However, the ionization
in a defective glass-envelope vacuum-capacitor is usually deep inside
the concentric meshed plates and can not be seen. Variable
vacuum-capacitors should be tested with the plates fully meshed. When
a vacuum-capacitor goes bad in an amplifier, reduced PEP in one
result. However, other things can cause the same problem. When
troubleshooting an amplifier, it is a good idea to routinely test all
of the vacuum capacitors and vacuum relays with a BVT.
Vacuum-capacitors that have been in storage for a long time may
develop "whiskers"--microscopic filaments of copper. Sure, it sounds
weird. This anomaly causes the breakdown-voltage to initially be
lower than normal. It is possible to burn-off these whiskers. During
the process, the capacitor may self discharge--as indicated by a
"tink" sound. Repeatedly forcing the capacitor to self discharge will
result in a decrease in breakdown voltage. Five tinks is usually a
good point to stop.
Semiconductor testing is important when building HV power supplies
that contain groups of series-connected rectifier diodes. If one or
more of the rectifiers fails during use, it could start a
"domino-effect" and short-out the other rectifiers in that group.
Shorted rectifiers deliver AC to the filter capacitors. This is not a
serious problem for non-polarized capacitors. However, with polarized
electrolytic capacitors, even a small episode of reverse-current can
damage the capacitor--or even cause it to explode. Thus, one
defective 10¢ diode can trigger the destruction of many dollars
worth of good parts. It is better to cull-out bad parts prior to
construction.
Silicon-rectifiers: Each diode should be tested individually where
possible. Increasing reverse voltage is applied to the DUT until a
leakage-current of approximately 2uA is detected. [high-current
diodes can withstand more reverse current] At this point it is
important to observe the current meter. If the leakage-current
randomly fluctuates without adjusting the voltage, the diode has a
manufacturing defect and it should be discarded. Since it is
difficult to mark each diode, I usually sort diodes into labeled
drawers according to PIV. Individual diodes which test greater than
about 1.3kV should be viewed with suspicion because this usually
indicates a doping problem. A forward voltage-drop test at the rated
current can be used to discover whether such a diode has a problem.
At 1A, the forward voltage drop in a silicon PN rectifier junction
should be less than 0.9V.
Transistors are now made with voltage capabilities that are similar
to silicon-rectifiers. Some transistors are rated at 1500V. Testing
these devices is similar to testing silicon-rectifiers--except that a
resistor of roughly 100 Ohm should be connected from the base to the
emitter, or from the gate to the source.
Air-variable capacitors: Identifying too-closely spaced points that
need realignment is easy with a voltage breakdown-tester.
Gridded power tubes need a good vacuum in order to function properly.
A vacuum test is made with no filament voltage applied. HV is applied
between the anode and a grid. A healthy 3 - 500Z will typically
exhibit less than 10uA of current-leakage at double the rated
anode-voltage.
The BVT can also be used to check the alignment of the filament in a
3-500Z. When its filament is cold, a healthy 3-500Z can withstand 7 -
8 kV between its grid and filament. If the filament is not concentric
with the grid, the breakdown voltage will be lower. This problem is
usually brought about by intermittent VHF parasitic oscillations--a
condition that generates a large pulse of cathode and grid currents.
The principle is simple: a flow of electrons is always accompanied by
a magnetic force. The larger the current, the stronger the force.
During an intermittent VHF parasitic oscillation, the magnetic force
is sometimes strong enough to bow the hot, tungsten filament wire
helices toward the grid. If the [cold] filament to grid
withstanding voltage of a 3 - 500Z is less than 6kV, a grid to
filament short may occur when the tube is hot.
Testing for parasitic damage in 8874s, 8877s, 3CX800A7s and other
oxide-cathode type tubes:
Such tubes have the following things in common: indirectly heated
strontium-oxide/barium-oxide cathode, high gain, ultra high frequency
capability, and gold-plated grid. The oxide-coating is an efficient
electron-emitter. The gold plating helps to reduce primary
electron-emission from the grid. This improves performance. There is
a tradeoff. If the gold evaporates [sputters], the loose gold
particles can cause serious problems.
In a vacuum, gold does not begin to evaporate unless it is heated to
more than 1000ºC [1832ºF]. Heating the entire mass
of the grid to >1000ºC requires more energy than is
available. However, if there were a way to heat the gold plating,
without heating the entire grid, gold evaporation would be possible.
VHF/UHF energy has a substantial "leg up" when it comes to heating
metal plating. VHF/UHF current travels exclusively on the surface.
During an intermittent VHF parasitic-oscillation, the VHF
grid-current can become so large that the surface of the gold plating
briefly becomes hot enough to evaporate gold. The resulting gold
vapour cloud can then move about freely inside the envelope. As the
gold cools, it solidifies into tiny balls. In a low power microscope,
they look like dew drops of water on the petal of a flower. Some of
the evaporated gold lands on the emissive coating. Gold poisons the
cathode's electron-emitting ability--causing a reduction in
anode-current and power output. [A copy of a letter from Eimac
describing this phenomenon is available on request to this
author.] Evaporated gold can also land on the inside of the
ceramic anode-insulator. This can cause arcs between the anode and
the adjacent (grounded) grid-ring. An arc is most likely during the
crest in the anode-voltage swing when the tube is not conducting--as
the HF tank-circuit/flywheel swings to its positive voltage peak. At
this instant, the peak anode-voltage is normally about double the
positive supply voltage. If the insulating ability of the ceramic has
been compromised by the presence of gold, trouble is probable. When
the anode arcs to the grounded grid, the HV-positive circuit is
virtually grounded. Thus, the HV-negative circuit tries to rise above
ground to the voltage in the HV filter capacitors. This typically
causes damage to components in the HV negative circuit. A common
problem is an arc between the cathode and the heater or an arc
between the cathode and the grid. Test for compromised voltage
withstanding ability between the heater and the cathode--and a burned
out heater.
The Loose Gold Test (W6IHA): There is a simple test that can
confirm the existence of loose gold particles without sawing open the
suspect amplifier-tube. The only piece of equipment needed is a BVT.
The principle behind the test: Like charges repel and unlike charges
attract.
Procedure: Remove the amplifier-tube from the amplifier. The positive
and negative DC-voltages that are applied between the anode and the
grid should be two to three times the operational anode-voltage.
Loose gold particles can be moved around by changing the polarity of
the anode-voltage. If the anode is positive, the gold particles are
attracted toward the anode-insulator. This causes the indicated
leakage current to increase. If the anode is made negative, the gold
particles are repelled and the leakage current will decrease. If the
leakage current is equal with either polarity, the presence of gas is
indicated.
Another method of confirming the presence of loose gold particles is:
apply positive anode-voltage and record the leakage current. Shut
down the BVT and, with the tube vertical and the anode-cooler up,
repeatedly bang the anode-cooler, both vertically and horizontally
with a approximately 2oz. soft face hammer. This will cause some of
the loose gold particles to fall to the bottom where they will be
less attracted by the positive voltage at the anode. Keep the tube
vertical. Re-apply positive anode-voltage. If the leakage current
decreases, you have loose gold and you are making progress. If the
leakage current does not decrease, either you need to bang harder or
no more improvement is possible. This procedure has been used by some
amplifier owners to get more operating hours out of gold sputtered
tubes. Banging also causes the gold to fall off the cathode coating.
This increases electron-emission. However, if the tube is turned
upside down, the loose gold becomes redistributed and the banging
process must be repeated to move the errant gold to a safer
place.
CIRCUIT ALTERATIONS:
Although T2 in the diagram has one side of the secondary grounded
internally, some transformers have no secondary terminal grounded.
This is a small advantage since it allows the BVT outputs to float if
needed.
There are many things that can be altered to accommodate available
parts--or to change the voltage capability of the tester. Ohm's Law
is the most important guide.
The wattage of the current-limiting lamp should be approximately
twice the wattage or VA rating of the HV transformer.
It is important to note that resistors have a dissipation-rating and
a voltage-rating. The voltage-rating always takes precedent over the
sometimes-phony dissipation-rating. For example: A 5.1M Ohm, 2W,
±5% resistor has a dissipation-rating that could lead a designer
to assume that the resistor could safely dissipate 2W. Using Ohm's
Law, E = [PR] 0.5 = [2W x 5.1M]
0.5 = 3193V at 2W. However, there's a catch. The resistor
is limited to 500V maximum by the technical specifications. This
means that the "2W" max. resistor is really {P = E2 / R, =
500v x 500v / 5.1M Ohm =} a 0.049W max. resistor! It pays to read the
specifications. High voltage resistors have a long, spiral,
resistive, deposited film that is designed to handle voltage.
- R8: The actual value of R8 is not critical. If you can not locate a
single resistor for R8, there is a commonly available spiral-film
resistor, that is marketed by ECG-Phillips, which has the right
properties. These resistors have a tolerance of ±2% and a
dissipation rating of 2W. They can be series wired on a perfboard to
build a satisfactory R8. One can be used for R9--whose function is to
help limit reverse. current through the DUT.
- C1 AND C2: It might seem necessary to use capacitors with a Working
Voltage [WV] rating that is equal or greater than the actual
V that will be encountered. However, voltage testing components is an
intermittent application that produces virtually zero ripple-current.
WV is a continuous duty rating--usually at an elevated temperature
and for a substantial amount of ripple current.
For BVT intermittent service at normal room temperatures, a WV-rated
capacitor can usually be used at 1.5 times its rating with no
difficulty. Sprague Vitamin-Q capacitors are very conservatively
rated. They can usually be used at double their WV rating in BVT
service.
If you are going to use tubular glass-case capacitors with metal
screw end-connections, it is important to put a stop-nut between the
capacitor and the mounting hole. The stop-nut prevents the screw
[which is soft-soldered to the end-cap] from being pulled out
of the capacitor when the outside fastening nut is tightened. Four
nuts are required to properly mount each capacitor. It is not
appropriate to mount this type of capacitor by only one end.
Metal-cased capacitors should be mounted without metal clamps on an
insulated surface. Use silicone rubber adhesive for mounting.
OPERATING NOTES:
Reasonable care should be used with a BVT. Although the
steady-current capability of this BVT is quite limited--even without
R8--a substantial [as in rudely awakening] current pulse can
be delivered by a charged capacitor.
When testing with voltages above about 8kV, it is possible to have a
false current indication caused by corona from sharp points. To check
for this problem, disconnect one end of the DUT, increase the voltage
and observe the current reading on M2. . . Rich, AG6K
(805)-386-3734.