making HV probes and using old TV probes

Old "TV set" HV probes can be converted for use as HV scope and modern digital meter probes. These usually have a good quality HV resistor of about 1GOhm (1 GigaOhm) and have a meter on them scaled for 30-40KV. They are also somewhat cheap because CRT TV sets are not as much in use as in the past.

The resistor's insulation is able to stand the full voltage continuously however the wattage may not be enough to do so. Many TV probes carried a warning that they were not for continuous use.

40KV -> 1GOhm = 40uA = 1.6W

1.6W does not seem like much but it's very important to avoid a lot of temerature changes in any resistors being used for calibrated functions.

Fortunately in most amateur radio work, the DC voltage is 5KV or less and the plate transformer no more than 10KV peak voltage, so these resistors can be used continuously.

I have converted two of them and they work great but there is a boogeyman in it.

Most scopes have a 1 MegOhm input resistance. Using a 1GOhm resistor in series makes for a 1000:1 voltage multiplier. It is convenient then to put the scope on the 1V per cm scale and use the probe to measure HV circuits. the 8cm scope screen becomes an 8KV measuring tool.

Set the sensitivity to 100mV per cm and the screen measures 800V. This is great, considering most scope probes are not rated above 500V, or even 100V. Modern very inexpensive probes have lower ratings. HV ratings are just not needed for most electronics today.

Modern scopes with solid state inputs and very small low power voltage dividers are sensitive to damage from high voltage. The old tube type scopes are much more durable, or in other words, forgiving of mistakes, when checking high voltage circuits.

About the boogeyman: Using just a 1G resistor with the cold end open is safe enough for human beings, but at zero current there is no voltage drop and the full voltage appears at the cold end, even though it is more like a static electricity charge at this point.

All is well as long as it is firmly connected to the scope's 1 Meg input. In case of a bad connection there, or in case of connection cuts while switching scope ranges, the full voltage can instantaneously appear. This can blow the scope input just as ESD can blow a semiconductor. The chance is remote but scopes are costly and sometimes hard to repair. Even the vacuum tube scope can be damaged if on the AC input, because its DC blocking capacitor is usually not rated above 1KV.

A good probe design should provide a definite path to ground from the cold end of the HV probe by means of a shunt resistor, whether or not the measuring instrument is attached so that's what we did. There's enough good advice to do this floating around that it didn't take long to see the good sense.

The probe circuit is not compensated, just resistive but it is good enough for DC and audio modulation use.

HV resistor R1
shunt resistor R2
scaling resistor R3
instrument resistance R4

---R1-------------R2-----GND
|
|----R3----instrument----GND