Test and Measurement Forum

Thanks for all the info on input protection.
I also work on old valve equipment, so I have made an HV box with a series 630V .22 capacitor, followed by two back-to-back zeners. They should have been 20V, but are currently 10v (serious over-protection!) but they were the only ones in my box of zeners. This HV box is meant to be used always with a 10:probe.

It is obvious, (now that I have found the manual) that one can raise these zeners up to 100V. Any charging current for the capacitor will come through a limiting resistor in the probe of 9M which should clamp the input fairly well.

A further project is to add a switchable 100k to ground at the input of the HV box. Together with the 10:1 probe, this will give 100:1 attenuation, and reduce the Dc voltage to one hundredth of the probe-tip value.

Paul Galpin

Having thought about this further, I think that I will go with the following (attached) circuit.

To explain how it works.

The 10M resistor R1 will bring the dc voltage at the probe tip down to just over half its value, removing some of the stress on the blocking capacitors in the box, and in the 3206. I seem to remember that the dc voltage distribution between two capacitors in series is inversely related to the sizes, so the smaller one will take more than half. It also gives a leakage path to ground for any charge left on the Capacitor in the box. However, the attenuation for ac will be the normal x10, as C1 can be considered to be zero impedance.

The switched 100k (R2) helps reduce the dc even further, and it will bring the ac attenuation down to 100x, useful if your circuit and gain allow you to do this.

The Zeners, Z1 and Z2, prevent the voltage going into the 3206 from rising above the permitted values. By making them no greater than 30, this can be used on the External Sync input, which is only protected up to 30V.

Some assumptions have been made, namely that the probe resistor is exactly 9M, and that the input circuit of the 3206 can be represented by one C and one 1M resistor. The effect of the 10M resistor R1 should be negligible on the ac voltage passed into the 3206.

The value of R2 may have to be played around with, to get a reasonably accurate 100:1 attenuation.


Any comments?

Hi Paul,
I am afraid that your circuit leaves out the effect of some capacitors present but not shown. There is a 4pF variable capacitor present in the 10:1 probe, parallel with the 9M ohm resistor. There is also a capacitance of about 12pF in parallel with the input 1M ohm to ground of the scope. The variable capacitor is used to trim the 10:1 probe AC response for the correct shape of a square wave input:-
http://www.picotech.com/applications/ho ... robes.html
Your add-on circuit will stop any such trimming and your AC response will be very poor.
Regards,
PeterF.