Convert x10 probe to x20

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
Hello all,
My oscilloscope is and old Hamag with input imedence of 1Mohm and capacitance of 25pF.
I have various x10 probes and a x100 probe, but I would like something in the region of x20.
Can I simply use a series resistor, and if so how do I calculate it?
Thank you,
Mark
 

MrChips

Joined Oct 2, 2009
21,818
This is what a x10 probe would look like:

1600628273747.png

9MΩ in series along with the 1MΩ input of the oscilloscope will give x10 attenuation.
Add 10MΩ to the probe (with capacitor for compensation) will give x20 attenuation.
 

SamR

Joined Mar 19, 2019
2,529
Would it be safe to do so? You would still be limited by the design specification MAX Voltage for the probe that should not be exceeded even with the added attenuation. What are you trying to measure and why do you want 20X?
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
Thank you MrChips.
When you say add 10MΩ, do you mean add it to the 9MΩ to give a total of 10 + 9 + 1 (existing at scope) = 20MΩ ?
 
@MrChips

I think you need to add 11M in series, not 10. 1/(1+9) = 1/10.

Add 10MΩ to the probe (with capacitor for compensation) will give x20 attenuation.
(1/(9+11) = 1/20 the 1 is the scope. the 9 is existing in the probe. 11M is what you add in series.

NOTE: I agree now, 11M is the wrong answer.

You could put that in a small box and attach to the probe inputs. Pomona makes one that will take 4-bolt BNC connectors, Male and Female. Don;t add cable. That adds capacitance.

You may not need any extra capacitance.

It's actually an impeadance divider. the scope is 1M||22pf then there is cable capacitance. You want the divide to be an impeadance divider.

Bandwidth MIGHT change, not sure. Bandwidth goes down without compensation.
 
Last edited:

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
Would it be safe to do so? You would still be limited by the design specification MAX Voltage for the probe that should not be exceeded even with the added attenuation. What are you trying to measure and why do you want 20X?
I need to look at 240vac. My x10 probe is rated at 600vp-p so you have a good point. It would exceed 240vrms.
 

SamR

Joined Mar 19, 2019
2,529
Std 1X is 1MΩ, 10X is 10MΩ, 20X would be 20MΩ. The scope input is internally 1MΩ and the probe for 10X switched in series a 9MΩ resistor for a total of 10MΩ. Then add 10MΩ to reach 20X.
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
80
I think you need to add 11M in series, not 10. 1/(1+9) = 1/10.

(1/(9+11) = 1/20 the 1 is the scope. the 9 is existing in the probe. 11M is what you add in series.

You could put that in a small box and attach to the probe inputs. Pomona makes one that will take 4-bolt BNC connectors, Male and Female. Don;t add cable. That adds capacitance.
Thank you, so I'll add 11MΩ.
 

Tesla23

Joined May 10, 2009
406
If we assume that the original x10 probe input has an input impedance of 10M || 15pF, then just adding a series 10M will make it x20 at DC, but without compensation the -3dB frequency will be about 2kHz. If this is OK for your application then proceed, otherwise add 15pF or so across your 10M. (Use a trimmer and adjust it using the CAL waveform on the 'scope.)
 

tautech

Joined Oct 8, 2019
124
I need to look at 240vac. My x10 probe is rated at 600vp-p so you have a good point. It would exceed 240vrms.
Just one word, WHY ?
A 10x probe hasn't sufficient insulation and also CAT rating to safely do this so it's wisest to use a 100x probe.
Even your old Hameg should have sufficient sensitivity to work just fine with a 100:1 probe.
 
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