# Convert x10 probe to x20

#### 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:

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?

#### 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 ?

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,196
@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.

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#### 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.

#### 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.

#### Danko

Joined Nov 22, 2017
988
(1/(9+11) = 1/20 the 1 is the scope. the 9 is existing in the probe. 11M is what you add in series.
Thank you, so I'll add 11MΩ.
Wrong!

Now it is:
R_scope/(R_scope+9MΩ) = 1/(1+9) = 1/10.

For 1/20 it should be:
R_scope/(R_scope+9MΩ+10MΩ) =1/(1+9+10) = 1/20.

So resistor 10MΩ should be added.

#### Mark Flint

Joined Jun 11, 2017
80
So resistor 10MΩ should be added.
OK, cool - I appreciate the help. Thanks.

#### 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.)

#### ronsimpson

Joined Oct 7, 2019
907
Turn the red knob and reduce the gain to 50% and you have the same effect.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,196
Yep, definitely made a mistake, sorry. it's 10M. I'm having a BAD day.

See this calculator: http://www.calculatoredge.com/electronics/voltage divide.htm

if you use Vin of 10 (/10) or 20 (/20) and Vout of 1 and Rb=1000000 (1M) you will get 9M and 19 M respectively . You already have 9, so add 10M to get 19M for 20:1 or a 1/20 divider.

Really sorry guys!

#### 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.

#### crutschow

Joined Mar 14, 2008
25,367
I need to look at 240vac. My x10 probe is rated at 600vp-p
A 10 megohm resistor in series with the probe will reduce the peak probe voltage by 1/2.

#### SamR

Joined Mar 19, 2019
2,529
Or a 10X probe in series with a 10MΩ shunt resistor to the device being measured to give 20X.

#### crutschow

Joined Mar 14, 2008
25,367
Or a 10X probe in series with a 10MΩ shunt resistor to the device being measured to give 20X.
How can it be both series and shunt?

#### SamR

Joined Mar 19, 2019
2,529
Ooops...