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x100 'scope probe

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George Herold
Guest

Tue Sep 25, 2018 3:45 pm   



I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Guest

Tue Sep 25, 2018 4:45 pm   



You need a 90 meg if coming off an X 10 probe, 99 to the
BNC.I think.

Off the top of my head I think you'll be down to 1/10th the upper limit bandwidth, and on AC coupling it will take much longer to settle to the center is there's DC on the input. Also not that in most scope front ends that will NOT give you 10 X the max DC input.

The sell X 100 probes, a bit more money per everything else. I used one for certain things when faced with a scope that lacked 20V/div. Yeah, they gave you more at the low end but took away from the high end. No more looking at the collector of an HOT/LOPT with a X 10 probe.

default
Guest

Tue Sep 25, 2018 4:45 pm   



On Tue, 25 Sep 2018 07:34:28 -0700 (PDT), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Well if the intent is to measure high voltages you'd want a resistor
or series of resistors to avoid "flash-over" of the outside of the
resistor with dirt or moisture in the air. If you are measuring
waveforms, the coaxial cable's capacitance will distort the wave form
because the resistor and cable form a R-C integrator.

https://www.electronics-tutorials.ws/rc/rc-integrator.html

Normally there's a variable cap in the probe that can neutralize the
cable capacitance.

Tom Gardner
Guest

Tue Sep 25, 2018 4:45 pm   



On 25/09/18 15:34, George Herold wrote:
Quote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance
to form the lower leg of the potential divider, then consider
what happens if you turn the scope to AC input coupling. The
*entire* input voltage appears across the scope's AC coupling
capacitor.

That can and should be avoided by having a probe with the
potential divider made from two resistors, one for the upper
leg and the other is (in parallel with the scope/s input)
the lower leg. Then if the scope is in AC mode the voltage
"seen" by the scope will be slightly wrong, but the voltage
will be limited by the potential divider.

George Herold
Guest

Tue Sep 25, 2018 4:45 pm   



On Tuesday, September 25, 2018 at 10:45:57 AM UTC-4, jurb...@gmail.com wrote:
Quote:
You need a 90 meg if coming off an X 10 probe, 99 to the
BNC.I think.

Off the top of my head I think you'll be down to 1/10th the upper limit bandwidth, and on AC coupling it will take much longer to settle to the center is there's DC on the input. Also not that in most scope front ends that will NOT give you 10 X the max DC input.

The sell X 100 probes, a bit more money per everything else. I used one for certain things when faced with a scope that lacked 20V/div. Yeah, they gave you more at the low end but took away from the high end. No more looking at the collector of an HOT/LOPT with a X 10 probe.


Right I was going to stick it one the end of a x10 probe... I don't
really care if it's a x110, probe... mostly want to look at what might be
a high voltage without blowing up the 'scope.
(maybe it's a bad idea.)

George H.

George Herold
Guest

Tue Sep 25, 2018 4:45 pm   



On Tuesday, September 25, 2018 at 10:50:39 AM UTC-4, default wrote:
Quote:
On Tue, 25 Sep 2018 07:34:28 -0700 (PDT), George Herold
gherold_at_teachspin.com> wrote:

I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.

Well if the intent is to measure high voltages you'd want a resistor
or series of resistors to avoid "flash-over" of the outside of the
resistor with dirt or moisture in the air. If you are measuring
waveforms, the coaxial cable's capacitance will distort the wave form
because the resistor and cable form a R-C integrator.

https://www.electronics-tutorials.ws/rc/rc-integrator.html

Normally there's a variable cap in the probe that can neutralize the
cable capacitance.


I was going to ignore the capacitance... a 1 ms time constant would be
OK... (100 meg ohm * ~16 pF ~ 1.6ms)

George H.

George Herold
Guest

Tue Sep 25, 2018 4:45 pm   



On Tuesday, September 25, 2018 at 11:06:40 AM UTC-4, Tom Gardner wrote:
Quote:
On 25/09/18 15:34, George Herold wrote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance
to form the lower leg of the potential divider, then consider
what happens if you turn the scope to AC input coupling. The
*entire* input voltage appears across the scope's AC coupling
capacitor.

Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope? (putting my DMM across my 'scope input
I measure 1 meg ohm for both DC and AC coupling.)

George H.
Quote:
That can and should be avoided by having a probe with the
potential divider made from two resistors, one for the upper
leg and the other is (in parallel with the scope/s input)
the lower leg. Then if the scope is in AC mode the voltage
"seen" by the scope will be slightly wrong, but the voltage
will be limited by the potential divider.


Tom Gardner
Guest

Tue Sep 25, 2018 8:45 pm   



On 25/09/18 16:20, George Herold wrote:
Quote:
On Tuesday, September 25, 2018 at 11:06:40 AM UTC-4, Tom Gardner wrote:
On 25/09/18 15:34, George Herold wrote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance
to form the lower leg of the potential divider, then consider
what happens if you turn the scope to AC input coupling. The
*entire* input voltage appears across the scope's AC coupling
capacitor.

Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope? (putting my DMM across my 'scope input
I measure 1 meg ohm for both DC and AC coupling.)


Not on the scopes I'm familiar with.

The BNC is connected to the capacitor, and the other side
of that is connected to the 1Mohm (not 10Mohm, doh!)
vertical sensitivity attenuators.

In DC mode the capacitor is shorted out so the BNC is
connected to the attenuators. In AC mode the BNC is
"floating".


Quote:

George H.
That can and should be avoided by having a probe with the
potential divider made from two resistors, one for the upper
leg and the other is (in parallel with the scope/s input)
the lower leg. Then if the scope is in AC mode the voltage
"seen" by the scope will be slightly wrong, but the voltage
will be limited by the potential divider.


amdx
Guest

Tue Sep 25, 2018 8:45 pm   



On 9/25/2018 9:34 AM, George Herold wrote:
Quote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.

I built an attenuator to measure some high voltage spikes 25 or 30
years ago.
I still have it, just checked the values to put on the picture in the
link below. I don't recall the attenuation, It might make a 10x be a 20x
or a 40x. The values make me think 20x but the cap has me not sure.
I was able to calibrate it using the scope calibrator for a good
square wave, but the shield was important to allow that.
I was always a little concerned for the scope probe coax arcing, but
at 800 volts, it never did.
> https://www.dropbox.com/s/iwk8yzmkx766ls6/Probe%20Attenuator.jpg?dl=0
Note: I put a T on the scope, then the probe off one side and the
attenuator off the other side.
If I get some time I'll set it up and check the attenuation.

Mikek


Guest

Tue Sep 25, 2018 8:45 pm   



Quote:
"Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope?"


Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example, in

http://bama.edebris.com/manuals/tek/7a18/

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent would form a capacitive divider and with even miniscule leakage of C 10 or equivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope's input cap.

George Herold
Guest

Tue Sep 25, 2018 9:45 pm   



On Tuesday, September 25, 2018 at 3:03:26 PM UTC-4, jurb...@gmail.com wrote:
Quote:
"Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope?"

Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example, in

http://bama.edebris.com/manuals/tek/7a18/

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.

Huh. OK... I didn't look at the schematic, but my TEK1001 floats the input
at AC, (as you say.) The new Keysight reads 1.0 meg ohm at DC 1.0 meg ohm at
AC. I put in a DC voltage through a 1 meg resistor, and measured the
voltage at the 'scope input (AC and DC). It was 1/2 the applied voltage.
The same in both cases.

So there is at least one 'scope that is different.
(I never knew... thanks)
Quote:

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent would form a capacitive divider and with even miniscule leakage of C 10 or equivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope's input cap.

Right, I've done that... I wanted DC
(well I don't need it any more.. voltage was not as high as I thought.)

George H.

George Herold
Guest

Tue Sep 25, 2018 9:45 pm   



On Tuesday, September 25, 2018 at 3:25:36 PM UTC-4, Tom Gardner wrote:
Quote:
On 25/09/18 16:20, George Herold wrote:
On Tuesday, September 25, 2018 at 11:06:40 AM UTC-4, Tom Gardner wrote:
On 25/09/18 15:34, George Herold wrote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor. Any thing I should look out for?

George H.


Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance
to form the lower leg of the potential divider, then consider
what happens if you turn the scope to AC input coupling. The
*entire* input voltage appears across the scope's AC coupling
capacitor.

Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope? (putting my DMM across my 'scope input
I measure 1 meg ohm for both DC and AC coupling.)

Not on the scopes I'm familiar with.

The BNC is connected to the capacitor, and the other side
of that is connected to the 1Mohm (not 10Mohm, doh!)
vertical sensitivity attenuators.

In DC mode the capacitor is shorted out so the BNC is
connected to the attenuators. In AC mode the BNC is
"floating".


Thanks Tom, I need to correct what I said about the keysight scope
(DSOX 1102G) On AC it's 1 meg ohm for gains of 500 mV/ div and less
and 1.2 Meg at 200 mV and higher.

George H.
Quote:



George H.
That can and should be avoided by having a probe with the
potential divider made from two resistors, one for the upper
leg and the other is (in parallel with the scope/s input)
the lower leg. Then if the scope is in AC mode the voltage
"seen" by the scope will be slightly wrong, but the voltage
will be limited by the potential divider.


John Larkin
Guest

Tue Sep 25, 2018 11:45 pm   



On Tue, 25 Sep 2018 08:07:40 -0700 (PDT), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
On Tuesday, September 25, 2018 at 10:45:57 AM UTC-4, jurb...@gmail.com wrote:
You need a 90 meg if coming off an X 10 probe, 99 to the
BNC.I think.

Off the top of my head I think you'll be down to 1/10th the upper limit bandwidth, and on AC coupling it will take much longer to settle to the center is there's DC on the input. Also not that in most scope front ends that will NOT give you 10 X the max DC input.

The sell X 100 probes, a bit more money per everything else. I used one for certain things when faced with a scope that lacked 20V/div. Yeah, they gave you more at the low end but took away from the high end. No more looking at the collector of an HOT/LOPT with a X 10 probe.

Right I was going to stick it one the end of a x10 probe... I don't
really care if it's a x110, probe... mostly want to look at what might be
a high voltage without blowing up the 'scope.
(maybe it's a bad idea.)

George H.


Amazon has decent 100 MHz 100x scope probes for around $35.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

John Larkin
Guest

Tue Sep 25, 2018 11:45 pm   



On Tue, 25 Sep 2018 13:04:38 -0700 (PDT), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
On Tuesday, September 25, 2018 at 3:03:26 PM UTC-4, jurb...@gmail.com wrote:
"Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope?"

Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example, in

http://bama.edebris.com/manuals/tek/7a18/

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.
Huh. OK... I didn't look at the schematic, but my TEK1001 floats the input
at AC, (as you say.) The new Keysight reads 1.0 meg ohm at DC 1.0 meg ohm at
AC. I put in a DC voltage through a 1 meg resistor, and measured the
voltage at the 'scope input (AC and DC). It was 1/2 the applied voltage.
The same in both cases.

So there is at least one 'scope that is different.
(I never knew... thanks)

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent would form a capacitive divider and with even miniscule leakage of C 10 or equivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope's input cap.
Right, I've done that... I wanted DC
(well I don't need it any more.. voltage was not as high as I thought.)

George H.


My Rigol measures 1M input on a DVM (to 5 decimal places) on either AC
or DC coupling.

Set to 50 ohms, it measures 50.37 whether AC or DC coupled.

Good to know, I guess.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

George Herold
Guest

Wed Sep 26, 2018 12:45 am   



On Tuesday, September 25, 2018 at 5:53:50 PM UTC-4, John Larkin wrote:
Quote:
On Tue, 25 Sep 2018 13:04:38 -0700 (PDT), George Herold
gherold_at_teachspin.com> wrote:

On Tuesday, September 25, 2018 at 3:03:26 PM UTC-4, jurb...@gmail.com wrote:
"Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope?"

Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example, in

http://bama.edebris.com/manuals/tek/7a18/

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.
Huh. OK... I didn't look at the schematic, but my TEK1001 floats the input
at AC, (as you say.) The new Keysight reads 1.0 meg ohm at DC 1.0 meg ohm at
AC. I put in a DC voltage through a 1 meg resistor, and measured the
voltage at the 'scope input (AC and DC). It was 1/2 the applied voltage..
The same in both cases.

So there is at least one 'scope that is different.
(I never knew... thanks)

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent would form a capacitive divider and with even miniscule leakage of C 10 or equivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope's input cap.
Right, I've done that... I wanted DC
(well I don't need it any more.. voltage was not as high as I thought.)

George H.

My Rigol measures 1M input on a DVM (to 5 decimal places) on either AC
or DC coupling.

Set to 50 ohms, it measures 50.37 whether AC or DC coupled.

Good to know, I guess.

I didn't know my 'scope could be floating at AC coupling....
That's even better to know.
(shame on me.)

George H.
Quote:


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


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