x100 'scope probe

Started by George Herold September 25, 2018
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. 
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.
On Tue, 25 Sep 2018 07:34:28 -0700 (PDT), George Herold
<gherold@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.
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. 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.
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.
On Tuesday, September 25, 2018 at 10:50:39 AM UTC-4, default wrote:
> On Tue, 25 Sep 2018 07:34:28 -0700 (PDT), George Herold > <gherold@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.
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.) 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.
>"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.
On 9/25/2018 9:34 AM, 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. >
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
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".
> > 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. >