[TechAssist] Re: Trouble with 10x Probe
- From: Bil Green <tv.vcrrepair@xxxxxxxxxxx>
- To: Wayne Vanaman <techassist@xxxxxxxxxxxxx>
- Date: Thu, 17 Mar 2005 05:09:45 -0800
Thanks Wayne,
My B&K goes up to 200V (2000V with the 10x probe). One day I got so
mad (because I needed to measure 1700V PP) that I used the bad probe
in the direct mode (1700V on the 200V input) and it didn't blow out
the scope.
I was able to at least see that the waveform was far higher than 200V.
It's an old scope, so I cared less about damaging it. Works fine still.
This place has a 50kV scope probe for only $47. Huge hippo clips, but
those could be changed out. http://ezprobe.com/products/HV/
I just couldn't wait. Some of the guys said there needs to be a very
small capacitance across the series resistors (9M). 20pF brought the
voltage up to normal (from only 10% or so).
I only need 2% or better accuracy when measuring the 1700V I'll be
testing. I believe most DMMs will do OK measuring the 2M or so
resistors required for the 10:1 circuit.
The probe's cable will only be exposed to 1/10 the voltage present at
the front end of the resistors.
Bil
PC 1000
Mammoth Lakes, CA
760-924-1000
M-Pulse 5000 magnetic pulse generator
Thursday, March 17, 2005, 4:37:06 AM, you wrote:
WV> Don't forget the maximum input capabilities of your scope! You don't
WV> want to blow that. Many scopes won't take 250v directly at the input.
WV> I have a Tektronix P5100 scope probe. 100x attenuation, 2.5KV voltage
WV> rating, 250MHz frequency response. They can be purchased from places
WV> like Techni-tool ($290.) or Contact East.
WV> The dielectric of the coax you use will have to be rated at that voltage.
WV> The only 1000x scope probes I know about are made by Tektronix.
WV> You would need an ohm meter capable of precision measurements at high
WV> resistance in order to match the resistors in the voltage divider.
WV> Wayne Vanaman
WV> Omega Electronics
WV> 6904 W Fairfield Dr
WV> Pensacola, Fl. 32506-3310
WV> Wayne@xxxxxxxxxxxxxxxxxxxxx
WV> WWW.PensacolaTVRepair.Com
WV> 850-456-5995 voice
WV> 850-458-6369 fax
WV> Bil Green wrote:
>> I am trying to find just what's needed to build a 10x adapter for a
>> scope probe.
>>
>> I need to measure up to 2kV AC PP and it's hard to find one that high
>> (and I need it right anyway). I will be checking a 5kHz AC voltage.
>>
>> My scope has a 1M input impedance, so adding 9M in series should do
>> the trick. But that doesn't work! The voltage reading is far less
>> than 10% measuring 250V PP horizontal signals (TV analyst). I tried
>> a carbon composition and carbon film resistor.
>>
>> Same problem using it with my PP voltmeter (designed for testing TVs,
>> and also has a 1M input impedance).
>>
>> The 10x probe for my scope went bad (reads way too low when set for
>> 10x). I was using it to measure an AC voltage which was over 2,000V
>> PP and didn't know that the probe was only rated at 600V.
>>
>> Bil
>>
>> PC 1000
>> Mammoth Lakes, CA
>> 760-924-1000
>> M-Pulse 5000 magnetic pulse generator
>>
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WV>
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