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4hv.org :: Forums :: High Voltage
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Extend the total voltage input on a oscilloscope

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101111
Fri Mar 23 2007, 02:40PM Print
101111 Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263
 		I want to extend the total voltage input on my oscilloscope, I have many 10 mohm resistor but don’t know what the best way is and what formulas I may use.

Any ideas? Some links or formulas.

EDIT: I've been searching in here, at wiki and google. Nothing found.

Regards: Waco
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Bjørn
Fri Mar 23 2007, 03:10PM
Bjørn Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
 		Have a look at the third picture for a very accurate circuit: http://www.probemaster.com/helpful.htm

If you simply connect the resistor in series with the input you will extend the DC voltage range by approximately 10 times if you consider 10% error to be good enough. You can always calibrate your oscilloscope to show the correct value. When it comes to AC it depends on the resistor and the frequency how well it will work. Some resistors are remarkably inductive.
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Steve Conner
Fri Mar 23 2007, 03:54PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		I like these 100:1 HV scope probes: Link2
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Hazmatt_(The Underdog)
Fri Mar 23 2007, 06:39PM
Hazmatt_(The Underdog) Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
 		Digging up another prehistoric thread: Link2

Vmax = 40KVDC from terminal to gnd.
The adjustable pots allow the divisor to be anywhere from 1000:1 to 1,000,000:1, and if you want to meter a TC while operational, you need a really high division ratio for when its ringing because it went way off scale for me at even 1000:1.
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ragnar
Sat Mar 24 2007, 05:55AM
ragnar Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
 		Hmm, I've a question that's appropriate here.

According to the reference manual on my PM3092, the scope's isolation is frequency depedent -- that means it can take 500V on the input at DC and audio frequencies.. but at 10MHz, it will only take about 10V.

Does that really mean I'm supposed to use a 100:1 divider probe, such that the scope is exposed to no more than 5V at this frequency? Won't the divider stuff my waveforms around?
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Fusion
Mon Mar 26 2007, 01:22PM
Fusion Registered Member #354 Joined: Thu Mar 30 2006, 10:12AM
Location:
Posts: 55
 		If you need a high frequency measure you can use a capacitor divider instead of resistor divider.
I am going to use resistors in parllel with capacitors in order to have DC and AC response
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HV Enthusiast
Sat Mar 31 2007, 02:16AM
HV Enthusiast Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
 		Make sure you add a compensation network as well at the scope input end. You'll need to compensate the network to get flat response across the measurement band.
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Fusion
Mon Apr 02 2007, 11:07AM
Fusion Registered Member #354 Joined: Thu Mar 30 2006, 10:12AM
Location:
Posts: 55
 		Ok, I have simulated now, thanks:

Link2
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Steve Conner
Mon Apr 02 2007, 11:54AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		347uF?! Don't you think that's a little high for a probe input capacitance? The last X100 probe I got was about 5pF.
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Fusion
Mon Apr 02 2007, 12:55PM
Fusion Registered Member #354 Joined: Thu Mar 30 2006, 10:12AM
Location:
Posts: 55
 		Yes it is very big, that is due I used one of the compensating resistor of a serial-parallel capacitor bank as sensing resistor. I supose you can scale it down if capacitances are well over 5pF scope input and resistance well under 1Mohm scope input.
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