[TechAssist] Re: Trouble with 10x Probe
- From: "Jerome Silverman" <greentron@xxxxxxx>
- To: <techassist@xxxxxxxxxxxxx>
- Date: Wed, 16 Mar 2005 22:46:10 -0500
There are two problems. First, small resistors have a voltage rating,
typically 500 volts DC, so you have to string several in series to avoid
exceeding the voltage rating. Thats what happened to the resistor in your
probe.
Second, the voltage division at high frequencies is determined by the
capacitance of the resistors, cable, and scope input rather than the
resistance value. To provide a voltage division independent of frequency or
waveshape the scope input needs to have a certain value of capacitor across
it. Thats why your probe had a adjustable trimmer capacitor in it and you
adjusted it for a good replica of the scopes calibration square wave. Small
resistors have a few picofarad of capacitance, say 5. If you put nine 1 meg
resistors in series, the string has 5/9 picofarad of capacitance. For a 10:1
voltage division you need a total of 50/9 picofarads scope input and cable
capacitance. Whatever enclosure and cable you use with your homemade probe
wil have some capacitance also, so a trimmer cap is necessary to adjust to
compensate.
Jerry Silverman
Greentron Inc
4 Newland Ave Greenville SC 29609
Fax/Phone 864 232 3889
greentron@xxxxxxx
----- Original Message -----
From: "Bil Green" <tv.vcrrepair@xxxxxxxxxxx>
To: ".TECHASSIST @ freelists . org" <techassist@xxxxxxxxxxxxx>
Sent: Wednesday, March 16, 2005 9:39 PM
Subject: [TechAssist] Trouble with 10x Probe
> 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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