# Measuring high voltage with oscilloscope help

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#### qsiguy

I have a Tektronix TDS2002C oscilloscope and need to measure high voltage in the range of 1000 ~ 10,000 volts AC, referenced to ground, in the frequency range of 1.1kHz - 3kHz. Perhaps also up to 25kHz or so in the future. I found the Tektronix P6015A high voltage probe. The voltage I am attempting to measure is on the secondary of a step up high voltage transformer feeding a capacitive load. Does anyone have experience with an application like this? I am trying to verify that this setup will do what I need before I spend $2000 for this probe. Any other ideas or comparable products that can do this I am open to suggestions. Typical high voltage probes such as the Fluke 80K-40 will not work as they only work near 60Hz. I've tried voltage divider circuits with several standard multi-meters and while I did get a reading the result was so far away from the expected voltage I do not think that it was accurate. The expected voltage on this test was around 5-6kv and the voltage divider came up with around 1.7kv. Thanks Shane #### Diver300 ##### Well-Known Member Most Helpful Member Even your multimeters might not be working correctly at 1.1 - 3 kHz. The problem is the capacitance of whatever you are reading the divided voltage with. With the high resistances needed for that sort of voltage, you end up with a time constant larger than the cycle time. Oscilloscope leads, including the high voltage one, have a capacitive voltage divider in parallel with the resistive one, and the adjustment is to compensate for the capacitance of the oscilloscope input. Getting that working at 10 kV is difficult, and it is very rare to need voltage measurement at those voltages and frequencies, so all of Tektronix's development costs for that probe are spread across just a few customers. I can't see an alternative to a probe like that #### crutschow ##### Well-Known Member Most Helpful Member How much of a resistive load can the transformer output tolerate before its voltage is significantly affected? #### Colin ##### Active Member Just use some 1M resistors to create a voltage divider. 10 resistors will give you 10:1 reduction. If the probe is 1M, you will see half the real amplitude. #### JimB ##### Super Moderator Most Helpful Member Just use some 1M resistors to create a voltage divider. 10 resistors will give you 10:1 reduction. If the probe is 1M, you will see half the real amplitude. What about frequency compensation? JimB #### Colin ##### Active Member You don't need frequency compensation at 3kHz. #### spec ##### Well-Known Member Most Helpful Member Here are a few thoughts about this task: 10KV sine wave = 14.14KV peak (scopes only display peak voltages). The maximum displayed trace on the scope is 8 vertical divisions of 5V per division = 40V maximum (without using the Y control and any voltmeter display) Divide this by 2 to get peak to peak From this it follows that the required minimum attenuation from 10KV sine wave to the input of the scope is, 14,14/20= 707 The minimum probe resistance would need to be 100M Ohm and preferably 1G Ohm. Any less than 100M Ohms will place an inordinate load on the unit under test (UUT) at 14.14KV. I hope the data and calculations are correct. I feel sure that a combination of the 2.5KV probe that I mentioned above and some 9M, 10M, 90M, and 100M Ohm resistors will do the job a lot cheaper than$2,000, but high voltage resistors will be required. Theses are big and expensive but dirt cheap compared to the Tek HV probe. These resistors are not usually designed for good frequency response though

I have done quite a bit of HV work, while making a range of scopes and automobile ignition systems, so just a word of caution. If you are rolling your own attenuator, you need to be very careful about the components and insulating materials you use, At 14KV even the wire has to be special. The layout is also critical.

To get a 25KHz bandwidth capacitive peaking will be required across the resistors. This may mean that you will need small 15KV capacitors.

spec

The maximum safe voltage on the input of a scope is typically 300V.
The maximum voltage on a 10:1 10M Ohm scope probe is 500V to 600V.

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#### Colin

##### Active Member
"The maximum displayed trace on the scope is 8 vertical divisions of 5V per division = 40V maximum "

My $5.00 CRO has 20v per division #### ronsimpson ##### Well-Known Member Most Helpful Member Tektronix P6015A high voltage probe http://www.testequipmentdepot.com/t...tm?ref=gbase&gclid=CMWRscy0gM0CFQkwaQod6dINyA http://www.ebay.com/itm/like/311596153402?lpid=82&chn=ps&ul_noapp=true I have at least one. Can't find it today. For years I made a clone of it. Good to 40kv. Can't find it either. Some where I should have the parts to make more. Maybe lost over the years. I think the 6015 is a good part! That is what I use to measure AC on a scope. For measuring DC I use a meter probe that is more accurate but does not work well with AC. #### spec ##### Well-Known Member Most Helpful Member "The maximum displayed trace on the scope is 8 vertical divisions of 5V per division = 40V maximum " My$5.00 CRO has 20v per division
Hy Colin, I was describing the OP's scope.

Is that a typo?