On 30 Jul 2006 19:54:04 -0700, Winfield Hill Gave us:
HV coax has a much higher capacitance per foot, and will allow you to safely arrive at the desired capacitance value using much less cable length. One must be careful to properly terminate the tail with HV heat shrink.
On Sun, 30 Jul 2006 22:32:41 +0100, "Reg Edwards" Gave us:
Nope. Also, not quoting what you are claiming is rather stupid as well.
Bugger off, bother boy.
Dumbass. That's what an HV probe does. It is a very high resistance presented to the load so that the meter's internal resistance does not present a load to the supply being probed.
It is Ohms law, and you have failed the test. If you knew anything at all about HV probes you would never have come back to post this utter crap.
You are the one that needs to BONE UP, and do some math.
Questions:
What loading does a 10Meg Ohm meter present to a 1200 Volt supply?
What loading does a 10Gig Ohm HV Probe monitored by a 10MegOhm meter present to the same source?
Get a high voltage differential probe for your oscilloscope, that is the best and safest way to measure it. Probably cheaper than finding a DMM that can do 28KHz True RMS too.
On Mon, 31 Jul 2006 02:27:04 -0400, "Paul E. Schoen" Gave us:
Nice little device. Probably hard potted in "Stycast". Those little shells are cool high dielectric strength suckers too. Nice design. The probe lead negates the need for a long probe tip. Needs to be attached before energization though.
One MUST make sure that pin one is tied to ground (read: HV supply return or "low" side in the case of a positive supply) as well as the return side of your meter, BEFORE energizing your circuit. Pin two will then be at LV potential. Pin 3 should also be already hooked up to the HV out of your circuit device under test (DUT).
Remeber also that a negative output supply will have its positive, "high side" lead as its "grounded" side. Make sure you know which you are testing and get your returns hooked up, or you can blow the meter if it "floats high" (or "low" for that matter).
What are some HV coax types (p/n) you like to use?
BTW, it should be pointed out that the coax in the home-made probe above does not see HV, only the 2pF capacitor does. It should also be pointed out that stray capacitance conducting any signal current to the coax center wire, besides the 2pF, will reduce the desired 100:1 division ratio. This means the 2pF and its LV output wire must be shielded.
The issues and techniques involving this shield and the method of creating the 2pF and the heat shrink to hold it all together and the nice but safe finger-hold and the sharp probe tip and the various schemes to adjust or calibrate the probe (take a breath) presents considerable opportunities to a clever mind.
One more comment. That 2pF had better never break down.
At the cost of some more capacitance, you can add a Sidac or other over voltage clamp. Lets say that the scope's input is rated at 500V. You can use a clamp rated for, lets say half that and then make sure you are making your measurment with an input much less than that.
As long as the tube lights, the voltage is kinda unimportant, compared to the CURRENT.
These CCFL tubes have a rather variable resistance, varies during each cycle, and varies as the tube warms up.
You'll probably find it much easier and helpful to measure the tube current.
Just insert a 1K resistor in one of the HV leads, put a good AC RMS meter across it, and go to it. Note that you'll have to suspend the meter in the air if you can't ground the HV lead going to the meter.
As others have noted, the voltage and current wavefornms are likely to be rather peaked, so even a "true RMS" meter of the simpler variety is likely to be a bit off. Check the meter's "Crest Factor" rathing. If it's less than 6:1 or not mentioned, that meter is unlikely to give you giood results. Get a real, true, RMS meter, such as the HP 3403C. They're not too pricey on eBay, just make sure you get a working one.
On 31 Jul 2006 05:57:08 -0700, Winfield Hill Gave us:
We used a military rooted, PTFE dielectric (IIRC), teflon jacketed SPC multistranded core with a full, tightly bound SPC shield. It was a little over an 8th inch in outside diameter. I think it was 30kV form factor, but we used it for ultra low noise supplies. You be amazed (or not) how much noise the supply injects into itself from the multiplier array. I developed arrays that were shielded all around, and the output node was a big round blob of solder. It helps when you are trying to get a 4kV supply down to 4mV ripple. :-] The whole thing was in a can, but the multiplier array was itself shielded from the feedback network and output lead and RC filtering. It does make a difference at those levels. I could more than double the output capacity by not incorporating the RC filter, and the supply would push it, no problem. We needed noiselessness, not brute power, however.
I can't recall exactly how many pF per foot we got from the coax, But we used them as HV loads, and various other uses, like for characterizing a feed forward needed in an HV feedback loop. Most were between 5 and 20pF across several hundred MegOhms or more of feedback resistor. Some supplies worked well with a little feed forward on the feedback... some needed none.
Yes... noise is easy to inject in small signal electronics in an unshielded environ.
Our stuff was DC, and the ripple was so small as to really be insignificant. We ran at frequencies anywhere from 17kHz to a couple hundred on our HV switcher driven supplies. We also made supplies for transmitters. The England facility made up to 400kV supplies. We still shielded things.
We used the left overs from potting sessions placed in a dixie cup. It makes great stand offs for HV. One could have a one inch or a six inch plug (drinking glass shaped) holding a circuit element... several actually. One can even cut V shaped grooves in it to help "hold" the wire or object in question,
One should always have a coax rated several times greater than the voltage being tested. Corona can poke holes right through teflon. Good thing coax doesn't have air gaps. Another reason why the HV shrink is needed over the tail.
The coax you suggested has a 2kV breakdown strength on the core insulator, IIRC.
Have to watch out for poke throughs on that stuff too. More even than my "Made for the purpose" coax. It's like RG-174 on steroids. :-]
On Mon, 31 Jul 2006 13:55:50 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith) Gave us:
We made "ripple checkers" that had 90V neon lamps as perfect little clamps in them. Want more? Series two together. Much better than a spark gap as they are "soft" clampers.
So any high transients that get through to the input get shunted in the ripple checker, not your scope probe or scope.
They may, but we don't care because the scope input is toast. We need a TVS, transient voltage suppressor, AKA zener diode, with its massive sub-ns response to overvoltage. They do have a capacitance penalty, but in this case we can live with it.
One trick I've used to mitigate Zener capacitance is to use a regular small-signal diode in series with the Zener (with the appropriate polarity). Typically the forward capacitance of the small-signal diode is much lower, so you get two capacitors in series. Very handy for a rude-and-crude clamp at an opamp's output, but I've never tried it in a transient-suppression application.
I guess its all a matter of priorities. Pretty colors vs working scope, you choose.
As I suggested elsewhere in this thread, a SIDAC would be good for this job. You will need to keep the operating voltage at the input of the scope well away from the voltage limit to allow room for the SIDAC, but that wouldn't be a major problem.
One of the nice things about SIDACs is that when tripped, they "breakover" and pull the voltage near ground. This reduces the amount of energy they have to absorb.
We also need an impedance in the path from the HV that we are fairly sure won't fail shorted. A lowish valued resistor would be good for this. Without it, we just get to see the TVS explode a few nS before the scope does.
If not:
Signal ----------+-----------+-----------------Scope ! ! V --- --- ^ ! ! +200V --/\\/\\---+ +---/\\/\\-- -200V ! ! +---[TVS]---+ ! ! --- V ^ --- ! ! GND GND
The TVS and 4 diodes are likely to be destroyed if the fault happens.
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