garnett@batcomputer.tn.cornell.edu (Roger Garnett) (11/16/90)
Does anyone have reccommendations for homemade High Voltage Oscilliscope probes? I would like to use my scope for checking automotive ignitions, so would like to build an inexpensive HV probe. Direct and inductive coupling are both of interest. Calibration, while desireable, is not critical. ______________________________________________________________ Roger Garnett (garnett@THEORY.TN.CORNELL.EDU) (607) 255-2522 Cornell University Agricultural Economics Ithaca, N.Y.
jchristy@hplred.HP.COM (Jim Christy) (11/22/90)
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jchristy@hplred.HP.COM (Jim Christy) (11/22/90)
I built a HV probe for my scope as follows: 10 10 megohm 1/2 watt resistors 1% 1 1" diameter PVC pipe 18" long 2 PVC end caps for above pipe 1 small nail for the probe tip 2 small machine screws about 1.5" long 2 nuts for the screws 2 blocks of paraffin wax to fill the tube The idea is to solder the 10 resistors in series. One end of the chain should be soldered to the nail (probe end), the other to one of the machine screws around the head (ground end). Tap off a lead between the first and second resistor (nearest the ground end) and solder it to the other machine screw head (measurement point). Drill a single hole in one end cap for the nail, drill two holes side-by side in the other cap for the two machine screws. Put the nail through the end cap and epoxy into place from the inside. String the resistors through the PVC pipe and secure the cap with the nail on the pipe. Melt the paraffin and fill the tube, providing extra dielectric strength to prevent arc-around the resistors and through the pipe to your hand. Finally, install the 2 machine screws in the other end cap and secure onto pipe. Be sure to mark which screw is ground and which is "hot". This makes a simple 10:1 HV prope assuming your scope has 100 Megohm input impedence. Mine has 1 Megohm input impedence, making this a 100:1 (approximately) probe when used with the scope. Current draw is under a 120 micro amps when measuring 10KV. Commercial probes from Beckman have similar specs (and use the same passive divder circuit, as far as I can tell), yet cost $100. This one will cost less than $10. Be very careful with the HV. Always keep one hand behind your back and wear rubber soled shoes. Needless to say, one mistake can be your last. Jim Christy HP Labs jchristy@hplren2.hpl.hp.com Palo Alto, Ca.
agn@bovic.Eng.Sun.COM (Andreas G. Nowatzyk) (11/23/90)
Building a high voltage resistor of 100 or 1000 Mohm solves only 1/2 of the problem if you want to measure anything besides DC. The problem is due to the input impedance of the scope, which has a capacitive component to it (say 1 Mohm and 5 pf). Furthermore, the high voltage resistor will have some parasitic capacitive component too. The net result is an R/C circuit that distorts your frequency response. As a first order approximation, you need to parallel the 100 Mohm with some C such that the total C (including parasitic C) is 0.05 pf. Combined with the 1Mohm || 5pf of the scope input, you end up with an resistive and capacitive divider of the same ratio, which doesn't distort. Unfortunately, there are also inductive components and the effect of the cable, etc. The cost of good scope probes has a lot to do with the compensation networks used to minimize distortions.