Homebrew HV hiZ scope probe

On a sunny day (Wed, 24 Oct 2012 22:17:26 -0800) it happened Robert Baer wrote in :

Safe to touch 7kV high power with that? For TV maybe (

You're an idiot.

First, you have no clue what "high power" is.

Yep; cheap, small, does the job.

Does the model act as the physical part scopes out?

RL

On Mon, 22 Oct 2012 22:04:28 -0800, Robert Baer wrote:

Takes forever to converge.

I softened the analysis a bit by making the pulse risetime 20ns, similar to your HP3312A, but it's still excruciatingly slow.

How sure are you of the resistor model? Have you considered making it a uniform RC line (U) model? I'll try that in a while, it might converge faster.

There's a pole at around 67 Hz that needs addressing.

I added 3 feet of RG179, which i have a model for, which is similar to RG175, for which I don't. The inevitable quarter-wave spikes appear at around 46MHz, et seq. 60 ohms in series with each end of the coax tames this, and we now have a 3dB rolloff around 70MHz, making risetime about

5ns. That's why resistive cable is used in commercial probes.

I'd do what Tektronix do, and do all compensation at the 'scope end.

Bear in mind that Tek's 40kV probes used to run their HV resistor in an atmosphere of Fluorcarbon 114 vapor, which needed topping up from time to time. I don't know what they use these days in HV probes, but I doubt it's FC.

Try this, it's your circuit with 3 feet of coax. Do an .ac analysis, with and without the 60 ohm resistors.

Version 4 SHEET 1 2064 680 WIRE -352 -16 -448 -16 WIRE -208 -16 -272 -16 WIRE -64 -16 -128 -16 WIRE 80 -16 16 -16 WIRE 224 -16 160 -16 WIRE 448 -16 304 -16 WIRE 592 -16 528 -16 WIRE 736 -16 672 -16 WIRE 880 -16 816 -16 WIRE 1024 -16 960 -16 WIRE 1280 -16 1104 -16 WIRE 1328 -16 1280 -16 WIRE 1408 -16 1328 -16 WIRE 1568 -16 1488 -16 WIRE 1760 -16 1664 -16 WIRE 1952 -16 1840 -16 WIRE 1984 -16 1952 -16 WIRE -352 16 -352 -16 WIRE -272 16 -272 -16 WIRE -208 16 -208 -16 WIRE -128 16 -128 -16 WIRE -64 16 -64 -16 WIRE 16 16 16 -16 WIRE 80 16 80 -16 WIRE 160 16 160 -16 WIRE 224 16 224 -16 WIRE 304 16 304 -16 WIRE 448 16 448 -16 WIRE 528 16 528 -16 WIRE 592 16 592 -16 WIRE 672 16 672 -16 WIRE 736 16 736 -16 WIRE 816 16 816 -16 WIRE 880 16 880 -16 WIRE 960 16 960 -16 WIRE 1024 16 1024 -16 WIRE 1104 16 1104 -16 WIRE 1568 16 1552 16 WIRE 1680 16 1664 16 WIRE 1280 32 1280 -16 WIRE 1328 64 1328 -16 WIRE 1952 64 1952 -16 WIRE 2000 64 1952 64 WIRE 2000 80 2000 64 WIRE -352 112 -352 80 WIRE -272 112 -272 80 WIRE -272 112 -352 112 WIRE -208 112 -208 80 WIRE -208 112 -272 112 WIRE -128 112 -128 80 WIRE -128 112 -208 112 WIRE -64 112 -64 80 WIRE -64 112 -128 112 WIRE -32 112 -64 112 WIRE 16 112 16 80 WIRE 16 112 -32 112 WIRE 80 112 80 80 WIRE 80 112 16 112 WIRE 160 112 160 80 WIRE 160 112 80 112 WIRE 224 112 224 80 WIRE 224 112 160 112 WIRE 304 112 304 80 WIRE 304 112 224 112 WIRE 448 112 448 80 WIRE 528 112 528 80 WIRE 528 112 448 112 WIRE 592 112 592 80 WIRE 592 112 528 112 WIRE 672 112 672 80 WIRE 672 112 592 112 WIRE 736 112 736 80 WIRE 736 112 672 112 WIRE 784 112 736 112 WIRE 816 112 816 80 WIRE 816 112 784 112 WIRE 880 112 880 80 WIRE 880 112 816 112 WIRE 960 112 960 80 WIRE 960 112 880 112 WIRE 1024 112 1024 80 WIRE 1024 112 960 112 WIRE 1104 112 1104 80 WIRE 1104 112 1024 112 WIRE 1280 144 1280 96 WIRE 2000 144 1952 144 WIRE -448 160 -448 -16 WIRE -32 160 -32 112 WIRE 784 160 784 112 WIRE 2000 192 2000 144 WIRE 1328 224 1328 144 WIRE 1552 224 1552 16 WIRE 1680 224 1680 16 WIRE -448 272 -448 240 WIRE -32 272 -32 224 WIRE 784 272 784 224 FLAG 1328 224 0 FLAG 2000 192 0 FLAG 1984 -16 scope FLAG 1280 144 0 FLAG -448 272 0 FLAG 784 272 0 FLAG -32 272 0 FLAG 1680 224 0 FLAG 1552 224 0 SYMBOL voltage -448 144 R0 WINDOW 0 8 7 Left 2 WINDOW 3 11 105 Left 2 WINDOW 123 11 123 Left 2 WINDOW 39 11 141 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 250 0 20n 20n 1m 2m 10) SYMATTR Value2 AC 1 SYMATTR SpiceLine Rser=50 SYMBOL cap 1296 96 R180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C31 SYMATTR Value 2478p SYMBOL res 1312 48 R0 WINDOW 3 30 126 Left 2 SYMATTR Value 4.008Meg SYMATTR InstName R31 SYMATTR SpiceLine tol=0.1 pwr=1 SYMBOL cap 1984 80 R0 SYMATTR InstName C32 SYMATTR Value 22p SYMBOL res 1968 160 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R32 SYMATTR Value 1Meg SYMBOL cap -368 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C1 SYMATTR Value {Cr} SYMBOL cap -288 16 R0 SYMATTR InstName C2 SYMATTR Value {Cr} SYMBOL res -368 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R1 SYMATTR Value {Rp} SYMBOL cap -224 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C3 SYMATTR Value {Cr} SYMBOL cap -144 16 R0 SYMATTR InstName C4 SYMATTR Value {Cr} SYMBOL res -224 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R2 SYMATTR Value {Rp} SYMBOL cap -80 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C5 SYMATTR Value {Cr} SYMBOL cap 0 16 R0 SYMATTR InstName C6 SYMATTR Value {Cr} SYMBOL res -80 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R3 SYMATTR Value {Rp} SYMBOL cap 64 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C7 SYMATTR Value {Cr} SYMBOL cap 144 16 R0 SYMATTR InstName C8 SYMATTR Value {Cr} SYMBOL res 64 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R4 SYMATTR Value {Rp} SYMBOL cap 208 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C9 SYMATTR Value {Cr} SYMBOL cap 288 16 R0 SYMATTR InstName C10 SYMATTR Value {Cr} SYMBOL res 208 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R5 SYMATTR Value {Rp} SYMBOL cap 432 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C11 SYMATTR Value {Cr} SYMBOL cap 512 16 R0 SYMATTR InstName C12 SYMATTR Value {Cr} SYMBOL res 432 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R6 SYMATTR Value {Rp} SYMBOL cap 576 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C13 SYMATTR Value {Cr} SYMBOL cap 656 16 R0 SYMATTR InstName C14 SYMATTR Value {Cr} SYMBOL res 576 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R7 SYMATTR Value {Rp} SYMBOL cap 720 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C15 SYMATTR Value {Cr} SYMBOL cap 800 16 R0 SYMATTR InstName C16 SYMATTR Value {Cr} SYMBOL res 720 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R8 SYMATTR Value {Rp} SYMBOL cap 864 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C17 SYMATTR Value {Cr} SYMBOL cap 944 16 R0 SYMATTR InstName C18 SYMATTR Value {Cr} SYMBOL res 864 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R9 SYMATTR Value {Rp} SYMBOL cap 1008 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C19 SYMATTR Value {Cr} SYMBOL cap 1088 16 R0 SYMATTR InstName C20 SYMATTR Value {Cr} SYMBOL res 1008 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R10 SYMATTR Value {Rp} SYMBOL cap -48 160 R0 SYMATTR InstName C21 SYMATTR Value {Cs} SYMBOL cap 768 160 R0 SYMATTR InstName C22 SYMATTR Value {Cs} SYMBOL ltline 1616 0 R0 SYMATTR InstName O1 SYMATTR Value RG179 SYMBOL res 1504 0 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R11 SYMATTR Value {R} SYMBOL res 1856 0 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R12 SYMATTR Value {R} TEXT 264 256 Left 2 !.ac dec 1000 1 200meg TEXT 264 -208 Left 4 ;40KV 2E9 ohms HV scope probe TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form TEXT 272 384 Left 2 !.model RG179 LTRA (\n+ len=3\n+ L=.1u\n+ C=19.5p\n+ R=252.5e-3\n+) TEXT 272 328 Left 2 !;.step param R 10 100 10m \n.param R = 60 TEXT 272 -176 Left 2 ;3 feet RG179 (similar to RG175) added to model\nBelden

83265 data used.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Fred Abse wrote:

** This is what i have, before i saw your response; not too bad considering simple compensation and implicit coax of indeterminate (but limited) length to scope end. Also posted at: .

Version 4 SHEET 1 2064 680 WIRE 1488 -64 1488 -240 WIRE -352 -16 -448 -16 WIRE -208 -16 -272 -16 WIRE -64 -16 -128 -16 WIRE 80 -16 16 -16 WIRE 224 -16 160 -16 WIRE 448 -16 304 -16 WIRE 592 -16 528 -16 WIRE 736 -16 672 -16 WIRE 880 -16 816 -16 WIRE 1024 -16 960 -16 WIRE 1168 -16 1104 -16 WIRE 1200 -16 1168 -16 WIRE 1312 -16 1280 -16 WIRE 1440 -16 1312 -16 WIRE 1568 -16 1440 -16 WIRE 1600 -16 1568 -16 WIRE -352 16 -352 -16 WIRE -272 16 -272 -16 WIRE -208 16 -208 -16 WIRE -128 16 -128 -16 WIRE -64 16 -64 -16 WIRE 16 16 16 -16 WIRE 80 16 80 -16 WIRE 160 16 160 -16 WIRE 224 16 224 -16 WIRE 304 16 304 -16 WIRE 448 16 448 -16 WIRE 528 16 528 -16 WIRE 592 16 592 -16 WIRE 672 16 672 -16 WIRE 736 16 736 -16 WIRE 816 16 816 -16 WIRE 880 16 880 -16 WIRE 960 16 960 -16 WIRE 1024 16 1024 -16 WIRE 1104 16 1104 -16 WIRE 1440 64 1440 -16 WIRE 1568 64 1568 -16 WIRE 1616 64 1568 64 WIRE 1312 80 1312 -16 WIRE 1616 80 1616 64 WIRE 1168 96 1168 -16 WIRE -352 112 -352 80 WIRE -272 112 -272 80 WIRE -272 112 -352 112 WIRE -208 112 -208 80 WIRE -208 112 -272 112 WIRE -128 112 -128 80 WIRE -128 112 -208 112 WIRE -64 112 -64 80 WIRE -64 112 -128 112 WIRE -32 112 -64 112 WIRE 16 112 16 80 WIRE 16 112 -32 112 WIRE 80 112 80 80 WIRE 80 112 16 112 WIRE 160 112 160 80 WIRE 160 112 80 112 WIRE 224 112 224 80 WIRE 224 112 160 112 WIRE 304 112 304 80 WIRE 304 112 224 112 WIRE 448 112 448 80 WIRE 528 112 528 80 WIRE 528 112 448 112 WIRE 592 112 592 80 WIRE 592 112 528 112 WIRE 672 112 672 80 WIRE 672 112 592 112 WIRE 736 112 736 80 WIRE 736 112 672 112 WIRE 784 112 736 112 WIRE 816 112 816 80 WIRE 816 112 784 112 WIRE 880 112 880 80 WIRE 880 112 816 112 WIRE 960 112 960 80 WIRE 960 112 880 112 WIRE 1024 112 1024 80 WIRE 1024 112 960 112 WIRE 1104 112 1104 80 WIRE 1104 112 1024 112 WIRE -448 160 -448 -16 WIRE -32 160 -32 112 WIRE 784 160 784 112 WIRE 1568 176 1568 144 WIRE 1616 176 1616 144 WIRE 1616 176 1568 176 WIRE 1616 192 1616 176 WIRE 1312 208 1312 160 WIRE 1168 224 1168 160 WIRE 1440 224 1440 144 WIRE -448 272 -448 240 WIRE -32 272 -32 224 WIRE 784 272 784 224 WIRE 1312 336 1312 272 FLAG 1440 224 0 FLAG 1616 192 0 FLAG 1600 -16 scope FLAG 1168 224 0 FLAG -448 272 0 FLAG 784 272 0 FLAG -32 272 0 FLAG 1312 336 0 SYMBOL voltage -448 144 R0 WINDOW 0 8 7 Left 2 WINDOW 3 11 105 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 250 0 1p 1p 10 20 1) SYMBOL cap 1184 160 R180 WINDOW 0 -37 59 Left 2 WINDOW 3 28 4 Left 2 SYMATTR InstName C91 SYMATTR Value 1170p SYMBOL res 1424 48 R0 WINDOW 3 -89 120 Left 2 WINDOW 0 -49 70 Left 2 SYMATTR Value 4.008Meg SYMATTR InstName R33 SYMATTR SpiceLine tol=0.1 pwr=1 SYMBOL cap 1600 80 R0 SYMATTR InstName C32 SYMATTR Value 22p SYMBOL res 1584 160 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R32 SYMATTR Value 1Meg SYMBOL cap -368 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C1 SYMATTR Value {Cr} SYMBOL cap -288 16 R0 SYMATTR InstName C2 SYMATTR Value {Cr} SYMBOL res -368 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R1 SYMATTR Value {Rp} SYMBOL cap -224 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C3 SYMATTR Value {Cr} SYMBOL cap -144 16 R0 SYMATTR InstName C4 SYMATTR Value {Cr} SYMBOL res -224 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R2 SYMATTR Value {Rp} SYMBOL cap -80 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C5 SYMATTR Value {Cr} SYMBOL cap 0 16 R0 SYMATTR InstName C6 SYMATTR Value {Cr} SYMBOL res -80 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R3 SYMATTR Value {Rp} SYMBOL cap 64 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C7 SYMATTR Value {Cr} SYMBOL cap 144 16 R0 SYMATTR InstName C8 SYMATTR Value {Cr} SYMBOL res 64 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R4 SYMATTR Value {Rp} SYMBOL cap 208 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C9 SYMATTR Value {Cr} SYMBOL cap 288 16 R0 SYMATTR InstName C10 SYMATTR Value {Cr} SYMBOL res 208 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R5 SYMATTR Value {Rp} SYMBOL cap 432 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C11 SYMATTR Value {Cr} SYMBOL cap 512 16 R0 SYMATTR InstName C12 SYMATTR Value {Cr} SYMBOL res 432 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R6 SYMATTR Value {Rp} SYMBOL cap 576 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C13 SYMATTR Value {Cr} SYMBOL cap 656 16 R0 SYMATTR InstName C14 SYMATTR Value {Cr} SYMBOL res 576 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R7 SYMATTR Value {Rp} SYMBOL cap 720 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C15 SYMATTR Value {Cr} SYMBOL cap 800 16 R0 SYMATTR InstName C16 SYMATTR Value {Cr} SYMBOL res 720 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R8 SYMATTR Value {Rp} SYMBOL cap 864 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C17 SYMATTR Value {Cr} SYMBOL cap 944 16 R0 SYMATTR InstName C18 SYMATTR Value {Cr} SYMBOL res 864 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R9 SYMATTR Value {Rp} SYMBOL cap 1008 16 R0 WINDOW 0 20 11 Left 2 SYMATTR InstName C19 SYMATTR Value {Cr} SYMBOL cap 1088 16 R0 SYMATTR InstName C20 SYMATTR Value {Cr} SYMBOL res 1008 0 R270 WINDOW 0 -24 71 VTop 2 WINDOW 3 68 60 VBottom 2 SYMATTR InstName R10 SYMATTR Value {Rp} SYMBOL cap -48 160 R0 SYMATTR InstName C21 SYMATTR Value {Cs} SYMBOL cap 768 160 R0 SYMATTR InstName C22 SYMATTR Value {Cs} SYMBOL res 1184 0 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R91 SYMATTR Value 1200 SYMBOL cap 1328 272 R180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C92 SYMATTR Value 900p SYMBOL res 1296 64 R0 WINDOW 3 -56 49 Left 2 WINDOW 0 -40 8 Left 2 SYMATTR Value 590K SYMATTR InstName R92 TEXT 264 256 Left 2 !.tran 0 10m 0 10u TEXT 264 -232 Left 4 ;40KV 2E9 ohms HV scope probe 2500:1 TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form TEXT 168 160 Left 2 !.PARAM Cr=2.048p, Cs=0.1p, Rp=200Meg TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form TEXT 1536 -152 Left 2 ;[SCOPE] TEXT 1320 -152 Left 2 ;[PROBE] TEXT 336 -184 Left 3 ;Tr~27nSec; 2% overshoot at 1mSec

I see you made Cs, capacitance from floating shields, a lot larger =

8pf instead of my wild guesstimate of 0.1pf. Those floating shields allow defined and predictable capacitive coupling across the resistors; the capacitance from them to "outer space" ground seems to be undefinable: coax capacitance runs (log(D/d))^-1 and with a theoretically infinite D (or very large D in reality),the capacitance is rather close to zero. I picked 0.1pf as an estimate to that "zero"; seems you picked a huge 8pf.

• I did not know of that uniform RC line (U) model; it would be an excellent choice; better than the pi-pad scheme. Values can easily be calculated on basis of resistor diameter, dielectric material and floating shield diameter. Seems that an eXplicit shield around the floating shield makes for more trouble.

No, I refer you to your previous post,:

"TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg"

Cs=7.9pF

Exact copy of your published schematic, with the addition of 3 feet of cable, and two resistors.

I wondered about that.

The same value occurs in the listing on your website.

I didn't pick it, you did ;-(

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

You can't ignore the effect of what is, in effect, an open-circuit quarter wave line, at a frequency within the intended bandwidth of the probe.

3 feet of 0,66 velocity coax, terminated in a large resistance will resonate at about 54MHz, and ring like a bell. That's why resistive cables are used.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

Modern automotive "spark plug wire". Has a "graphite core". Specific R per foot value. Meant to be an emission suppressive passage for HV pulses being fed to the plugs, something which generally causes a pretty big magnetic spike to emanate.

There were folks in the '50s and '60s (hams) who even went so far as to fully ground shield each of their spark plug wires to reduce the problem. Then they (those 'they' people) came out with the better, resistive wire and the problem sufficiently subsided..

May not be exactly what you seek, but that is the only version I am aware of.

Probably easiest to buy a scope probe and cut the ends off!

See: Patent number: 2883619 Issue date: Apr 21, 1959

We had an old car here once that I decided to tackle a blower problem in the heating and ventilation system. THe problem, no blower operation.

After tearing out the blower box under the glove compartment, which was a big job getting to, I found there was nothing wrong with the blower motor. The plug was too hard to get to so it wasn't an easy option to simply test it before hand. So I went to the control switch, that worked ok. So I then started ripping apart the harness, they had put a resistor wire in the harness for the blower to reduce the brush noise in the electrical system.

At that point, I really didn't care much for the car, it was getting old, so I just hung a copper wire outside of the harness.. I can say this, not only was that wire there to reduce noise, it also served as a voltage drop to the blower, that thing was like a wind tunnel afterwards :)

Jamie

maybe you need this? copper clad conductor..

Jamie

This is my best birthday present.

grrrrrrrr.

how many pine trees had to be cut down to build that wall for you?

Jamie

I suspect that probe manufacturers get their resistive cables made to order. That implies a few thousand feet to get a cable manufacturer interested.

*Almost* the same effect can be got by putting a small (tens to hundreds of ohms) in series with each end, if all you want is to damp resonances in a probe.

You might try a few of the "usual suspect" cable manufacturers. There are a few specialist cable companies in Germany who claim to make small quantities of specials to order.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

It's the scope probe type cable I'm after indeed. I contacted Draka to see if they could deliver any. They said they were willing to make it to my specs, provided I'd buy at least 1km. I'd need a dozen or so five-meter pieces. Oh well. Maybe Spehro was right: Just chop of the unwanted ends off a scope probe.

I tried that. Not good enough.

Thanks, I'll shop around.

Jeroen Belleman

Don't know. The entire cabin is knotty pine, inside and out. But pine trees are mostly farmed, a renewable resource.

The cabin is sort of a small-scale ripoff of a Frank Lloyd Wright church in Madison, Wisconsin. Go see it if you're even in the neighborhood. The church, I mean.

The baer was done by an elementary school kid, part of a fund raiser up in Truckee. My other birthday present was the snow.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

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