1G resistor

Den fredag den 20. november 2015 kl. 18.21.58 UTC+1 skrev John Larkin:

it is only 100x :P

how baout these?

-Lasse

On Fri, 20 Nov 2015 12:06:36 -0500, Phil Hobbs wrote:

A pcb-mount SMB with no cap would be OK, I think. I'll measure one. It might be tricky to get the prudent HV creepage distance on the pads. There must be other parts that are accidentally small HV capacitors.

HP built a sampling head that had a small screw sticking up off a PC board, as the blowby compensation adjust. One just cranked it up and down with a screwdriver.

I was thinking of using an ACPL227 optoisolator as a 1 pF capacitor. I'm using them already. Rated 3KV RMS, 37 cents.

Maybe I'll do that. I will look weird on the schematic, but so does the PCB capacitor.

Version 4 SHEET 1 1072 680 WIRE 128 0 -160 0 WIRE 448 0 128 0 WIRE 528 0 448 0 WIRE 576 0 528 0 WIRE 976 0 880 0 WIRE 976 32 976 0 WIRE 880 64 880 0 WIRE 128 80 128 0 WIRE 448 96 448 0 WIRE -160 160 -160 0 WIRE 880 192 880 144 WIRE 128 208 128 160 WIRE 288 208 128 208 WIRE 448 208 448 160 WIRE 448 208 288 208 WIRE 848 208 448 208 WIRE 976 224 912 224 WIRE 1024 224 976 224 WIRE 1040 224 1024 224 WIRE 848 240 800 240 WIRE 128 256 128 208 WIRE 288 272 288 208 WIRE 448 272 448 208 WIRE -160 304 -160 240 WIRE 800 320 800 240 WIRE 976 320 976 224 WIRE 976 320 800 320 WIRE 128 400 128 336 WIRE 288 400 288 336 WIRE 448 400 448 336 WIRE 592 400 448 400 WIRE 800 400 800 320 WIRE 800 400 672 400 WIRE 880 400 880 256 WIRE 448 432 448 400 WIRE 880 560 880 480 WIRE 448 576 448 512 FLAG -160 304 0 FLAG 128 400 0 FLAG 528 0 PULSE FLAG 1024 224 MON FLAG 880 560 0 FLAG 976 32 0 FLAG 448 576 0 FLAG 288 400 0 SYMBOL res 112 64 R0 WINDOW 0 -65 42 Left 2 WINDOW 3 -68 81 Left 2 SYMATTR InstName R1 SYMATTR Value 1G SYMBOL res 112 240 R0 WINDOW 0 -68 40 Left 2 WINDOW 3 -87 85 Left 2 SYMATTR InstName R2 SYMATTR Value 1Meg SYMBOL cap 432 96 R0 WINDOW 0 -71 5 Left 2 WINDOW 3 -79 36 Left 2 SYMATTR InstName C1 SYMATTR Value 1.1p SYMBOL cap 432 272 R0 WINDOW 0 67 15 Left 2 WINDOW 3 66 48 Left 2 SYMATTR InstName C2 SYMATTR Value 1.5n SYMBOL voltage -160 144 R0 WINDOW 0 -78 146 Left 2 WINDOW 3 -260 208 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 1200 1u 10u 10u 1m) SYMBOL Opamps\\UniversalOpamp2 880 224 M180 WINDOW 0 35 46 Left 2 SYMATTR InstName U1 SYMATTR Value2 Avol=1Meg GBW=3Meg Slew=3Meg SYMBOL voltage 880 384 R0 WINDOW 0 65 37 Left 2 WINDOW 3 73 72 Left 2 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL voltage 880 48 R0 WINDOW 0 -86 39 Left 2 WINDOW 3 -80 75 Left 2 SYMATTR InstName V3 SYMATTR Value 5 SYMBOL res 432 416 R0 WINDOW 0 -79 49 Left 2 WINDOW 3 -105 83 Left 2 SYMATTR InstName R3 SYMATTR Value {500*P} SYMBOL res 688 384 R90 WINDOW 0 -14 58 VBottom 2 WINDOW 3 45 52 VTop 2 SYMATTR InstName R4 SYMATTR Value {500*(1-p)} SYMBOL cap 272 272 R0 WINDOW 0 61 21 Left 2 WINDOW 3 49 50 Left 2 SYMATTR InstName C3 SYMATTR Value 270p TEXT -464 64 Left 2 ;1000:1 HV PICKOFF TEXT -408 192 Left 2 !.tran 2m TEXT -448 120 Left 2 ;JL Nov 20, 2015 TEXT 504 504 Left 2 ;500 ohm pot TEXT -344 440 Left 2 !.param P=0.45 TEXT 488 128 Left 2 ;ACPL227 TEXT 744 176 Left 2 ;OPA171

I still think that a cutout below the resistor would give you a nice stable capacitance of a reasonable value, while improving your creepage, and cost nearly zilch. Put it on a paddle if you're worried about board flex cracking the resistor

Cheers

Phil Hobbs

Cheers, James

On Fri, 20 Nov 2015 12:41:51 -0500, Phil Hobbs Gave us:

Put a piece of heat shrink on a round thru hole HV resistor and place a single turn of mag wire on that... and make a super-regenerative feedback. :-)

This is suitably weird.

The Mardi Gras colors are company standard, over my objections.

WHat I find weird is the the fact your opto's are connected in such a way..

basically shorted wtih no common reference on the bottom? what's that all about ? or am I seeing something others are not?

Jamie

On Fri, 20 Nov 2015 15:25:48 -0500, M Philbrook Gave us:

You're an idiot, and others are not.

Hey, back off. Not everyone can be born geniuses like us.

He's using it as a capacitance. Really. (Someone's going to reverse-engineer this some day, and John will know when he reads an obituary about a competitor's head exploding.)

Cheers, James Arthur

Hahaha

On Fri, 20 Nov 2015 21:26:21 -0500, "Tom Miller" Gave us:

Current tv commercial with exploding heads and purple vapor coming out...

It's crazy that the ACPL is a 1 pF cap, rated 3000 VRMS. It has two LEDs, two phototransistors, lead frame, wire bonds, internal insulation, potted, laser marked. For 38 cents, a tiny fraction of the price for a 3KV ceramic cap, which is just a blob of stuff.

Pickoff_ACPL_1.jpg

That reminds me of the time I used a schottky diode with A and K shorted together. It didn't do much electrically, but the big rectangular black epoxy case was the perfect size for blocking the light bleed between two front panel LEDs.

The design review was fun.

Allan

Nice idea. We've had LED bleed problems. Light pipes and such cost more than schottky diodes. What part has white sides? A trimpot maybe? A boxed film cap?

I bet I could make a nice low-pF cap by using two surface-mount tantalum caps, physically parallel, some modest distance apart.

Or two film caps, which could stick up higher off the board.

Or two turret terminals. But caps are cheaper.

There could be a whole category of parts used for not-intended uses.

As a kid I used to use low-ohms carbon comp resistors as firecrackers (with the aid of a TV cheater cord).

Does that count?

Cheers

Phil Hobbs

Sure. Did you ever use firecrackers as resistors?

Nah, mine only had one lead. And the iron was too hot. ;)

Cheers

Phil

But a capacitor has better specs than the ACPL capacitance. It says .6pF nominal and 1pF max. No min specified. That's a pretty wide range.