Very high value resistors

It seems there are resistors around in the range of 50Gohm! Having never encountered anything beyond 10M., I never imagined there could be such a stratospheric value but apparently there is:

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What on God's good earth would these possibly be used for?

Reply to
Chris
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Imaginary currents >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I have some sample surface-mount 1T resistors.

My old Keithley will measure up to 1e14 ohms.

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High-value resistors are useful in photodiode circuits, high-voltage dividers, leakage measurements, femtoammeters, all sorts of stuff.

A 1G or 10G radial resistor makes a cheap high-voltage probe for a DVM.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I don't recall the values but at work there was some equipment that used them They were in part of an amplifier for a radiation detector similar to a very large geiger counter. It used a detector with a 4 foot long tube.

Reply to
Ralph Mowery

There's a small stash of 100M resistors in my collection.

For anything in the G Ohm range - you should probably check the leakage resistance of whatever PCB/substrate you're mounting it on.

At the very least, you'll need guard slots cut in any PCB around the termination points.

Reply to
Ian Field

Yup. A couple of years back, I did a front end for a surface potential tool for semiconductor measurements that had switchable 1G and 50G feedback resistors.

There are lots of subtleties in a circuit like that--I had to bootstrap the coil of the latching relay, and switch it with another (nonlatching) relay. Also the 1G resistor had to be shorted out when the circuit was in the 50G mode, because otherwise its current noise would have gone straight through the capacitance of the open relay contacts (~0.2 pF) and trashed the measurement.

And, of course, you have to fix up the ridiculous RC rolloff in the second stage.

The good news about that is that the intrinsic SNR of the measurement isn't that good, so the RC can roll off a pretty long way before the second stage's noise becomes important. That's true of high-Z measurements in general.

The customer (Qcept Technologies) ran out of dough last summer and had to shut its doors with a bang. A pity--they made good stuff.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Yikes, 50 big ones:

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That happened in the big .com bust here. The landlord or the cleaners or somebody came by and everyone was gone.

It's gonna happen again.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Here's a small PCB deliberately glopped with rosin flux and fingerprints.

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It's pinning the needle on the 1e14 ohm range.

1G is easy. Just don't use water soluble flux, or if you do, clean it really good.
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John Larkin         Highland Technology, Inc 

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John Larkin
[...]

Nice to see these wonderful old vintage instruments still in use outside of ham circles. What's it like accuracy wise?

Reply to
Cursitor Doom

Been a long time since I used anything in the 100 Meg range or above, but I recall that there were a lot of recommended precautions. One was to clean the glass body very carefully (with alcohol, I think) to remove fingerprints which would otherwise trap dust, which would then allow humidity to cause a low-resistance shunt. You also want the body to be elevated above the board surface for the same reason. I also seem to recall something about teflon mounts/feedthroughs at the lead ends, but I can't recall exactly what that was supposed to do.

Best regards,

Bob Masta DAQARTA v9.10 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

I've got this "worry" in the back of my head that the leakage won't be linear with voltage. It would be fun to see pcb with rosin flux leakage at

1V, 3V, 10V, 30V, 100V....

George H.

Reply to
George Herold

Sounds like urban myths. 100M is regular PC board turf, axial or surface-mount parts. Fingerprints seem to have no effect in 1G ohm circuits.

Glass resistors are overkill in the low Gohm range.

High voltage breakdown is another matter. Fingerprints and dust can reduce breakover voltages, in the many-kilovolt domain.

Water-wash flux is evil. It can leave hygroscopic crud on and under parts.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

It's hard to tell, but it agrees pretty well with purchased or sample high-ohm resistors, usually to around 5% or better. It's better on the lower (

Reply to
John Larkin

Sure is. So is garden-variety IPA solvent. In high-Z circuits, I usually used guard traces top and bottom (with the solder mask removed) and a slot under the photodiode and the feedback resistor (and cap, if any). Being able to get solvent under the components helps a lot with the 1/f noise and stability.

The popcorn noise you get from a board washed with drugstore IPA has to be seen to be believed.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

I'd believe it. The drugstore stuff I've used (lots of it over the years) seems to have denaturing agents added or some other alcohols even though it's not needed.

Optical cleaning grade IPA doesn't seem to have the other "stuff" in it.

Reply to
Cydrome Leader

Isopropanol (IPA) has low surface tension, so it forms a film. It's also hygroscopic, so it becomes (after some evaporation) a water film full of any crud you can imagine. And when it dries, all the empty board space looks clean, because the dissolved stuff is deposited at the capillary-attractive sites you DON'T see. Water, by comparison, has higher surface tension, forms droplets you can remove with airflow.

Wash with IPA, rinse with distilled water, blow dry (with clean air) then bake dry at low temperature for an hour, if you really want to clean.

Reply to
whit3rd

On my bench, if I'm too lazy to take a board downstairs to the solvent cleaner machine, I use acetone. Cleans rosin flux nicely.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Exotic chemicals and solvents are hard to come by in the UK - car spay paint thinners is less difficult to obtain.

It shifts rosin flux very well, but is destructive to some components.

Reply to
Ian Field

Acetone is considered exotic? Here (in the US) we can walk into any hardware store and buy Acetone/ alcohols/ other stuff.

George H.

Reply to
George Herold

Many women use Acetone every day. It is often used to clean the nail polish off. I just bought some in a can at Walmart in the paint department, a store that sells most everything including food.

Reply to
Ralph Mowery

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