50 Gigohm Resistor?

[Boris Mohar]

Punny!

Yes, it would have to be small.

 

[Ancient_Hacker]

Some folks need to measure really low currents, like picoamps or even
femtoamps. For instance, if you are peeking at a faraway star, you're
only getting a few photons per second. Even after being enhanced by a
photomultiplier tube, you're still talking picoamps. You need a few
gizillion ohms as your amplifier's input impedance to get any
measureable voltage from those few electrons.

If you need any parts like that and you're not working for J. Paul
Getty, go to eBay and bid on any old Keithley electrometer, say the 6xx
series. You'll probably snarf it up for $5.99 or so. Inside you'll
find resistors from 1 ohm to 1 gigohm, all mounted on a
teflon-insulated and gold-plated switch. Drat, I gave away my source
for otherwise pricey components!

Regards,

A_H

 

[Winfield Hill]

Ancient_Hacker wrote...

Some folks need to measure really low currents, like picoamps or even
femtoamps. For instance, if you are peeking at a faraway star, you're
only getting a few photons per second. Even after being enhanced by
a photomultiplier tube, you're still talking picoamps. You need a few
gizillion ohms as your amplifier's input impedance to get any
measureable voltage from those few electrons.

If you need any parts like that and you're not working for J. Paul
Getty, go to eBay and bid on any old Keithley electrometer, say the
6xx series. You'll probably snarf it up for $5.99 or so. Inside
you'll find resistors from 1 ohm to 1 gigohm, all mounted on a
teflon-insulated and gold-plated switch. Drat, I gave away my
source for otherwise pricey components!

Yeah, but probably like me, you have a high pile of them already.
Actually, I only have one in its original condition, my technician
disassembled the rest for a nice collection of high-quality parts.

 

[Ken Smith]

"Ken Smith"



** How mysterious .........

What does it take to get you to spill the beans ??

I think I already did. Didn't I? Someone has elected to go with a higher
value. I'm sure that you've already guessed why.

 

[www.interfacebus.com]

I stand corrected, and high voltage as well.

 

[Joerg]

Hello Michael,

22 meg was readily available in the old carbon comp days in 5%, 10%
and 20%.

I remember those from the tube days but that's long gone (not the tubes
though). Recently I could really have used an 18M resistor in a circuit
but the client told me they had great difficulty sourcing even those and
asked if I could get by with 15M. It was for battery monitoring and
there every microamp counts.

Regards, Joerg

 

[Spehro Pefhany]

Hello Michael,


I remember those from the tube days but that's long gone (not the tubes
though). Recently I could really have used an 18M resistor in a circuit
but the client told me they had great difficulty sourcing even those and
asked if I could get by with 15M. It was for battery monitoring and
there every microamp counts.

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes
have dropped, 10M is about as high as you can easily get from stock in
0603, 1% or 5%. Of course you can always use multiples in series, and
often the manufactures will make higher values to order for the same
price, just maybe a bit more relaxed specs, if you buy enough.


Best regards,
Spehro Pefhany

 

[GregS]

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes
have dropped, 10M is about as high as you can easily get from stock in
0603, 1% or 5%. Of course you can always use multiples in series, and
often the manufactures will make higher values to order for the same
price, just maybe a bit more relaxed specs, if you buy enough.

I was using the same value for the same reasons for a photodiode amp, 33M.
I ordered some high ohms from K&M Kobra many years ago, and still have some.
I recall having to order many values at a fairly high cost. The trick about using them
in some current measurments was getting them in low capacitance varieties.

greg

 

[Joerg]

Hello Spehro,

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes
have dropped, 10M is about as high as you can easily get from stock in
0603, 1% or 5%. Of course you can always use multiples in series, and
often the manufactures will make higher values to order for the same
price, just maybe a bit more relaxed specs, if you buy enough.

They can do custom but I found that to be a hassle with production in
Asia. They like everything off-the-shelf from half a dozen released vendors.

You can also take a trimmable resistor and do some aggressive meander
cuts with the laser but only if laser trim is required anyway for other
stuff.

Regards, Joerg

 

[Rich Grise]

Hello Michael, [who said]

22 meg was readily available in the old carbon comp days in 5%, 10%
and 20%.

I remember those from the tube days but that's long gone (not the tubes
though). Recently I could really have used an 18M resistor in a circuit
but the client told me they had great difficulty sourcing even those and
asked if I could get by with 15M. It was for battery monitoring and
there every microamp counts.

Hi, Joerg. The thing that popped up in my mind was a memory from about
a millennium ago, of me paging through the Allied catalog with almost
as much lust as I used to page through the Sears Catalog, which, let's
face it, was a lot! I used to drool looking at the Sears tractors with
PTO options, and other farm stuff and household stuff, and, well, I don't
know which part of the Allied catalog maps onto the Sears underwear
section, but hopefully everyone's got the point by now) and in the order
section, you select the shipping method, and one option was "Freight (rail
or truck)" - yeah, back in those days Sears sold stuff so big it had to be
shipped by train! ;-)

But the point of this post is, I remember looking at the ordering
scheme for the Allied resistors, and there was a table of normalized
values, with sort of a caption: "10 ohms to 22 megs."

Brought back memories.

Cheers!
Rich

 

[Rich Grise]

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes
have dropped, 10M is about as high as you can easily get from stock in
0603, 1% or 5%. Of course you can always use multiples in series, and
often the manufactures will make higher values to order for the same
price, just maybe a bit more relaxed specs, if you buy enough.

Not necessarily seriously, when you've got 10M in a (an?) 0603 SMT, do
you have to take heroic measures to deal with ordinary leakage?

Thanks,
Rich

 

[Joerg]

Hello Rich,

... I don't
know which part of the Allied catalog maps onto the Sears underwear
section, but hopefully everyone's got the point by now) ...

Wrong catalog. That would be Victoria's Secrets

But the point of this post is, I remember looking at the ordering
scheme for the Allied resistors, and there was a table of normalized
values, with sort of a caption: "10 ohms to 22 megs."

22M was popular as a resistor from gate to ground. But with the
widespread demise of tubes those went away. I still have some.

Regards, Joerg

 

[Spehro Pefhany]

Not necessarily seriously, when you've got 10M in a (an?) 0603 SMT, do
you have to take heroic measures to deal with ordinary leakage?

Thanks,
Rich

Nope, not at 10M. Well, unless you're trying for 0.1% or something
like that (10G in shunt blows your error budget).


Best regards,
Spehro Pefhany

 

[Eric]

Service to my country? Been there, Done that, and I've got my DD214 to prove it.

Thanks for your service, Michael!

 

[Phil Allison]

"Ken Smith"

I think I already did. Didn't I?

** Nope.

You pompously alluded to a mysterious product.

Proves you are a liar or a wanker.

Or both.




........ Phil

 

[Winfield Hill]

Spehro Pefhany wrote...

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes
have dropped, 10M is about as high as you can easily get from stock in
0603, 1% or 5%. Of course you can always use multiples in series, and
often the manufactures will make higher values to order for the same
price, just maybe a bit more relaxed specs, if you buy enough.

We used to order 100M in 1k to 10k quantities for our instruments,
at standard prices and delivery. Who knows, it's possible the
distributor stocked them due to our steady, if small, orders.

 

[Winfield Hill]

Ken Smith wrote...

I think I already did. Didn't I? Someone has elected to go
with a higher value. I'm sure that you've already guessed why.

They wanted response down to 0.1Hz?

 

[Rich Grise]

Hello Rich,

Wrong catalog. That would be Victoria's Secrets

Twarn't none o' that Victoria's Seecrit nonsense back then! Just good ol',
practical, cotton and canvas foundation garments. ;-D Which, by the way,
is _like_ VS, to a pubescent boy who was raised in the forest by a family
of cows. ;-)
....

22M was popular as a resistor from gate to ground. But with the
widespread demise of tubes those went away. I still have some.

One day I'll look in my "random resistors left over from breadboard
projects that I haven't bothered to sort yet" bin - I'm pretty sure
I've got a couple of 1M; don't know if I have any higher, or why
I would, for that matter. ;-)

Cheers!
Rich

 

[Rich Grise]

Thanks for your service, Michael!

Yeah, I was in the service pretty much around the same time as MAT was,
but I did my Army-dodging the smart way: I joined the Air Force. ;-)

The closest I got to combat was one time in Thailand an airplane came
back to the base with a .50 cal bullet hole in it. About 40 or so guys
took a break from the shop and swarmed around the airplane, oohing
and ahing - "Wow! They're really shooting real bullets over there!"

Then, they fixed the plane, and we all went downtown and got drunk and
stoned and played with the pleasure ladies. ;-)

Before I went on that assignment, I had a few weeks or so of specialized
training on the equipments that would be there, at Keesler AFB, Biloxi,
MS. A coule of my buddies had already been to Thailand, so I asked them
to tell me about it. They were rhapsodic. It made my eyes red just
thinkin' about it. %-}

And, of course, one of them mentioned the khatoys, aka "ladyboys" -
pre-op transsexuals (or maybe they have no intent of changing, but they
can be as pretty as some girls - boy Thais have kinda feminine features
anyway, which is probably why the straight ones take up kick-boxing and
kung fu and stuff.) who offer blow jobs in the street for a couple bucks.
One of the fellows quipped, "What the hell, a blow job's a blow job!"

Cheers!
Rich
--
Elect Me President in 2008! I will:
A. Fire the IRS, and abolish the income tax
B. Legalize drugs
C. Stand down all military actions by the US that don't involve actual
military aggression against US territory
D. Declare World Peace I.

 

[John Larkin]

Ken Smith wrote...

They wanted response down to 0.1Hz?

I once designed a telemetry system to pick up the sound of the Saturn
S1B rocket engine static tests, with pickups scattered about in the
backwoods of Mississippi. We used a closed-loop-heated GR
piezo-ceramic microphone and just dumped it into the gate of a jfet
follower [1], no resistors at all, and got below 0.1 Hz. That was
analog FM'd and fed over dedicated land-lines back to the MTF site.
That's when I learned about Bessel functions and FM distortion in
bandlimited systems, before I had that sort of stuff in school.

The locals liked to shoot the boxes off their poles, so there was a
decent replacement market.

NASA thought we had an intermittent subsonic motorboating problem, but
it turned out to be the mating call of male alligators.

Gosh, I *have* done a lot of silly things.

John

[1] a *silicon* jfet. TI was actually making some germanium jfets at
the time, and they were pretty good for low-noise RF front-ends.