50 Gigohm Resistor?

wrongaddress@att.net · Feb 13, 2006

I had the experience of looking at a circuit board with a resistor
marked 50G, indicating 50,000 Megohms. One of the resistor leads was
mounted on a teflon pad since the resistance of the board would
probably be a good portion of 50 Gigs.

Any idea what application would make use of a resistor so high?

-Bill

Winfield Hill · Feb 13, 2006

I had the experience of looking at a circuit board with a resistor

marked 50G, indicating 50,000 Megohms. One of the resistor leads
was mounted on a teflon pad since the resistance of the board
would probably be a good portion of 50 Gigs.

Any idea what application would make use of a resistor so high?

To work with small currents, of course. I have resistors to
1T in my inventory. When using these I find that air is better
than teflon.

Sometimes one has to replace the resistor with a capacitor, if
you get my drift.

t kidd · Feb 13, 2006

I agree

Steve Noll · Feb 13, 2006

I had the experience of looking at a circuit board with a resistor
marked 50G, indicating 50,000 Megohms. One of the resistor leads was
mounted on a teflon pad since the resistance of the board would
probably be a good portion of 50 Gigs.

Any idea what application would make use of a resistor so high?

-Bill

One application is a feedback resistor for a transimpedance amp for a
photodiode.

Steve Noll | The Used Equipment Dealer Directory:
| http://www.big-list.com
| Peltier Information Directory:
| http://www.peltier-info.com

Joerg · Feb 13, 2006

Hello Win,

To work with small currents, of course. I have resistors to
1T in my inventory. When using these I find that air is better
than teflon.

And after turning it on, don't breathe

Regards, Joerg

www.interfacebus.com · Feb 13, 2006

Wouldn't that be more than the resistance of air?

I think your reading it wrong. I think the highest resistors only make
it to 10's of Meg, and not 1000's

http://www.interfacebus.com/resistor_table.html

Joerg · Feb 13, 2006

Wouldn't that be more than the resistance of air?

I think your reading it wrong. I think the highest resistors only make
it to 10's of Meg, and not 1000's

15M, occasionally 18M is usually the highest readily available value.
Anything higher I'd count under boutique parts. Might be needed for
highly specialized measurements but should not be used in day to day
designs IMHO.

Regards, Joerg

Phil Allison · Feb 13, 2006

I think your reading it wrong. I think the highest resistors only make
it to 10's of Meg, and not 1000's

** Wrong:

http://www.ohmite.com/cgi-bin/showpage.cgi?product=hvc

Condenser microphones typically use 1Gohm resistors to bias the capsule and
FET preamp.

.......... Phil

Ken Smith · Feb 13, 2006

Any idea what application would make use of a resistor so high?

One application is a feedback resistor for a transimpedance amp for a
photodiode.[/QUOTE]

I can imagine a situation where you'd want one that high on a
transimpedance amp. Consider an op-amp with a 1nA bias current.

50E9 * 1E-9 = 50V

That's more offset voltage than the Vcc limit of most op-amps.

Spehro Pefhany · Feb 13, 2006

Wouldn't that be more than the resistance of air?

I think your reading it wrong. I think the highest resistors only make
it to 10's of Meg, and not 1000's

http://www.interfacebus.com/resistor_table.html

Nah, 50G is not all that high.

http://www.ohmcraft.com/pdfs/sm.pdf
(SMT parts up to 300G in 0603)

http://www.srt-restech.de/high_value___high_voltage_lead.php
(up to 100T = 100,000,000 Megohms)

Best regards,
Spehro Pefhany

Ken Smith · Feb 13, 2006

Phil Allison said:
Condenser microphones typically use 1Gohm resistors to bias the capsule and
FET preamp.

I know one design uses quite a bit more than even that.

Winfield Hill · Feb 13, 2006

Spehro Pefhany wrote...

Nah, 50G is not all that high.

http://www.ohmcraft.com/pdfs/sm.pdf
(SMT parts up to 300G in 0603)

http://www.srt-restech.de/high_value___high_voltage_lead.php
(up to 100T = 100,000,000 Megohms)

Are there distributors who carry either of these guys parts?

Phil Allison · Feb 13, 2006

"Ken Smith"

Phil Allison

I know one design uses quite a bit more than even that.

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

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

........ Phil

Steve Noll · Feb 13, 2006

One application is a feedback resistor for a transimpedance amp for a
photodiode.

I can imagine a situation where you'd want one that high on a
transimpedance amp. Consider an op-amp with a 1nA bias current.

50E9 * 1E-9 = 50V

That's more offset voltage than the Vcc limit of most op-amps.

--[/QUOTE]

I've built photodiode transimpedance amps using as high as 100G-ohm
resistors before. It takes some care, but is definitely do-able.
It's interesting and dramatic to watch the offsets decrease when the
box containing the circuit is flushed with dry nitrogen.

Steve Noll | The Used Equipment Dealer Directory:
| http://www.big-list.com
| Peltier Information Directory:
| http://www.peltier-info.com

Sven Wilhelmsson · Feb 13, 2006

Joerg said:
Hello Win,

And after turning it on, don't breathe

Regards, Joerg

But you can look! 1V/1Tohm = 6 Millions electrons per s.
Your eyes are sensitive to a few hundreds photons per s.
For a particle physicist 1 Tohm is shortcut.

Robert Baer · Feb 13, 2006

Winfield said:
To work with small currents, of course. I have resistors to
1T in my inventory. When using these I find that air is better
than teflon.

Sometimes one has to replace the resistor with a capacitor, if
you get my drift.

Punny!

Robert Baer · Feb 13, 2006

www.interfacebus.com said:
Wouldn't that be more than the resistance of air?

I think your reading it wrong. I think the highest resistors only make
it to 10's of Meg, and not 1000's

http://www.interfacebus.com/resistor_table.html

Of the more common hi-R resistors, note that DigiKey and Mouser carry
the various Ohmite series; to 5,000Megs and 20KV rating at 1% and 5%.
...and Ohmite is not the only maker...

Michael A. Terrell · Feb 13, 2006

Joerg said:
15M, occasionally 18M is usually the highest readily available value.
Anything higher I'd count under boutique parts. Might be needed for
highly specialized measurements but should not be used in day to day
designs IMHO.

Regards, Joerg

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

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Roger Hamlett · Feb 13, 2006

Michael A. Terrell said:
22 meg was readily available in the old carbon comp days in 5%, 10%
and 20%.

Meggitt electronic components, do 0805 SMD cermet resistors, in a range
from 15M, to 50G. Maybe even further than this, but this is the highest I
have used. The tolerance, is only 30% on the higher values, and with
slightly worse than normal temperature coefficients, but readily available
(at least in the UK - both RS, and Farnell do them).

Best Wishes

Frank Bemelman · Feb 13, 2006

Spehro Pefhany said:
http://www.srt-restech.de/high_value___high_voltage_lead.php
(up to 100T = 100,000,000 Megohms)

And if that isn't enough, you can always put a bunch in series

50 Gigohm Resistor?

50 Gigohm Resistor?

[email protected]

Winfield Hill

t kidd

Steve Noll

Joerg

www.interfacebus.com

Joerg

Phil Allison

Ken Smith

Spehro Pefhany

Ken Smith

Winfield Hill

Phil Allison

Steve Noll

Sven Wilhelmsson

Robert Baer

Robert Baer

Michael A. Terrell

Roger Hamlett

Frank Bemelman

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