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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Dec 2019, 12:00 am   #1
elanman99
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Default Giga Ohm resistors, What would these be used for?

This is a question I should be able to answer because I bought them in 1995!


They are new Welwyn glass encapsulated precision high value (1GOhm and 10GOhm) 1% tolerance and rated at 100V. I am baffled as to what I needed them for and I can only think it might have been something to do with a Photo-multiplier tube as they require a high voltage.



High value resistors are used as dividers for measuring high (kilovolt) voltages but at 100V I dont think these would be any use.


Ian
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Old 29th Dec 2019, 12:06 am   #2
Skywave
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Question Re: Giga Ohm resistors, What would these be used for?

If you intended to design & build a very high input impedance voltmeter using IGFET transistors, that design might have called for resistors of that magnitude.
That voltmeter would not necessarily been required to measure high voltages.

Just a thought . . .

Al.
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Old 29th Dec 2019, 3:44 am   #3
TonyDuell
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Default Re: Giga Ohm resistors, What would these be used for?

Wasn't one similar type of component called the 'Morganite Megistor'?

Photomultiplier tubes normally need a chain of resistors to set the voltages on the dynodes, but they are generally in the megohm/10's of megohms range. And the 100V limitation on each resistor would rather limit the use of your resistors there.

I think Skywave's suggestion of a high impedance voltmeter (possibly to use with some specialised electrode, a pH probe or something) is a good guess.

Incidentally I was told to always handle these by the wires, never touch the glass. A bit of contamination on the glass can form a conducting path that is comparable to the resistance of the component, meaning they appear to seriously change in value.
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Old 29th Dec 2019, 12:37 pm   #4
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Default Re: Giga Ohm resistors, What would these be used for?

I've recently used some 1GΩ resistors with FET's in a condenser microphone circuit.
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Old 29th Dec 2019, 12:42 pm   #5
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Default Re: Giga Ohm resistors, What would these be used for?

Electrometers/ratemeters?
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Old 29th Dec 2019, 4:22 pm   #6
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Default Re: Giga Ohm resistors, What would these be used for?

Mass spectrometers?
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Old 29th Dec 2019, 5:27 pm   #7
elanman99
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Default Re: Giga Ohm resistors, What would these be used for?

It has come to me what the resistors were for!

At the time we were measuring the hydrocarbon content of exhaust emissions using a FID (Flame Ionisation Detector). It had an electrode in the flame connected to a high impedance Opamp and I think I was going to experiment changing the gain but never did.

Thanks anyway for the various suggestions.

Ian
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Old 29th Dec 2019, 6:07 pm   #8
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Default Re: Giga Ohm resistors, What would these be used for?

They were also used in the construction of the amazing "Contamination Meter No.1" - a rare example of the use of a complex circuit constructed from cold thyratrons.
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Old 29th Dec 2019, 8:28 pm   #9
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Default Re: Giga Ohm resistors, What would these be used for?

Ah, yes, flame sensing circuits -- a.k.a. "borrowing a few charged particles from a nearby chemical reaction, to prove said reaction is happening"!

You can pass a few microamps down a strong natural gas flame -- the electron flow is one-way, from the (earthed) burner to the anode (which is energised from the mains via two big resistors in series, in case one fails short-circuit). If the same electrode is also used for ignition, there needs to be a VDR in the earthy end of the secondary of the ignition transformer. (But if you use separate spark and sense electrodes, you can make sure the flame has propagated across the whole burner.)

With a much weaker mix of some inflammable gas and air, I guess you would be talking nanoamps of flame current -- and you need a gigohm of resistance to develop a volt from a nanoamp.
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Old 29th Dec 2019, 10:57 pm   #10
Karen O
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Default Re: Giga Ohm resistors, What would these be used for?

I'm sure I've read about 'flame triodes' at some point in the past...
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Old 29th Dec 2019, 11:04 pm   #11
Malcolm G6ANZ
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Default Re: Giga Ohm resistors, What would these be used for?

Usually used for the input in ion chamber electrometers. http://www.techlib.com/science/ionpage2.html
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Old 30th Dec 2019, 8:46 am   #12
Oldmadham
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Default Re: Giga Ohm resistors, What would these be used for?

Quote:
Originally Posted by Karen O View Post
I'm sure I've read about 'flame triodes' at some point in the past...
http://www.sparkbangbuzz.com/flame-amp/flameamp.htm
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Old 30th Dec 2019, 10:12 am   #13
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Default Re: Giga Ohm resistors, What would these be used for?

They were used in ionisation chamber Radiacs.

Cheers,

Colin.
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Old 30th Dec 2019, 10:30 am   #14
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Default Re: Giga Ohm resistors, What would these be used for?

As part of my Electronics Engineering degree course at Bangor back in 1970 to 73, my project involved controlling a gas flame in a Bunsen burner using a probe in the flame to sense its conductivity. A major problem was if the flame size changed enough so the probe fell into one of the different 'cones' in the flame. The electrical properties of different regions of the flame were quite different.

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