Re: tubes Vs fets, ohmmeter schematics



Chris Hornbeck wrote:

On Sun, 10 Jun 2007 14:24:06 GMT, Patrick Turner
<info@xxxxxxxxxxxxxxxxxx> wrote:

It possible to apply 9V from a batter to two terminals, and have a
voltage divider to
work the input to a darlington pair of signal bjts to give some
indication which works a meter.
My friend's was like this but again, no calibration.

Does anyone have a schematic which will read say 0-30M, 30M to 100M,
100M to 300M, 300M to 1,000M, &above 1,000M ?

The proven workable schematic should be based around applying a low
voltage of say 9V across the DUT,
and the measurement based on the amplification of small dc current flow.

How about a 9 volt battery and a collection of shunts
across a DVM, the resistance under test and the DVM/shunt
forming a simple voltage divider?

Assuming a 10 MegOhm DVM with a usable sensitivity of
10mVolts full scale, required shunts would be (very
roughly; values would need to be trimmed for scale
accuracy) :

100MegOhm FS = 100K Ohms shunt

1000MegOhm FS = 1Meg Ohm shunt

etc. I hope I haven't made my usual three orders of magnitude
error somewhere. Arf.

Much thanks, as always,

Chris Hornbeck

OK, let us suppose we have 1,000M, ( 1 giga-ohm ) to measure.

My Fluke has a 0-300mV dc millivolt range which does not work anymore,
but has LCD display plus digits and auto ranging for mV expressed down
to
0.001mV.
The 1M shunt would make the meter have about 800k of input resistance,
so 1.2M would give a round figure of 1M.

So the series R between 9V and 0V with meter and 1,000M to measure =
1,001M,
and I should have I = 9V / 1,001,000k which = 9uA,
thus producing 9mV dc across the 1M of the meter input R.
1mV would indicate R = 10Gohms
1.0Vdc would indicate quite low resistance

This will move around if the connection isn't constant, so a cap of
say 0.033uF across the meter would remove fast V changes.

Have I answered my own question?

A j-fet source follower input with following opamp with gain = say 100
so 1Gohm gives a 1V reading, with a less sensitive meter. A diode
limiter would be needed.

Assuming the above arrangement is OK, I can make a probe for measuring
resistance of the membrane coating for ESLs using a pair of coins as
electrodes soldered
to two threaded rods mounted in a piece of clean polycarbonate plastic
so hand effects are eliminated.

If I used 100V dc for the resistance test, the same gear can be used
without fear of meter
overload, or fusing the coating on a membrane easily.

BTW, my Fluke is very vague about R < 1ohm, and using a voltage source
and shunt R of
say 10.00ohms would tell me current in 1ohm, and the voltage across the
tested R
is divided by the current and R more accurately found.

0.01 ohms with 1Amp flow gives 10mV across R, and read as 10V across the
meter.
R = 0.01V / (10V / 10ohms)
0.1 ohms with 1A gives 0.1V, and 10V across the meter
1ohm gives I = 10V / 11ohms = 0.909 A = 9.1 V approx, and 0.91V across
the 1ohm,
so R is calculated at 0.91V / ( 9.1 / 10 ) = 1.0 ohms.

More simply R tested = 10 x VR / Vmeter


My temporary dc bench supply fitted with a 10V regulator should
do the trick without smoke generation.
For low wattage resistors, R shunt on the meter could be 100ohms.

So 0.01ohms gives R = 100 x 0.001V / 10V ohms.

Patrick Turner.
.

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