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Viewing ohms change on oscope???

S

scanner80

Jan 1, 1970
0
Hello,
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
will need to be able to see a change as small as 1 ohm if possible , but the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I need
a way to exactly measure the change.
I will be greatful for any help.
Thank you,
Jeff
 
S

scanner80

Jan 1, 1970
0
I just want to be able to measure the time it takes from when the resistance
leaves 1355 ohms and returns to 1355. I can do it with a meter and a
stopwatch, but I was looking for a more verifiable way to do it. I want to
remove human error. This is for calibration purposes.
Is there any type of simple timer or timer circuit that can will trigger as
the ohms changes and stop when it returns?
Jeff
 
B

Bob Monsen

Jan 1, 1970
0
scanner80 said:
I just want to be able to measure the time it takes from when the resistance
leaves 1355 ohms and returns to 1355. I can do it with a meter and a
stopwatch, but I was looking for a more verifiable way to do it. I want to
remove human error. This is for calibration purposes.
Is there any type of simple timer or timer circuit that can will trigger as
the ohms changes and stop when it returns?
Jeff

Build or buy a precision current source to source 1mA, then use a
multimeter. My cheapo sears multimeter can resolve 1 mV, and it only
cost $30.

A fairly reasonable current source can be made with a TL431. Use a 1%
2.49k resistor from ref to anode, put your thingy from ref to cathode,
then connect a 1k resistor to a 5V source. Your ohms output is then 1000
* V(cathode) - 2500.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
 
S

scanner80

Jan 1, 1970
0
I have a Flike 5500 calibrator to source with and a Fluke 87 for mA
measurements.
The device I'm testing will use an opamp circuit to create the resistances
and the
change. Will the current setup work with that type of circuit without
causing it to malfunction?
Normally the device under test requires a ohmmeter with 10uA or 100uA of
source current for ohms measurements.
Jeff
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
scanner80 said:
I just want to be able to measure the time it takes from when the resistance
leaves 1355 ohms and returns to 1355. I can do it with a meter and a
stopwatch, but I was looking for a more verifiable way to do it. I want to
remove human error. This is for calibration purposes.
Is there any type of simple timer or timer circuit that can will trigger as
the ohms changes and stop when it returns?

You can make a window comparator that puts out a logic 1 if the input
voltage is within a certain range. The resistor under test can be
supplied by a precision current source to produce a voltage signal. Or,
if the comparator reference voltage divider and your resistor are driven
from the same voltage source and precision resistors are used, a supply
with reasonably stable voltage regulation will suffice.

The comparator output can drive a simple timer circuit, or an LED if
stopwatch precision will suffice.
 
B

Bob Monsen

Jan 1, 1970
0
scanner80 said:
I have a Flike 5500 calibrator to source with and a Fluke 87 for mA
measurements.
The device I'm testing will use an opamp circuit to create the resistances
and the
change. Will the current setup work with that type of circuit without
causing it to malfunction?
Normally the device under test requires a ohmmeter with 10uA or 100uA of
source current for ohms measurements.
Jeff

The device may have some kind of current limit, but 1 mA probably wont
be a problem.

If you are afraid of higher currents, you'll probably have to amplify
the resulting voltage. Use the following or something similar so as not
to disturb the voltage across the device too badly:

http://www.ecircuitcenter.com/Circuits/instamp1/instamp1.htm

A 100x amplifier will give you good results with 10uA. You could also
use a sample and hold circuit to match the voltage before the test, and
do a comparative measurement between the two during the test. That way,
you can amplify 1000x or even 10000x, since you will only be amplifying
the change, which should be less than 200uV at 10uA.

There is an autonulling dc lab amplifier schematic in Art of
Electronics, second edition that might be perfect (perhaps a more than
you really need...) for your application. The schematic is Figure 7.1 on
page 393. Set the gain, null it out, and start the test.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
 
F

Fred Bloggs

Jan 1, 1970
0
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
will need to be able to see a change as small as 1 ohm if possible , but the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I need
a way to exactly measure the change.
I will be greatful for any help.

You can almost make this into a Radio Shack project using the circuit
below- the only matching required is on the two 1% 12.4K ohms resistors.
As shown, the circuit will produce slightly less than 1mV per ohm change
in your 1355 RUT to within a few percentage points, and you can gain
this up as required. Adjust Rpot for Vout=0V while RUT is in quiescent
state.
View in a fixed-width font such as Courier.
 
F

Fred Bloggs

Jan 1, 1970
0
Fred said:
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355
ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19
seconds. I
will need to be able to see a change as small as 1 ohm if possible ,
but the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but
I need
a way to exactly measure the change.
I will be greatful for any help.


You can almost make this into a Radio Shack project using the circuit
below- the only matching required is on the two 1% 12.4K ohms resistors.
As shown, the circuit will produce slightly less than 1mV per ohm change
in your 1355 RUT to within a few percentage points, and you can gain
this up as required. Adjust Rpot for Vout=0V while RUT is in quiescent
state.
View in a fixed-width font such as Courier.

.
. +----------+--------+
. | + | |
. | | ---
. | Vref | /// V+
. | [12.4k] |
. _|_ - | U1:B |
. / \---------+---------------------|+\
. --- LM385 | | >--+-> Vout
. | Q2 e +---|-/ |
. | |/ | | |
. | Q1 e---| 2x PN4250A | | |
. | |/ |\ | V- |
. +--| c | |
. | |\ | +-[12.4k]-+
. | c | |
. | | | U1:A Q3 c
. | +-----+--------|+\ |/
. | | | >---| Vref
. [100k] || +--|-/ |\ Vout= ---- x d(RUT)
. | 100uA|| | e RUTo
. | v| | |
. | | +-----------+ _
. | | | /|
. | [RUT] [Rpot]
. | | /|
. | (1355)| |
. | | |
. +----------+-----------------+
. |
. ---
. V-
.
.

That would be a dual single-supply OA running off +/-5 or so, low offset
type preferable, Q3 is high gain.
 
J

John Larkin

Jan 1, 1970
0
Hello,
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
will need to be able to see a change as small as 1 ohm if possible , but the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I need
a way to exactly measure the change.
I will be greatful for any help.
Thank you,
Jeff

The quick way to do this is to use a 9-volt battery or two and make a
4-resistor wheatstone bridge, with your test resistor as one of the
legs, and go directly into the scope. Crank up scope gain as required.
You may need to hang a big capacitor across the scope input if you
pick up too much hum.

Refinements: a zero-null pot, and a switchable 10-ohm cal check
resistor.

John
 
F

Fred Bloggs

Jan 1, 1970
0
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
will need to be able to see a change as small as 1 ohm if possible , but the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I need
a way to exactly measure the change.

Here is how you can gain that sensor ohm change by x10 with T-feedback
around the output difference amplifier. It may not look like it, but
this is a bridge. The "cal" switch is either an NC momentary or a jumper
strap, open it and record the change in Vout to get the exact gain
constant. Do this after you zero the output with Rpot. Rpot will be
adjusted to be equal R.U.T. within 1-2% for zero, so you figure out the
range you need.
View in a fixed-width font such as Courier.
 
K

Ken Taylor

Jan 1, 1970
0
scanner80 said:
Hello,
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms.
It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds.
I
will need to be able to see a change as small as 1 ohm if possible , but
the
most important thing is to see the reading change from 1355 and return to
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I
need
a way to exactly measure the change.
I will be greatful for any help.
Thank you,
Jeff

Not sure if I'm missing something here - what's wrong with a 'meter with
IEEE-488 bus and a PC so-equipped. Takes a few minutes to write the program
with HPIV or VB and you can graphically display it. Alternative is to use a
higher-end Fluke or similar with RS-232 bus. The rate at which you can read
and precision of the reading is then a function of how much you spend on the
meter.

Ken
 
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