"John Fields" wrote in message
news
On Fri, 5 Aug 2005 21:12:21 -0500, "DBLEXPOSURE"
wrote:
"scanner80" wrote in message
...
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
Jeff:
To measure resistance you need to apply a voltage and then
measure the current. Another way is to use a "voltage divider", put your
changing resistance in series with a known resistance, apply a voltage to
the circuit and monitor the voltage dropped across the known resistor. As
your changing resistance decreases current in the circuit will increase
and
the voltage drop across your known resistor will increase. This will all
be
linear so you will be able to make notations on your scope graticule so
that
you can directly readout the display in ohms.
---
That won't work, for several reasons.
The first is that the resistance change is too small to give a
detectable deflection of the scope trace with the scope vertical
input voltage range being whatever it needs to be to keep the trace
on the screen. For example:
E1
|
[R1]
|
+---E2
|
[R2]
|
0V
If we let E1 equal 1 volt and R1 and R2 equal 1000 ohms and 1355
ohms, respectively, then with R2 at 1355 ohms we'll have:
E1R2 1V * 1355R
E2 = --------- = --------------- = 0.575 volts
R1 + R2 1100R + 1355R
and with R2 at the high end, (1375 ohms) W2 will be equal to 0.579V.
That's a change of only four millivolts, which would give you a
deflection of two boxes with an input sensitivity of 2mV per box.
That might be OK, but look at what that 4mV signal is riding on: a
voltage about 150 times higher, so no matter how you adjust the
vertical sensitivity and trace position controls, I don't believe
you're going to wind up with anything that works.
The second problem is going to be triggering the scope at the
instant the resistance starts to rise, unless an external trigger
can be rigged using a comparator, and the third problem is going to
be the accuracy of the timebase and marking the screen properly when
the trace crosses the reference. In other words, when was the last
time you had your scope calibrated and how good are you with that
grease pencil/sharpie?
BTW, that 4mV change _won't_ be linear, but it doesn't matter since
all he really wants to do is look for the crossing.
---
You will want to use a regulated voltage source. A look into Ohms law
will
provide the math for you to make the calculations..
---
Woudn't have hurt for you to run the numbers, and it would have
saved me a post. 
Just as an aside, there _is_ a way to do it using a couple of
voltage dividers to make a bridge, like this:
E1--------+----------------+
| |
[R1] [R3]
| |
+---E2 E3---+
| |
[R2] [R4]
| |
0V--------+----------------+
What happens here is that when the ratio of R1 to R2 equals the
ratio of R3 to R4, then E2 will equal E3 and you can connect a scope
between E2 and E3 and measure the voltage between them without
having to worry about the offset you get with a half-bridge.
For ease of use, R3R4 could be a pot which could be adjusted for
precisely zero volts across E2 and E3 when R2 was at 1355 ohms, or
for greater resolution,
E1--------+------------------+
| |
[R1] [R3]
| |
+---E2 E3---[POT]
| |
[R2] [R4]
| |
0V--------+------------------+
One caveat, the supply should be floating or, if the scope can do
it, it should be set to display E2 minus E3
--
John Fields
Professional Circuit Designer
Ø The first is that the resistance change is too small to give a
detectable deflection of the scope trace with the scope vertical
input voltage range being whatever it needs to be to keep the trace
on the screen.
If a 30V source is used (My bench supply is 0-60Vdc regulated, BK 1623A) and
1K for the known resistance a difference in 20 Ohms of the resistor under
test will result in a rise of 100mV across the known resistance
30Vdc
|
|
R1 (1355-1375 Ohms)
|
|
R2 1K Ohm -à
|
|
Gnd------------à
1355+1000=2355 Ohms
30/2355=12.7mA
12.7mA X 1000 Ohms = 12.7V
And the high end
1375+1000=2375 Ohms
30/2375=12.6mA
12.6mA X 1000 Ohms = 12.6V
A difference of
12.7-12.6 = 100mV
Ø
The second problem is going to be triggering the scope at the
instant the resistance starts to rise, unless an external trigger
can be rigged using a comparator, and the third problem is going to
be the accuracy of the timebase and marking the screen properly when
the trace crosses the reference. In other words, when was the last
time you had your scope calibrated and how good are you with that
grease pencil/sharpie
Ø will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19
seconds.
If this whole process takes 19 seconds, triggering shouldn't be an issue.
Remember, he was using a stopwatch to measure the time, so, just use a quick
time base and watch the green line rise and fall.
10mV/cm gives me full deflection and I have plenty of range on the Vert.
position to bring the trace to the bottom of the screen. My scope is now
measuring 2 ohms/cm. I don't know why this would not be linear. Scope is
calibrated annually.
As for not running the number last night, I was doing other things and
fiddled with this while on break. And, I didn't want to steal your thunder.
I knew you would come up with a more elegant way of doing this. I'm just a
hack anyway..
I do like your bridge idea...