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A Simple Lab Demo of Resistor Noise EMF.



At 13:44 3/18/01 -0500, John Denker wrote:
At 11:25 AM 3/18/01 -0600, brian whatcott wrote:

Let me know how fine an oscilloscope it takes when you use the blow torch
I specified in my initial post. My scope cost me $50 (used).
You might need to put out a dollar or two for the ceramic resistor too....

Brian is obviously the expert here. Perhaps *he* will tell us
-- how much bandwidth you can get for $50.00 these days
-- how much input sensitivity (volts per cm)
-- how much instrumental noise, referred to the input
(i.e. noise figure or noise temperature)
-- where to get a megohm resistor that will withstand a blowtorch
(radio shack doesn't seem to carry them)

Better not to follow one of these famous red-herrings.....

Instead, here's a simple, cheap, straight forward protocol I hope
may be useful in the high school lab:

1) Go to RadioShack.
Ask for metal film or metal oxide resistors in assorted values.
They don't always carry them. Splash out on a 20 cent
sample or two from Farnell if necessary.

2) While there, pick up an autoranging DVM.
Ensure that it can at least register 2 volts AC in 3 and one
half digits or more so that its least significant digit
represents at most 1 millivolt.

3) At the hardware store, pick up a propane blowtorch.

4) Connect each meter lead to one lead of a resistor of
at least 120 kilohms.

5) Heat the resistor red hot. (If the resistor goes open circuit,
you will see that the DVM drifts up to 100s of millivalts.
Replace the resistor in this case.)

6) Note the reading of 4, 5 or 6 millivolts.

7) Remove from flame and check that there is a readable resistance value.

8) Repeat from step 5)

This is a reasonable classroom exercise. Just a hint of danger to keep
the students interested. Good to ask what maximal peak to peak value they
would expect from a 1 mv rms noise signal. If they cannot answer
'over 3 mv', explain the concept! Ask them if they could see this on an
oscilloscope.

p.s This afternoon I DID use Radio Shack resistors. I DID use a
Radio Shack DVM, it took approximately 3 minutes to set up.
Why not give it a try? :-)



brian whatcott <inet@intellisys.net> Altus OK
Eureka!