Daqarta and Function generator

[George Herold]

On Mar 18, 8:14 pm, John Fields <jfie...@austininstruments.com> wrote:

On Tue, 16 Mar 2010 13:14:36 -0500, John Fields





jfie...@austininstruments.com> wrote:
Another way is to use a resistor of a known value in series with the
unknown cap or coil:

                                        +-------+
     +-----[10K]----+<----[X10 PROBE]---|VERT   |
     |              |                   |       |
     |              |                   |       |
[FUNCTION GEN]    [DUT]                 | SCOPE |
     |              |                   |       |
     |              |                   |       |
     +--------------+<------------------|GND    |
                                        +-------+

I've gotta go do some chores, but I want to get this off instead of
waiting until later to send the whole thing, so I'll finish it when I'm
done...

OK.

For the cap, set up your equipment like this:

      E1                                 +-------+
      |<---------------------------------|VERTA  |
      |              E2                  |       |
      +-----[10K]----+<----[X10 PROBE]---|VERTB  |
      |              |                   |       |
      |              |                   |       |
 [FUNCTION GEN]    [DUT]                 | SCOPE |
      |              |                   |       |
      |              |                   |       |
      +--------------+<------------------|GND    |
                                         +-------+

Using the sine wave output of your function generator, set its output,
E1, to any convenient level and then tune it to the frequency which
causes E2 to be 1/2 the voltage of E1.

Then solve:

             1  
     C = ----------
          2pi f Xc

where C will be the capacitance in farads,
      2pi is 6.28,
      f is the frequency which causes E2 to be 1/2 of E1, in Hz,and
      Xc is 5800 ohms.

Why 5800 ohms?

That's the capacitive reactance you need in order to get E2 equal to
half of E1 at any frequency.

Ergo, with a constant reactance, capacitance will go inversely with
frequency.  

JF- Hide quoted text -

- Show quoted text -

If you've got a 'scope you can also just hit the RC or RL with a
square wave and find the time constant. (The point where the voltage
falls (or rises) by 1/e).

To the OP, If you want to do more than run stero speakers you can also
buy power opamps. I like the OPA544 (I think that's the number.)
( It can do at least 2 amps... more if you don't ask it to dissipate
too much power. Hmm, Just checking digikey and they are more
expensive than I remember ($18 for one) and also out of stock.

George H.