In article <> (who3) writes:

>   Hoping this is the right place.

>   I am looking for information on what i think is called a "curve-tracer"
>   BUT I'm really not sure at all of the name of the device. It's function
>   is to test transistors & diodes, but also works with capasitors.
>   it is a simple circuit when used with an O-scope produces a
>   lazious(sp?) pattern, a circle, or a calapsed circle that shows
>   the knee of a junction graphically on the scope. I know the circuit
>   was published in 2 magazines, but have no way of finding which
>   or when or where.

>   Please, any direction/correction would be much appreciated.
>   Thanks
>   Wes

Curve tracer design:

The design of a curve tracer is simple in principle (description here
for bipolar transistors):

For the horizontal (collector supply) you need a variable ramp generator.
If your scope has a sweep output, then you can derive it from this - if
you are not interested in frequency response, an audio amplifier may be
adequate with a volume control to adjust the amplitude.

For the base drive you need a programmable current source capable of putting
out a series of constant currents for the base drive.  Here, a counter driving
a D/A set up for a current output mode.  Use the trigger output or sweep
output of the scope to increment the counter so that it sequences through
a set of say, 10 current settings.

Then, you need some way of sensing collector current to drive the vertical
channel - a small series resistor in the emitter circuit, for example.

For simple diode tests, you can just use a variable AC voltage source
like a variable isolation transformer with a current limiting resistor
resistor across the diode.  X of the scope goes to the output of the
transformer.  Y of the scope goes across the diode under test.  See the
section: "Quick and dirty curve tracer".

Then, you can jazz it up with microprocessor controlled on-screen display.

Quick and dirty curve tracer:

I threw the following circuit together in about 10 minutes.  With minor
modifications, it is capable of displaying V-I curves for diodes, zeners,
transistors, resistors, capacitors, inductors, etc.

I used a 12 VAC transformer just because it was handy.  You can use anything
you like as long as you understand the safety implications of higher voltages
and make sure the components you use can withstand the power that might be
dissipated in them if the Device Under Test (DUT) is a dead short.  In
addition, it is bad form to blow out the DUT while testing it!  A signal
generator driving a small audio transformer could also be used if it is
desired to test components at frequencies other than 60 (or 50) Hz.

      o-------+             R1
               )|| +------/\/\/\------+-------o Horizontal input of scope
               )||(                   |            (Voltage display)
               )||(                   o
Adjustable AC  )||(                   +
 from Variac   )||(                  DUT
               )||(                   -
               )||(                   o
               )||(         R2        |
               )|| +--+---/\/\/\------+-------o Scope ground
      o-------+       |
                T1    +-----------------------o Vertical scope input (inverted)
                                                   (Current display)

CAUTION: turn down the intensity of the scope so the spot is just barely
visible so that when there is no input, you don't end up drilling a hole
in the face of the CRT!

R1: Current limiting and phase shift control.  I used 500 ohms which works
    well for small signal semiconductors and capacitors around 1 uF.

R2: Current sense.  I used 10 ohms and put the scope on the one of the
    .1, .2, or .5 V/cm ranges.

T1: Small power transformer.  I used the 12 VAC wall wart from an obsolete
    modem.  This will supply a voltage of up to about 17 V peak to your DUT.

A Variac provides a convenient method of adjusting the voltage applied to the

Modify these values (selector switches might be nice) for your needs.

Try this with a 5 V zener diode first since you will know what that should
look like - just as in the textbook!

For transistors, a source of base current is needed.  You can be fancy
or simple.  For a simple source, I used a variable 0 to 15 V power supply and
a current limiting resistor.  Since we know that the voltage drop across
the B-E junction is fairly constant at around .7 V (for silicon), the output
of the supply can be calibrated in terms of base current.

--- sam

Date: 28 Sep 1996 00:47:35 GMT

Original Subject: Re: Curve Tracer?

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