Curve tracer and component tester

Here is my implementation of a simple component tester, called the octopus.

I came to know about this from one of w2aews’ video in YouTube (https://www.youtube.com/watch?v=Gwo3pEH7hUE). I highly recommend his channel, wonderful stuff there.

You just connect channel 1 of the scope to the X terminal and channel 2 to the Y terminal of the circuit. Now set the scope to display in XY mode, and that’s it. You can now connect whatever device, a diode, capacitor, resistor, or even an IC to the Device under Test (DUT) terminal and you will see the voltage vs current characteristic curve on the scope. Here are some of my experiments.


Short circuit


When DUT terminal is open.


1N414 diode characteristics


BE junction of BC547 transistor. Notice the BE junction breakdown at about -11v.


Normal output pin of a 74LS14 IC


A damaged output pin in the same 74LS14


1K Resistor

To get the corresponding current for some voltage, you just measure the voltage on Y axis and divide it by the current sensing, R3 resistor value.

Workings

The R2 is a 10k potentiometer for current limiting. The R3 is current sensing resistor and the R1 is there to limit the current even when R2 is set to a low value.

The sine wave from the transformer is used to do a voltage sweep on the DUT, and you use the scope to see the response.
The probe at X measures the voltage across DUT, and the one at Y measures the voltage across the current sensing resistor R3.

Note: Because of the structure of this circuit, the current measured will be of opposite polarity as you would expect. So you need to invert the polarity of the channel 2 (Y) in the scope, or leave it as is and deal with it in your head.

I have used a fixed output transformer but a variable output one would be better.

When the DUT terminal is open, there is no current in the circuit and so Channel 2 (Y) will read 0 volts across R3, Channel 1 (X) however, will see the open circuit voltage across the DUT terminal. Therefore, you see only a horizontal line on the display.

Similarly, you will see a vertical line on the scope, when the DUT terminal is shorted.


My Implementation

My circuit

With max current of 57mA (R.M.S), the R1 resistor has to be 1W, instead I used 5, 1/2 W resistors in parallel, because I didn’t had any 1W 200\Omega resistors lying around, though if I had done my math correctly at the time I was soldering, I would have used 5, 1/4W 1k\Omega resistors for the task.

Please let me know your comments and question and have fun building it.

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