10$ curve tracer

Simple, quick and dirty curve tracer for diodes measurement

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I watched discussion on local forum, bunch of guys were discussing how curve tracer should look and work.
Classical feature creep (or scope creep?) in action. From simple 100EUR gadget to measure small transistors and diodes (to be finished in weeks or months) it grew into disgusting mess of super-truper accurate measurement unobtainium, capable of plotting characetristics up to bazilion volts and amps with zero point zerozerozerozero whatever precision, worth of 10-year average salary... And project stopped, as one could expect.

Before those guys decided, what kind of display to use, I designed and built my own curve tracer, totally out of specs, but... finished and working.
Design and build took me two evenings - one to solder all the components on two pieces of scrap perfboard, while second evening spent on the simple software itself. I designed it with regard to maximal simplicity and low price, however using components actually to be found in my drawer.

There is not very much magic behind this curve tracer. It works by impulse measurement - so, the voltage (or current) to draw IV curve is not applied step by step by means of adjustable voltage (or current source), but in single impulse from charged capacitor. This allows to have measurement range of ~0,5A and 35V without the need of bulky 18W power source and no DUT cooling needed. Measurement range could be easily increased to few amperes and hudreds of volts, this implementation is just simple case study.

The whole gadget consists of two logical parts - analog and digital; separated on two board. It is not needed to have it separated, but I had those two pieces of scrap perfboard on hand.

Analog board:

There is 35V source made by jelly-bean MC34063 (I didn't even bother to draw complete schematics). This source is permanently on, however separated by reed relay. It can be engaged to charge 22uF capacitor (by transistor on A input). This capacitor could be discharged into serial combination of Rs and DUT by two-transistor switch, engaged by input B. Voltage on DUT is measured on A0 output and combined voltage on A1 output. If Rs is known, current flowing through DUT is given by D*(A1-A0)/Rs, where D is ratio of voltage dividers (in this case - 560k and 39k, giving ratio approx 1/20).

And this is how it looks:

Digital board has to do measurement in three steps:

1, Turn on A, wait for a while to charge 22uF capacitor and turn it off.

2, Turn on B and immediately start sampling both A0 and A1 (both inputs at once) inputs into two long arrays, wait for a while and terminate sampling, turn off B

3, Draw X-Y plot of those two arrays.

(4, ... 5, Profit comes to mind)

To achieve those goals, not that much has to be done

There is not much more than MCU and display.

It looks like this:

Typical measurement profile on A0 and A1 inputs looks like this

You can see huge spike on blue line (A1) meaning fast discharge (high current), while voltage on yellow line (A0, voltaeg on DUT) doesn't change that much. This is typical IV characteristics of Zener diode (in this case approx 12V Ur). When drawn in XY mode, it looks like this

The unmarked axis goes like on would expect - voltage on X (horizontal) and current on Y (vertical). 5V/dev at X and 50mA/div at Y. We can see the same thing - voltage doesn't change very much when going from few mA above, looking quite similar to what could be find in datasheets, like

Final remarks:

Yes, it works. 

Current/voltage range is not that great, but OK for demonstration. Higher resolution display wouldn't harm, to allow more detail and X/Y axis legend. The dsPIC I used is dying from boredom in this application, it is 70MIPS DSP, some lower-end one should be better (it has to have dual sample and hold circuit, or even dual ADC).

  • Thoughts of transistors measurement

    jaromir.sukuba04/27/2014 at 20:37 0 comments

    I see there is a few people following this project, so just to know it is not dead - I thought of transistors measurement. Measuring 2N3904 at 20mA is simple task, but what about 2N3055 at 10 amps? Yes, it can be done with impulse measurement - this way I can measure it without heatsink at huge collector currents. What more, I don't need bulky power supply, just good capacitor.

    The devil is in details, as usually. Switching between NPN/PNP requires a few relays, connections to transistor have to be quite bulky, so the simplicity of my first design is gone.

    When I'll finish my part of work on JJ tricorder (deadline of HaD sci-fi contest is in two days) I'll try to sey up simple test bench for transistors.

  • Some more photos

    jaromir.sukuba03/24/2014 at 13:24 0 comments

    Digital board:

    Analog board:


    Complete set in action:

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Comedicles wrote 04/16/2016 at 20:58 point

Nice project. 

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counter.culture wrote 05/04/2015 at 19:53 point

"The dsPIC I used is dying from boredom in this application" - I can't stop chuckling about this...I'm getting a visual of a cartoon cpu die yawning and buffing his nails...

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Aqib Idrees wrote 08/23/2014 at 13:57 point
Do you have a link to the website where you've mentioned other people's IV curve tracer designs.

Nice work!

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jaromir.sukuba wrote 08/23/2014 at 15:58 point
Thank you.

Here it is - in Slovak and Czech language, not sure if it will help you much. This is mail-list, with online archive.
Google translate may help you, somehow.

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