# V-I Tester (aka ASA or Octopus or curve tracer)

I want to build a simple V-I tester and based on this article (or this) I did this easy circuit:

R1 = 560Ω
R2 = 100Ω
R3 = 1kΩ

where R1 and R2 is a voltage divider to set the $$\V_{(DUT+V_{R_3})}=1VAC\$$ and R3 limit the current in the Device Under Test.

I used this transformer, with 6.3V, 10 VA output (is dual, but I use only one output).

My question:

1. If I change the voltage divider resistors to $$\R_1=56Ω\$$ and $$\R_2= 10Ω\$$ (especially), isn't better? With 10Ω I will reduce the output impedance of the circuit that feed the DUT, thus I don't load the source because it is in parallel to the $$\R_3+R_{DUT}\ge1000Ω\$$ and then $$\R_3+R_{DUT}\gg R_1\$$, isn't?
BTW the current I draw is $$\I =\frac{6.3}{66} \approx 100mA\$$ and the output power is at the "safe" value of: $$\6.3V\cdot100mA = 0.63VA\$$
2. With this voltage divider I have a voltage less than 1VAC across the DUT. The article says that I can test also Diode, Zener, etc. How is possible with this small voltage?
• Just to say that R3 "limit" the current through device, but is also the "measure" sensor of this current. Point 2: diodes need only 0.7V ... current is always "exp", and the scale doesn't matter. You can't determine the Zener point if the voltage is too low. Apr 21, 2022 at 17:15
• Point 1: changing values is not really useful, because R3 limits the current. NB: adding a battery, one pot, and one resistor to control current, should also be used for testing and tracing curves of quasi any type of "transistor" ... Apr 22, 2022 at 7:09
• @Circuitfantasist - Divider R1- "R2 variable" only to limit the excursion of max voltage on DUT (some kind of current protection). For low current, it is enough and easy through R2. For high currents, a variable transformer (or a "fast" triangular voltage pulse with a low duty cycle) should be needed to adjust the voltage applied on DUT (as on a professional Curves-tracer), but with a scheme of "power" protection (so the calculation of v*i on the DUT is necessary to remain under Pmax) ... Apr 22, 2022 at 18:19
• @Circuitfantasist It is a very good exercise in a lab and helps to understand some drawing functions (mathematical with scope) and electronic characteristics of DUT. This simple circuit is very useful. I used this simple curve tracer about 55 years ago with my "own scope made" ... and a professional tool Philips (radiomuseum.org/r/philips_transistor_curve_tracer_pm_6507.html) 5 years later at university. Apr 22, 2022 at 19:56
• Right. I forget that the axes for the "tunnel" curve are reversed! I used only the load line for the curves of UJT (V vs i) ... :-( Apr 23, 2022 at 8:46