How to see the transistor characteristic curve with DSO-138 oscilloscope - Page 1
Forum section: electronics / beginners

author: yaratry
author_rank: Newbie
timestamp: August 09, 2025, 06:17:01 am
content: Is there a way to observe the transistor characteristic curve with a DSO-138 oscilloscope?I have been searching the internet for ways to observe the transistor characteristic curve and all of them uses xy mode. But DSO-138 doesn't have that:( I only want to observe the curve with a fixed base current and not multiple curves for different base currents. I have a 12v 2A variable power supply and a circuit that outputs a sawtooth wave.I saw here:https://github.com/ardyesp/DLO-138that the firmware of the stock DSO-138 can be upgraded to get 2 extra digital channels but I have no idea if that helps. TIA.

author: MathWizard
author_rank: Super Contributor
timestamp: August 09, 2025, 07:14:15 am
content: So you would want a fixed base bias, and say the emitter of a NPN was on GND, then you want to sweep up the supply voltage to the collector, and use some current limiting collector resistor too. For low currents, nothing near 2A, I'd try making some ramp circuit with op-amps or a 555 timer and some pass transistor, that would output the collector supply voltage. That voltage could ramp up, and then you have your VCE sweep.And for some nice round number of the collector resistor, if you had 2 probes, you could display the current as the Voltage of Rcollector .But yeah, all DSO138's I've seen, only have 1 channel.Do you use any circuit simulators ? I'd say try and simulate the experiment 1st, and get used to that, if you are worried about damaging your BJT.

author: yaratry
author_rank: Newbie
timestamp: August 09, 2025, 07:54:10 am
content: I think this is the circuit you are suggesting. But where should I probe in order to get a curve on the oscilloscope?

author: Aldo22
author_rank: Super Contributor
timestamp: August 09, 2025, 09:20:05 am
content: Quote from: yaratry on August 09, 2025, 06:17:01 amSCO2 is a cheap oscilloscope with XY mode.https://www.aliexpress.com/item/1005008043120167.html

author: voltsandjolts
author_rank: Unknown
timestamp: August 09, 2025, 09:32:00 am
content: https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/https://www.eevblog.com/forum/beginners/can-i-build-this-curve-tracer-circuit-with-an-awg/https://www.eevblog.com/forum/beginners/help-for-setting-up-the-scope-for-curve-tracing/...etc...

author: yaratry
author_rank: Newbie
timestamp: August 09, 2025, 10:09:48 am
content: Quote from: Aldo22 on August 09, 2025, 09:20:05 amWell I already have a DSO-138 and was trying to use that. But I guess it cannot be used to see the IV curve.

author: yaratry
author_rank: Newbie
timestamp: August 09, 2025, 10:12:28 am
content: Quote from: voltsandjolts on August 09, 2025, 09:32:00 amAll of those links describe using two channels and xy mode but DSO-138 doesn't have that capability.

author: armandine2
author_rank: Frequent Contributor
timestamp: August 09, 2025, 10:27:07 am
content: ...interested to hear you have a working stable DSO-138 (genuine one I assume) where this test facility was at least an aspiration.

author: yaratry
author_rank: Newbie
timestamp: August 09, 2025, 11:01:07 am
content: Quote from: armandine2 on August 09, 2025, 10:27:07 amStable DSO-138's are that hard to come by?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 12, 2025, 09:30:47 am
content: All that's needed is ramp generator with the same period as the timebase.You apply the ramp to the DUT.You gate your ramp and the sweep simultaneously. Trigger from the falling edge of the gate.Create the gate with a 555. (Did I really say that?)Active low gate.Add as much gain as you need to get the ramp amplitude required.The circuit below will create a linear ramp of ~6V over 1s with the components show.The period is determined by the gate on time. In the example 1 second.Add voltage and/ or curent gain with a driver amplifier.You can fiddle gate time and R2, C1 to get the ramp time required. Set R1=0.1*R2

author: armandine2
author_rank: Frequent Contributor
timestamp: August 27, 2025, 07:59:14 pm
content: Quote from: yaratry on August 09, 2025, 11:01:07 amI only bought one - Treedix? from Amazon - it was flaky

author: MrAl
author_rank: Super Contributor
timestamp: August 28, 2025, 12:27:07 pm
content: Quote from: yaratry on August 09, 2025, 06:17:01 amHi,Depending on what kind of sawtooth it is and what the amplitude is you may be able to use that. If the amplitude is not right, then you need to either amplify it or attenuate it or even use a voltage to current converter circuit if you need to drive the input with a current rather than a voltage.The main idea with any curve test is to apply a linear ramp or logarithmic ramp to the input to be swept, then look at the output to be viewed. For static conditions you would use a very low frequency like 100Hz. Set the horizontal time base to match the period. If you can't do that exactly then you would have to put up with a slight artifact near the end of the sweep, but you'd still be able to see the curve up to that point. If you take a photo you could always edit the photo later to remove any artifacts.

author: ledtester
author_rank: Super Contributor
timestamp: August 28, 2025, 04:25:59 pm
content: Here's a design using an Arduino:https://www.instructables.com/Transistor-Curve-Tracer/Besides the Arduino you'll need a MCP4802 DAC, some LM358 opamps, a boost regulator to get 12V and a IL9341 display.