probing a NPN Transistor - Page 1
Forum section: electronics / beginners

author: manytoolsmike
author_rank: Contributor
timestamp: January 10, 2021, 01:07:46 pm
content: Hello everyone,I want to use 2 channels of my scope (DS1104) to probe a transistor. Specifically I want to compare the PWM signal on the Digital Output pin of a micro to the switched signal on the collector of the NPN transistor.because of the common ground when I probe the LED I can only see the 5 volt rail. How can I see both signals on the scope?Ultimately this circuit will control the speed of a small DC motor.This is my first post, but I have been following Daves Channel for many years now.Thanks,Mike

author: DC1MC
author_rank: Super Contributor
timestamp: January 10, 2021, 01:09:59 pm
content: So why did you put the second scope channel to the 5V power supply and not on the transistor collector if you want to see the signal there?

author: manytoolsmike
author_rank: Contributor
timestamp: January 10, 2021, 01:35:32 pm
content: Quote from: DC1MC on January 10, 2021, 01:09:59 pmActually I did something even worse.... I connected the Ground clip to the collector and the probe to the rail; thinking that would show me the signal... Now I see I was shorting the Collector to ground through the 2 ground clips. good thing I set the current limit on the power supply to 500 mA.So now I am probing the collector with the second channel and not using the ground clip. It shows the signal as being inverted! I am really confused now.

author: DC1MC
author_rank: Super Contributor
timestamp: January 10, 2021, 01:52:51 pm
content: Quote from: manytoolsmike on January 10, 2021, 01:35:32 pmNow the oscilloscope is connected as good as it can be and the signal is as it should be, because the transistor is a switch in this configuration and you're not measuring the voltage over the LED, but over the transistor, you need to read a bit about transistors, I will start here for a quick introduction:https://learn.sparkfun.com/tutorials/transistors/applications-i-switches

author: manytoolsmike
author_rank: Contributor
timestamp: January 10, 2021, 02:04:50 pm
content: OK,I think I understand the inverted signal. When the Transistor is off the scope is reading the positive rail through the LED (minus the voltage drop) when the transistor switches ON the probe is brought to Ground. Therefore the PWM signal appears inverted on the scope.So.... how would I compare the rise time of the transistor vs the PWM signal?Here is the signal I have now.. Yellow trace is PWM from Micro, Blue is the Collector.

author: DC1MC
author_rank: Super Contributor
timestamp: January 10, 2021, 02:51:42 pm
content: First watch and understand the videos here:https://www.rigolna.com/scopebasics/then this one:Practically decrease a bit the time base (20ns should be OK), use vertical cursors and place the first where the signal is at 10% of its value and the second one at 90% of the value, the time between the cursors is your rise time as the scope sees it, keep in mind that what you see is very dependent of the actual scope rise time, of the probes capacity, where do you place the ground points and what kind of contacts you're using and many other factors, but for your intended purpose of controlling a motor it should be enough.

author: rstofer
author_rank: Super Contributor
timestamp: January 10, 2021, 04:47:45 pm
content: Put the probe ground clip(s) on the negative rail - the emitter of the transistor, for example. Unless I missed something where circuit ground is way above earth ground. Since you are using a lab supply with current limiting, I would back it down to a more reasonable number - what current does the meter measure? 50 mA? Then set to 100 mA.If you want to be absolutely certain of the circuit ground voltage relative to earth ground, use a shorting clip at the power supply to short between the black and green jacks. I'm assuming you have a legitimate earth ground at the input to the power supply. Since the scope clips are grounded through the scope power cord, you might as well short the negative rail to earth ground at the power supply. Or not... You should get the same results either way.Page 6-60 discusses rise time measurements:https://www.batronix.com/pdf/Rigol/UserGuide/DS1000Z_UserGuide_EN.pdfPush the up-arrow at the bottom left of the screen to get to the 'other' Horizontal menu.Then press the 3rd button down for "Rise Time" or the 4th button down for "Fall Time".You select the active channel as always, just tap the channel button once until the channel label at the bottom of the screen is highlighted. Or just display a single channel.The User Manual also shows the use of cursors for measurements. The thing is, you can't find the 10% and 90% points visually nearly as well as the scope can find them mathematically.