RL Circuit Waveforms - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: September 04, 2017, 01:09:41 pm
content: Hi,I was wondering if someone could help me read my scope and reconcile the difference between what I'm seeing and my expected results.I have a 10k ohm resistor in series with a 1mH inductor. When I apply 5V DC, I expected to see the ERto ramp up exponentially and ELto decay as in the photo of the book below. However, my scope shows ER(yellow) as decaying and ELas having a brief spike in the middle, but pretty much staying at 0V.My circuit is below. The red lead is coming from a DC 5V source. The top green lead at the resistor is hooked to Ch1 (yellow). The lower green lead at the inductor is hooked to Ch2 (blue). The orange leads, inductor, and black lead are hooked to the ground of the power source.This is my scope with a trigger set to the leading edge of Ch1.This is a photo of a book with the waveforms I'm trying to duplicate. I expected ERtoo look like the ramp-up from t1 to t2 and ELto look like the decaying graph below.Thanks in advance,Carl

author: rstofer
author_rank: Super Contributor
timestamp: September 04, 2017, 03:54:45 pm
content: A schematic is much more useful than a picture of a breadboard with leads moving out of the picture.Your yellow trace looks a lot like the middle graph.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 04, 2017, 04:34:03 pm
content: Thanks for replying. Here's the schematic. The yellow graph does look like what my book has for EL, but I have Ch1 wired to measure ER. I tried moving the Ch1 ground too.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 04, 2017, 04:42:01 pm
content: Please draw in that circuit where and how you connected the oscilloscope probes.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 04, 2017, 05:01:57 pm
content: Hi,I tried the Ch1 ground leads in two places: at the power source ground and in between the resistor and inductor.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 04, 2017, 05:27:32 pm
content: Ground of all probes from all channels are internally tied together, andare all connected to the outlet ground of the oscilloscope, so always take that into consideration, and never try to visualize the waveform from a power outlet.If you put the Ch1 GND between R and L, then you will short circuit the L, because Ch1 GND and Ch2 GND are tied together inside the oscilloscope.1. To visualize something like in that book, put both channels on DC. In your osc screen capture, CH2 Coupling is set as AC. You need both channels coupling as DC.2. Since you can not put a probe directly on the R, you need to add a 3rd MATH trace on the oscilloscope, with the difference CH1-CH2.3. Even so, you will not see traces exactly like in the book, because a switch does not instantly make perfect contact. In reality, any mechanical contacts of a switch will vibrate (bounce) before making a perfectly close or perfectly open contact. That is why, when you want to read haw many time a switch was pressed, you need a debouncing circuit. (Seehttp://www.labbookpages.co.uk/electronics/debounce.htmlfor more details about debouncing)

author: StillTrying
author_rank: Super Contributor
timestamp: September 04, 2017, 05:37:32 pm
content: You'll also need the scope's timebase on about 5 us/Div. At 100 ms/Div all you're looking at is the scopes AC/DC coupling capacitor charging and discharging.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 04, 2017, 05:39:06 pm
content: Thanks! I put the couplings to DC and made sure the ERscope ground was at the power source ground. This looks like what I expected.

author: eev_carl
author_rank: Regular Contributor
timestamp: September 04, 2017, 05:42:23 pm
content: Yes. In fact, I ended up zooming in to 2 microseconds