RL Circuit Waveforms

[eev_carl]

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 E_R to ramp up exponentially and E_L to decay as in the photo of the book below.  However, my scope shows E_R (yellow) as decaying and E_L as 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 E_R too look like the ramp-up from t1 to t2 and E_L to look like the decaying graph below.



Thanks in advance,
Carl

[rstofer]

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.

[eev_carl]

Thanks for replying.  Here's the schematic.  The yellow graph does look like what my book has for E_L, but I have Ch1 wired to measure E_R.  I tried moving the Ch1 ground too.

[RoGeorge]

Please draw in that circuit where and how you connected the oscilloscope probes.

[eev_carl]

Hi,

I tried the Ch1 ground leads in two places: at the power source ground and in between the resistor and inductor.

[RoGeorge]

Ground of all probes from all channels are internally tied together, and are 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. (See http://www.labbookpages.co.uk/electronics/debounce.html for more details about debouncing)

[StillTrying]

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.

[eev_carl]

Thanks!  I put the couplings to DC and made sure the E_R scope ground was at the power source ground.  This looks like what I expected. 

[eev_carl]

Yes.  In fact, I ended up zooming in to 2 microseconds