How do i connect an oscilloscope to a brushless speed controler? - Page 1
Forum section: electronics / beginners

author: rhinocat
author_rank: Contributor
timestamp: April 11, 2016, 03:00:53 pm
content: I wont to see if my BSC is working properly, and I'm also curious to see the output on my scope. I was thinking ether put the two grounds on L1 and channel 1 to L2 and channel 2 to L3, or put all of the grounds to the negative battery terminal, and CH1 to L1, CH2 to L2, CH3 to L3. Are either of these right?

author: danadak
author_rank: Super Contributor
timestamp: April 11, 2016, 03:10:25 pm
content: If you are going to look at circuits whose ground reference is not "mains" related,use the scope in 12 channel mode, second channel inverted, and use the twoprobes on the winding of interest, leave probe grounds unconnected.The two scope channels should be configed as CH1 - CH2, eg subtraction.http://web.aeromech.usyd.edu.au/MTRXLAB/ref/instruments/Differential%20Oscilloscope%20Measurements%20(51W_10540_1).pdfhttp://www.tiepie.com/en/classroom/Measurement_basics/DifferentialMeasurementsRegards, Dana.

author: rhinocat
author_rank: Contributor
timestamp: April 11, 2016, 05:30:37 pm
content: This is for a DC speed controller out of an R/C helicopter. I would like to view all three phases at once if possible.

author: stmdude
author_rank: Frequent Contributor
timestamp: April 11, 2016, 06:27:34 pm
content: I'm not so sure this will be a good idea.. Especially if you plan on having the motor connected.I imagine the EM kickback from the motor can be quite high, and the analog frontend on oscilloscopes are somewhat sensitive..If you want to go ahead anyways, connect one ground from the scope (all grounds on the scope are connected together, no need to connect each probe, unless you're measuring very tiny voltages) to the negative on the battery. Then probe the three phases with one probe each.But, again, I wouldn't recommend it.

author: danadak
author_rank: Super Contributor
timestamp: April 11, 2016, 08:15:39 pm
content: Are you just trying to see PWM signals to power fet gates or the windings themselves ?If you are trying to look at winding voltages/current, typically these are in H bridges,so they have high CM signal to deal with. But the H bridge typically has clamps on it.You could try a R divider to create a 1000X effective probe, to get an idea of V you aredealing with.http://how-to.wikia.com/wiki/How_to_make_a_100X_oscilloscope_probeThe other approach is a magnetic sniffer, something like -https://www.eevblog.com/forum/projects/diy-magentic-field-probes/Regards, Dana.

author: rhinocat
author_rank: Contributor
timestamp: April 11, 2016, 08:44:57 pm
content: I just want to look at the output without the motor connected.

author: danadak
author_rank: Super Contributor
timestamp: April 11, 2016, 10:24:53 pm
content: Then a scope on pin supplying PWM is just a standard single channelscope connection.Regards, Dana.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: April 11, 2016, 11:58:12 pm
content: Quote from: rhinocat on April 11, 2016, 03:00:53 pmApproach 2 would be better. You would, of course, need a 4-channel scope. It's quite likely you're going to have a hard time figuring out what's the correct signal and what's the noise picked up by the ground lead, so keep the ground lead as short as possible.Most ESCs won't do much without a motor connected. They watch for the back EMF from the winding that's not driven to synchronize commutation. At startup, it will drive the windings with a test pattern to determine the commutation order and without a back EMF pulse, it will not proceed.

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2016, 11:37:38 am
content: If you are trying to confirm the H bridge switch then sub a 100 ohm R for the motor load,and just probe the load connection points..Regards, Dana.

author: rhinocat
author_rank: Contributor
timestamp: April 12, 2016, 05:29:38 pm
content: So, the problem is when I throttle up the speed controller the motor does not start until I give the shaft a turn. I figure that either the motor has a short in one of its windings or the speed controler has a bad leg. I know I could just check the motor for resistance (and I will) but I just got my first oscilloscope (DS1054Z) and I wanted to try it out. I would like to see all of the phases at one time on the scope. Should I connect resistors across all of the legs, and how? L1 to L2, L2 to L3, L3 to L1, or tie one side of three resistors together and the other side to one leg each?Thanks for all of your help.

author: Kilrah
author_rank: Unknown
timestamp: April 12, 2016, 06:45:50 pm
content: The ESC will not give a meaningful output you can make sense of without a motor connected. It will only give "random" pulses trying to start the motor but never reach steady running state.You can probably put 3 resistors and look at each phase separately and confirm you get states of 0V, +VBAT and -VBAT with regard to another phase, x3... but that's about it. Shold confirm teh MOSFETS are all OK.Besides that, your issue might be with timing settings or wire lengths.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: April 13, 2016, 05:51:43 am
content: Sorry if I'm stating the obvious, but swap another motor in. If the problem follows the motor you likely have a shorted/open winding. You may want to power the ESC from a bench supply w/ current limiting.You could also probe the FETs with a multimeter in diode mode. They often fail shorted, so if you see a dead short from the phase outputs to V+ or GND, the FET's burned out. Normal FETs should have a reverse-biased diode (body diode) from output to rails.

author: danadak
author_rank: Super Contributor
timestamp: April 13, 2016, 10:07:21 am
content: This might help -http://www.irf.com/technical-info/appnotes/mosfet.Regards, Dana.