January 27, 2017


A quick build which is incredibly useful.  Somehow I only thought of this now.

Recently Bayley and I were helping the FSAE team do some motor-debugging.  To get a good oscilloscope shot of their motor's phase currents, I borrowed the big LEM closed-loop hall current sensor from the motor dyno buck converter, wired into a ±15V supply and a sense resistor.  It proved to be extremely helpful, and in the process I realized that having a small, high-current, medium-bandwidth current probe would be incredibly handy, especially for motor/motor control stuff.

I threw together a simple current probe made from a smaller LEM current transducer powered by a 9V battery and DC-DC converter IC.  Pass your wire through the LEM, and the current signal goes out the SMA connector in the back straight to your oscilloscope.  The particular sensor I used is good for ±70A at DC to 200 kHz, making it fast enough to see effects like current ripple from switching on electric motors.

Since it's around the Chinese new year, I couldn't order boards from 3PCB in any reasonable amount of time, so I etched a board on the MITERS mini-CNC router to test the design:

I forgot to add the power switch to the Digikey order, so I tested with a piece of wire, and replaced that with a jumper later.

Here are the Digikey bits for populating one, minus the actual current sensor.  I used an LA 55-P (because I scavenged 3 a while ago),  but it could be populated with a variety of transducers depending on your desired measurement range.

I 3d-printed a little box for it:

Here are some random scope shots from an assortment of motors and controllers lying around MITERS.

This one's from someone's electric skateboard, which has an airplane ESC and a small outrunner on it.  Just spinning the wheel, the switching ripple is the same amplitude as the phase currents, thanks to the combination of slow switching frequency and very low inductance motor.  Not a good-looking waveform.

And this funny looking one is a sensorless Jason trying to commutate a small internal permanent magnet motor.

Once 3PCB comes back from holiday I'll get some proper boards made and assemble a couple more.  Here are the Eagle and gerber files if you're interested in building your own.


  1. Hello I need to build a current sensor using a LEM current transducer for my work.
    But when I look for current transducer online I can't find any info on the output voltage on any that will fit my needs (100A max). I found this info for a 80A one but seems to be missing for 100A sensor.
    I see in your schematic that you're connecting the output of the current transducer directly to the BNC so I guess the one you have used is outputing 2.5V max.
    Because if it's not ouputing 2.5V that will damage the scope then.

    1. This style of LEM sensor outputs a ratiometric current, not voltage. I connected the output to a precision resistor and measure the voltage drop across the resistor.

  2. Hii, can you tell me the details of +15,-15 voltage regulator?