A very handy circuit for diagnosing brushed motors such as in power tools

Thread Starter


Joined Jan 31, 2017
My old faithful 1500W AEG rotary core drill stopped working a while ago. This was from the era when AEG used to make high quality tools in Germany now it’s owned by TTI. I decided to give it a go with repair as the unit was very expensive (500£) and replacement parts are still available.

I started with the usual suspects, check the plug, cord and the switch, all seemed fine. Next, I checked the carbon brushes, gears and also the speed control unit. They were also in a good condition. At this point the only remaining suspects were the armature and stator. First, I visually inspected the stator and no burn marks or charred wire on either stator or armature. The Ohm reading of both of the windings on the stator were also consistent.

The usual test for an armature is to successively read the resistance of the adjacent commutator bars. Any huge variation is indicative of either a shorted or open coil. Unfortunately, this is not a good test as you are working with low ohm range and instrument or user error can mess up the result. Additionally, if you have only one or two shorted wires within each coil (very common), like in my case, you can’t tell with a multi meter. The inductance also didn't show any meaningful variations.

In the past they used a device called Growler to diagnosis armatures but they are rare now. I continued my search for a tool that I could actually build and would be able to reliably detect fault and preferably in the form of an electronic device. I came across this gentleman on YouTube who had such a device and was kind enough to show the circuit diagram. The issue was that the video was in Russian and also, they wound their own custom inductors using magnet wire.
After several trial and error, I came up with a working device using a modified circuit that works with jellybean parts. It detects shorted coil within armature and stator and it is very sensitive and reliable.

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Please read the notes in the diagram and make sure you also follow the steps in the instruction note. The device seems to work better with the radial type (shown in the circuit diagram) inductors. The optimal size is 8x12mm or 9x12mm. Not very critical, but seems to have better sensitivity. If you have smaller inductors (6x8mm), you might need to lower the distance between inductors from 27mm to 21mm. Further testing needed.

Theory of operation

I am not an engineer, just a hobbyist who like to make functional and useful circuits. This is how I think the device works so my apology for any error or mistake.
The left side of circuit, with L2 highlighted as transmitter in the diagram, is a sine wave generator. The oscillator generates a signal of 10-100KHz, depending on the values used in this part. I get 54KHz as shown on my oscilloscope. Using larger values of C5, C6 gives lower frequency, lower amplitude but cleaner sine wave. I didn’t dive very deep to determine the optimum values, what is shown worked for me.

The receiver section with L1 receives the signal and this is adjusted using a multi-turn pot, after which, the signal is rectified through D1. The presence of shorted coil between L1 and L2 affects how the L1 receives the signal depending on the mode of operation explained in the instruction notes. When the signal reception is disrupted to L1, Q3 and Q2 receive no base current so they are in off state, and the green LED is on. With the signal of sufficient strength, Q2 turns on. In this condition, the red LED shines and the base of Q1 is grounded so the green LED is off. See the instruction file for more photos and how to operate the device.

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I tested my suspected armature. It showed shorts in four locations which means a pair of coils shorted. I noticed the iron body of armature was shiny on front side. I suspect the back bearing went bad and the body of armature was rubbing against inside of stator. The resulting friction caused overheating and few winding wires were shorted as the result. I tested a few good armatures and the device works like a charm. No false positive at all. You can test the device by artificially shorting two commutator bars and see how your device functions. Or place a piece of closed loop wire on stator winding.

Thank you for your time reading this and I hope you make this device and have a success like I did. It's rare to see a DIY circuit works this well. You never know when you might need such a device but since universal motors are so ubiquitous in power tools, blenders and many household items, it would be very handy to have such an item.

Armature tester circuit diagram.png