Electronics > Beginners
Mysterious things while hacking a Makita BLDC motor trigger
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steveinfrance:
Firstly 'Hello to All'
I've been tinkering with electronics for 50 years and am still alive to tell the tale although, in my youth, I did have some unfortunate contacts with anode voltages of Military wireless sets.
I will try to explain the problem - please tell me if I'm not making things clear.

I would like to use a Makita BLDC cordless drill motor and gearbox for some equipment we're making.
It offers a compact and reliable motor and gearbox which will require a housing made but that's not a problem.

The plan is to throw away everything else and drive the controller from a PLC and use an external power source.

The battery supply has a small third terminal which, I think, goes to a temperature sensor in the battery.
Connecting a 220  \$\Omega\$ resistor to +ve makes the drill happy to run off a bench PSU (it is an 18 V drill).

Next, and this is where the fun starts, the trigger.
It is marked as an Omron C3JW-4B but (apart from buying replacements at 1/3 the price of the whole drill) there's nothing about it on the Internet - trust me !
Contacting Omron they tell me Makita bought out their division that makes similar switches and regard its workings as a proprietary secret so no datasheet.

The trigger has two components - a set of beefy contacts to supply +V to the controller and a 5 pin miniature connector.
There's a first pressure which switches power and a second pressure which starts the motor and makes it spin faster the more it's pressed.
There's also a forward/reverse (F/R) switch.
Nothing too difficult so far.

Disconnecting +ve in and +ve out from the switch and jumpering them together lets the motor start on second pressure and run as expected.
So, a 5 pin 'black box' to sort out.

I cut the wires and took them to a breadboard - the drill still worked !

With the controller connected there were three easy ones - GND which was common with I/P GND
One which sat at 18 V until second pressure then fell to 0 V no matter what so it was there to detect 'second pressure'
One which sat at 3.5 V with the F/R switch in 'Forward' and 0V with the switch in reverse.
Disconnecting these from the trigger and jumpering them to GND as required gave the expected results - motor starts on 'second pressure' in the direction set by the F/R switch.

This left 3 connections to the trigger - GND, grey and white (colour of wires).
With the controller connected to the trigger white sat at 2.5V no matter what and grey went from 2.2 to 0V as the trigger was pulled.
Easy !
White is the excitation voltage for the speed pot and grey is the 'wiper' O/P.

So, disconnect the trigger completely and look at resistance values with my trusty Fluke DVM.
I should say for all resistance measurements I took them with the DVM leads both ways round in case of a diode type junction somewhere.

The 'second pressure' detect went from O/C to 'closed' as expected, as did the F/R switch.

White to GND read a constant 15  \$\Omega\$ while grey to GND went from 15 to 0  \$\Omega\$ so that seemed to fit in with white supplying 2.5 V across the pot which was passed back to the controller by grey on the slider to give speed.

15  \$\Omega\$ seems a bit low but anyway let's connect two 10  \$\Omega\$ resistors between white (2.5 V) and GND so we should get 1.25 V at the junction and connecting that to grey should give half speed (or thereabouts).

Now this is where the mystery starts (hope you weren't holding your breath).

With the two resistors between white and GND white fell to 0V so it wasn't happy feeding 20 \$\Omega\$ let alone 15.

I completely disconnected the trigger from the controller.

I put a 220  \$\Omega\$ resistor in series with white and the DVM read 15  \$\Omega\$ between the free end of the resistor and GND.

I put a 220  \$\Omega\$ resistor in series with grey and the DVM read 15 to 0  \$\Omega\$ between the free end of the resistor and GND.

In other words the DVM couldn't see the 220  \$\Omega\$ resistor.

How is this possible ? The 220  \$\Omega\$ resistor measured correctly.
I checked white to GND and  grey to GND for volts (in case of a capacitor or backup battery) and diode (both ways) but nothing.

I repeat there was no voltage supply of any sort (apart from the DVM) to the trigger - it was literally in my hand.
HELP (Please).










xavier60:
 I can't make much sense of that. This is what I traced out on my DHP481 a few years ago.
steveinfrance:
Thanks
Your schematic is exactly what I expected and partially what I found.
My problem is that  yellow (gnd) to white on your diagram should give the resistance of the speed pot which measures with a DVM as 15  \$\Omega\$.
The exciting voltage applied to white by the controller is 2.5V.
I tried to explain that if I put 20 (2x10  \$\Omega\$) between gnd and white the voltage fell to near zero so the pot can't really be 15  \$\Omega\$ - which seems far too low anyway.
I didn't believe the value so, among other things, I put a 220  \$\Omega\$ resistor with one end connected to the white lead and the other to the DVM.
If the pot was really 15  \$\Omega\$ one would expect to see a reading of 235  \$\Omega\$ but it still read 15  \$\Omega\$ - as if the resistance wasn't there - which seems impossible to me.
I can't work out what could possibly be inside the trigger to 'confuse' the DVM.
Sorry about the glare on the DVM, I was allowing 5  \$\Omega\$ for the DVM leads so I said 15  \$\Omega\$ while the DVM reads 20
capt bullshot:
Just a wild guess:
There's no stinkin' pot inside the trigger, but a linear magnetic sensor (Hall element) with some signal processing. These are available as single chip solutions today and have three pins: GND, supply and output voltage proportional to magnetic field.
This would (Edit: not) explain your DVM measurements as far as I could follow your description.
Does the controller work if you just apply a variable voltage to the speed control (wiper?) input?
xavier60:
I think that there is something wrong with your meter. Seems it is trying to tell you that the pot is 20300 ohms.
Try measuring the 220 ohm resistor and some other values.

Edit: I see where you said earlier that the 220 ohm resistor measured correctly. But look at the 2 readings in the 2 photos, the 22 difference.
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