Need electronics help to slow a drill

[Lemosteam]

Just bought this lovely drill, suggested by others here. Love it but I would like to slow it down more than the reastat already does; slowest speed is 8500 RPM.



Below are pictures of the cord with the electronics. Can I add a high wattage resistor in there somewhere to slow the motor or replace the reastat with one that has more range?

Topside of circuit board:


Bottom side of board:

[wvgca]

not really, you don't have a great deal of torque to begin with, and slowing it down more greatly reduces the torque ..

[Chris333]

Is the output to the drill AC or DC?

[DKS]

Very likely the drill motor is DC. You could try putting a PWM dimmer in between the existing controller and the drill, or run the PWM with a wall wart.

https://www.ebay.com/itm/16A-Knob-Manual-Dimmer-Switch-Controlller-for-3528-Single-Color-LED-Strip-Light/173512363701

[peteski]

This is an odd looking power supply for sure.  It appears to be a variable switching power supply.  So the output voltage adjustment is done on the primary (120V AC) side.  Weird!  Never seen anything like it!

The output filter cap is rated 330uF, 50V, so the maximum voltage is likely 40V DC, or less.  Any variable DC voltage power supply (with enough amperage) should be able to power this tool.  But as others mentioned, lower voltage and slower rpms means less torque.   Adding power resistor will not be a good idea, because it will further reduce the torque.  The more load you put on the tool, the more voltage drop will occur across the resistor, resulting in drop of the rpms (under load).

If you are messing around with it, be very careful. There is likely around 150V on the primary side!  Very dangerous!

There might be some hope to slow it down.  Post a closeup photo of the bottom, showing R11, 12, 13, U3, R2 (all in a single photo).  Any identifying marks on the VR (the speed adjustment pot)?

[Point353]

Also, a close-up photo showing the markings on U1, the device mounted to the heatsink.

[peteski]

Also, a close-up photo showing the markings on U1, the device mounted to the heatsink.

I don't need that one. The component on the heatsink is almost certainly a hybrid component: a switching volt. reg. IC, and the power MOSFET that drives the step-down transformer's primary winding.  U2 is the opto-isolator, and U3 is either another transistor, or maybe even another voltage regulator controller).

All I need is the values in the resistor voltage divider circuit.

[Point353]

I don't need that one.

Maybe someone else would like to see it.

[mmagliaro]

I'd be inclined to forget that controller,  put a meter on the outputs to the motor, see what the max voltage is at top speed, and then just buy a new PWM motor controller off eBay that takes 110v AC in and puts out up to that same maximum voltage.

I'll hazard a guess that the motor in that tool might not be very happy if the voltage drops too low and it stalls ( think brown smoke or white smoke).   

[peteski]

Maybe someone else would like to see it.

Maybe . . . and my response was that I don't need it.   
I'm thinking of "cowboying it".

[Point353]

I'm thinking of "cowboying it".

Yippee-i-o-ki-ay.

[Lemosteam]

Please note that in this image, the LEFT side of the vertical yellow stripe has the pot and the output to the drill.  The heat sink is on the right side of the stripe  and this side is the input from 110v AC.   This is reversed from the second image in the in OP. The transformer spans the yellow stripe.



Also note that this drill is NOT small. The drill body is 40mm OD.  I don't think I am going to lose enough torque to prevent modeling with it.



Here are the requested images and I have added the photo of the obligatory input/output voltage markings:

@peteski "R11, 12, 13, U3, R2 (all in a single photo).  Any identifying marks on the VR (the speed adjustment pot)?"  No markings on the pot.



 @Point353 "Also, a close-up photo showing the markings on U1, the device mounted to the heatsink.":



Rear cover of I/O markings:

[peteski]

As I see it, R11 is the key to controlling the low side of the adjustment.  While not 100% clear, I think it reads 1501 (not 01501), so that is 1500 ohms.  Lowering its value will likely lower the minimum voltage output.  I would replace it with a 750 ohm resistor and see of that lowers the speed.  Or just solder another 1500 ohm resistor on top of R1 (in parallel). That will result in the new total resistance of 750 ohms.

Just be careful when handing that circuit.  The large filter cap on the high-voltage side will likely hold high voltage for few minutes after unplugging the unit from the wall outlet.  If you have a voltmeter, measure the DC voltage across it to be sure it is safe to handle the board.  If it is less than 20V DC then you should be ok to handle that board.  You can measure that voltage across the bridge rectifier (withotu needing to flip the board out of the case. Just measure across the + and - leads of the bridge rectifier BD1  (it is in parallel with the large cap).

[Iain]

What drill is this?

[Lemosteam]

You mean R11 (is it 1501 or 1051?) not R1 right? R1 is on the hi voltage side (Far right in closeup) and reads 823