Electronic Projects

Make an Adjustable Voltage motor controller for bench top motor testing.

Warning!  The use of this project is up to you! If you let the smoke out of something it's not my fault.

One of the nice things about flying electric is you can test run your motor in the house, workshop, or in the yard and the neighbors won't complain. The problem is how to control the speed of the motor. One way is to use the plane, radio, and speed control with the motor in it. That's no fun! Mount the motor on the bench and just plug the battery into the motor. That's even worse! It's hard on the motor and gearbox.

To solve the "It's hard on the motor and gearbox" problem I made myself an Adjustable Voltage Control unit (speed control) so I can slowly bring the motor up to speed.

Control Schematic:

The 3055s are rated at 10 amps ea. if on a good heat sink, I recommend (to start with) 30 - 40 amp fuse on the input side of the controller.

How It Works:

VR1 is used as a voltage divider to drive Q5, Q5 then drives Q1, Q2, Q3, and Q4. As VR1 is turned clockwise the voltage increases on the base of Q5. Q5 then increases the voltage on Q1, Q2, Q3, and Q4. As the base voltage increases on the on Q1, Q2, Q3, and Q4 the voltage will increases to the motor. As Q1, Q2, Q3, and Q4 are used in parallel .1 ohm resistors are placed on the output of the 3055s to make an attempt to balance the load between them. The 12 volt regulator is used to supply a constant 12 volts to the fan when using voltage above 12

3055 Pin layout, Each one is rated at 10 amps

Prototype of the Adjustable Voltage Control. Note the 12" of 14 awg used for .1 ohm resistors (red wire).  To help keep the 12 volt regulator cool I mounted it on the exhaust side of the Heatsink. (not shown)

I have tested this Adjustable Voltage Control and have found that the 3055s generate quit a bit of  heat, even mounted to a large fan cooled Heatsink. The voltage drop across the voltage in to voltage out is about .10 volt per amp or 1 volt for every 10 amps. To make up for the voltage loss I made a series connector to run two battery packs to double the input voltage.

The maximum voltage of the regulator is 35 volts and the 3055s are 70. 

Note the home made Heatsink at the top

Below are the dimensions of the Heatsink I used for the Adjustable Voltage Control, anything smaller may not be able to dissipate the heat.

The maximum temperature rating of the 3055 is 302 deg. f. I ran the Adjustable Voltage Control with a load of 21 amps  for about 30 minutes and measured the temperature at the mounting screw hole of the 3055 at160 deg. f .the Heatsink exhaust temperature measured in at 110 deg. f at a room temperature of  70 deg. f.

Parts used in Project


Radio Shack Part No.

Price Ea.


TIP3055 Transistor 276-2020 1.99


+ 12 VDC Voltage Regulator 276-1771a 1.49


12 VDC Fan 273-24 14.99


10 k Potentiometer 271-1715 1.49


Control Knob 274-402 1.39


Heatsink (A large hunk of aluminum may do) N/A N/A
1 12 ft 14 AWG Wire 1.20

Total 27.31

If you plan to take on this project, you will find all updates listed in the table below. Please check back often.

Revision Table
Date Revision Description of Changes or Additions
12/22/97 1.0 Posted on this site for the first time
01/02/98 1.1 Added 30 - 40 amp fuse on the input side of the controller.
08/08/98 1.2 Changed price of  TIP3055 Transistor from $7.95 ea. to $1.99 ea.