A Quick-and-Dirty DC Motor Controller

By: 
October 31, 2016

Years ago my pal Steve Titus wanted to experiment with some drone rotor designs. He needed a simple motor controller that could operate from 8V to 12V and deliver amperes of current to a DC motor used in radio-controlled aircraft (figure 1).

I whipped out a design based on the classic TL494 PWM (pulse width modulation) chip (figure 2). This Texas Instruments chip is second-sourced by ON Semi, as well as others. Distributors carry the chip in small quantities with pricing from 24 cents in 1000s to 60 cents each (2016). I learned to love this chip when I consulted to Teledyne. I designed power supplies for radar jammers on F-16 fighter jets. The TL494 came in a ceramic DIP package so you could use it on mil-spec projects or for ultra-reliable industrial applications. These days I would design in an SOIC for the small size and vibration resistance. The part even comes in TSSOP for micro-miniaturization.

To get the large amperage Steve required, I had the chip drive a large TO-3 metal-can PNP Darlington transistor that can handle 100V and 16A. Since the inductance of the motor and the wires connected to it could generate switching spikes even higher than 100V, its necessary to put a robust clamp diode across the motor (figure 3).

Figure 4; The circuit applies pulses to the motor (red). This is based on the control voltage reaching a set threshold (yellow). The current into the motor gets up to 4 Amperes, and then falls through the off-time (green) . Small oscillations in current are due to the inductance of the motor and wiring interacting with the capacitance of the snubbing diode.

You effect motor speed control by chopping the DC input voltage into variable-width pulses. I picked C3 and R5 to give an operating frequency of 8.7kHz. If the IC doesn't chop at all, it applies full battery voltage to the motor. If it applies brief narrow pulses to the motor, the average voltage is low, and the motor runs slowly (figure 4). With pulses that are vanishingly narrow there is no voltage applied to the motor, and it stops rotating. With no load on the motor, the voltage waveform can look squirrely as the back-EMF (electromotive force) of the motor creates voltage as a generator.


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While it might appear brutal to apply these pulses to the motor, remember that motors run on magnetism and magnetism is made with current, not voltage. So while the controller apples voltage pulses, the motor inductance turns this into current ramps that are proportional to the voltage.

For mechanical design I bought some heat sinks at a salvage yard that were already drilled to mount a TO-3 transistor. I used mica insulators and thermal grease when I mounted them. The studs from the transistor mount also spaced off and mounted the circuit card. I also picked up some 10 kilo-ohm slide pots at the salvage yard. I laid out the board in ProCAD. To make the circuit board I used pre-coated boards that I applied transparencies from my laser printer. Once the photo-resist was exposed, I plopped them in a tray of heated ferric chloride.

Regrets, I've had a few. Today I would use a surface-mount chip, and not use any tantalum or electrolytic capacitors for the best reliability. There is no other way to get the power of a TO-3 transistor on a big metal heatsink. But this was a tremendous over-design. Next time I would look at using a low-on-resistance MOSFET so I would not need the metal heatsink or the TO-3 case. A surface mount D2PAK is equivalent to a TO-220 package, and soldered to a hand-sized circuit board should be able to take away the heat. Big downside here is that the circuit board would not be isolated from the transistor drain, so maybe a TO-220 or TO-3 case with an insulator would still be a good idea.

I also would just order the PCB from a decent quick-turn fabrication shop rather than messing with photo-resist PCB material and transparencies and ferric chloride. At some point it is just easier to have the boards made, especially now that I know the basic design works OK.

Another regret I learned the hard way. I once designed a prototype with salvage yard parts to get it to the customer as soon as possible. He loved it and asked for 50 more. When I went back to the salvage yard they did not have those quantities available. They had no idea where to get more of the same parts. Now I stick with main-line distributors like Allied, so I know I can get as many parts as I need, they are not counterfeit, and my fellow engineers and hackers can order the exact same parts. Reproducibility is an essential part of the scientific method. The BOM (bill of material) below has available parts rather than the salvage yard heatsink and pot and other parts in this prototype.

My final regret is not making it really really clear to my friend Steve that everybody knows the red wire goes to the plus terminal of the 12-volt battery. He promptly hooked it up backwards and blew up one the prototypes before any use (figure 5). Thankfully I have learned to make 3 prototypes. Two that I can compare design changes and one that Marketing steals and ships to a show in Kirghistan.

Click here to download the full build instructions, including ProCAD PCB files as well as DXF and DWG versions.

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DC motor controller PARTS LIST

Component reference

Component/Material

Cost

Source

Motor

Motor, DC Ceramic Permanent Magnet, Long Stack - Ball Bearings, 24 VDC

$127.07

Allied Stock #: 70217726

Q1

TRANSISTOR, NTE250 PNP SILICON DARLINGTON 100V IC=16A TO-3 CASE

$8.74

Allied Stock#: 70215870

Q1 insulator

RS Pro Heatsink Transistor Mount Kit For TO-3

$1.83

Allied Stock#: 70637780

Q1 grease

Aavid Thermalloy Thermal Grease, 2 Oz., Thermalcote

$10.82

Allied Stock#: 70115245

PCB

MG Chemicals Board; Copper Clad; 9 x 6 in; 1/16 thk; double sided; presensitized

$21.50 (makes 4 PCBs)

Allied Stock#: 70125853

PCB etchant

MG Chemicals Chemical; Ferric Chloride Copper Etchant; 17oz liquid

$9.25

Allied Stock#: 70125782

D1

Diode, MR752 6A 200V Silicon Rectifier

$1.14

Allied Stock#: 70911267

U1

TL494 SWITCHMODE Pulse Width Modulation (note- SOIC package!)

$0.27 ea (20 min)

Allied Stock#: 70341601

Pot1

Bourns Potentiometer, Panel Control, Carbon, 100mm, 10 Kilohms, 20%

$3.91

Allied Stock#: 70155184

Pot knob

RS Pro Slide Knob; Body: Black with a White Indicator; 1.2x4mm Shaft

$0.39

Allied Stock#: 70644546

Heatsink

Wakefield 423K Heatsink; TO-3; 47C @ 50W; 2.625 in.; 0.5C/W;

$20.79

Allied Stock#: 70236723

C1

Capacitor; Tantalum; 33uF; Tap Series; Radial; Case J; 10%; 25V; 33VDC; 1.2 Ohms ESR

$2.18

Allied Stock#: 70195926

C2

Capacitor; Ceramic; Cap 1uF; Tol 10%; Radial; Vol-Rtg 25V; Dielectric X7R

$0.770

Allied Stock#: 70096303

R1. R2

Vishay Dale Resistor; Metal Film; 133 Ohms; 0.5 W; Tol 1%; Axial; Military (qty 2)

$0.44 ea

Allied Stock#: 70200270

R3

RESISTOR METAL FILM 1/4W 3.16K OHM 1% AXIAL LEAD

$0.05

Allied Stock#: 70722839

R4

Resistor; Metal Film; Res 1.13 Kilohms; Pwr-Rtg 0.125 W; Tol 1%; Axial

$1.02

Allied Stock#: 70205979

C3

Nichicon Capacitor Polyester Film Cap 0.01uF 100V 10% Radial 6.5X10.5 LS 5mm

$0.142

Allied Stock#: 70188286

R5

Vishay Dale Resistor; Metal Film; Res 19.1 Kilohms; Pwr-Rtg 0.125 W; Tol 1%; Axial; Military

$0.09

Allied Stock#: 70200553

J1, J2

TE Connectivity UNIV M-N-L HEADER, 02P UMNL HDR ASSY R/A 94VO LF

$1.66 ea

(qty 2)

Allied Stock#: 70087875

P1, P2

TE Connectivity Connector, Soft Shell; Universal MATE-N-LOK; 2; Plug; Nylon; White; 19 A

$0.25 ea

(qty 2)

Allied Stock#: 70083521

P1, P2 contacts

TE Connectivity Universal MATE-N-LOK II Socket Contact, 24-18AWG, Gold (30) over Nickel

$1.26 ea

(qty 4)

Allied Stock#: 70041811

P3

Pomona Electronics Banana Jack; Banana Plug; 15 A; Black; Beryllium Copper; 5000 Vrms

$5.24

Allied Stock#: 70197210

P4

Pomona Electronics Banana Jack; Banana Plug; 15 A; Red; Beryllium Copper; 5000 Vrms

$5.24

Allied Stock#: 70197211

Q1 screws

APM Hexseal Hardware, Phillips Pan Head Seelskrew, 1 Inch, 6-32 Thread Size, SS

$0.51 ea

(qty 2)

Allied Stock#: 70156385

J1, J2 screws

APM Hexseal Hardware, Phillips Pan Head Seelskrew, 1/2 Inch, 6-32 Thread Size, SS

$0.43 ea

(qty 4)

Allied Stock#: 70156382

Q1, J1, J2 nuts

Keystone Electronics Nut; Hex 6-32 Threaded ; Steel; Machined; OD .250

$0.070 ea

(qty 10)

Allied Stock#: 70279752

Q1, J1, J2 lock washers

Keystone Electronics Terminal Lug; Lockwasher; Brass, Tin Plate; Length .875; Hole Size #6

$0.120 ea

(qty 6)

Allied Stock#: 70386216

[All images via Paul Rako, unless otherwise noted]

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