Motor amplifier board

Michael Peshkin, 2006-09-18

This printed circuit board (PCB) allows you to supply high currents to a brushed DC motor, or a solenoid, or a similar high power device.

(click on the image to enlarge)

The PCB has circuitry for two different kinds of control, depending on how you populate the PCB. The complete circuit is at right, including traces for current mode (the current through the load is in proportion to the analog voltage input, also known as torque mode), and for voltage mode (the voltage across the load is in proportion to the analog voltage input). It also has traces for negative resistance (never mind.)

For voltage mode, include Rvfdbk

Use a jumper in place of Rsense and Rnoninv, which aren't needed. The circuit simplifies as shown at right.

The voltage gain (ratio of load voltage to input voltage) is –2 volt/volt. This is 20K/10K with an inverting configuration. You can change these resistors to get another gain if you prefer.

For current mode, include Rifdbk and Rsense. You can leave out (leave uninstalled; open) Rvfdbk. You can use a jumper in place of Rnoninv, which isn't needed. The circuit simplifies as shown at right.

The gain is now a ratio of output current to input voltage, and it is –0.5 amps/volt as shown. This is 1K/(10K*0.2?) in an inverting configuration. You can change this gain by changing the resistors. The power dissipated in the current sensing resistor is I2Rsense, so be sure its value and power capacity are appropriate. The one in the photo is rated for 3W continuous.

In all cases the power circuit is the same. The OPA177 opamp drives the base of the NPN 2N6045 transistor positive to accomplish positive output voltages, and drives the base of the PNP 2N6042 transistor negative to accomplish negative output voltages.

The opamp cannot get its output very close to the supply rails ("rails" means whatever the dual supply voltages are, V++ and V--). For instance with a power supply of ±12V, the output swing of the opamp is only ±10V, and the additional 1.2V loss due to the transistors' Vbe reduces the maximum voltage across the load to ~±9V. Also, if you have a non-zero Rsense, there may be some voltage drop across that.

There are rail-to-rail output opamps available (LT1636, LT1638), and if you use one of those you can get considerably more output voltage swing from this motor amp circuit.

However, if you want to get the most motor voltage out of a given power supply, this is probably not the motor amp for you anyway. You will want a motor amp with an H-bridge output so that you can use a single power supply and still drive the motor in both polarities. You will want a motor amp that doesn't require 3V of headroom (also called overhead). And you will probably want a switching amp, rather than a linear amp like this one, so that its power transistors run cool.

Cb and Rbe are chosen to stabilize the circuit. Because of strong nonlinearities and inductive loads, circuits like this can be unstable. You may not know it is unstable unless you check what it is doing with a scope. The values shown seem to work for this opamp for the kinds of loads we have tried.

Similarly, the values for Cmotor and Rmotor seem to work. If in your application the amplifier is unstable, try changing them. Faster opamps may be less stable, too.

Inexpensive +12 & -12 supplies

You can use surplus 12volt (at up to 3amp) "power bricks" as 12volt supplies

There is a slight complication. The "ground pin" on the plug-into-the-wall side of each supply is connected to one of the 12V outputs; specifically to the barrel-exterior; the "minus" side of the output.

You can literally break off the ground pin on the plug-into-the-wall side, and use it as a floating 12V supply. (see photo; click to enlarge)

The hot & neutral pins on the plug-into-the-wall side are isolated from the 12V outputs, as they should be.

For safety, you still want to earth-ground your equipment. The easiest way to do this is to vandalize only one of your bricks.

Break off the ground pin on the brick that supplies -12 V, so it can float. Leave the ground pin on the brick that supplies +12 V, so that its minus side is still earth grounded.