Power amplifier

[nnnnnnnn]

Hi!

I'm currently trying to amplify the power so that the power from SCB68 can turn/ drive my motor using LM675 as in the first page of this datasheet:
http://www.national.com/ds/LM/LM675.pdf

I just replaced the RL with my motor, but how can I:
1. Set the gain of the power amplifier? Is it by changing the 20k resistor only?
2. How can I know/ verify what is the power amplifier gain without having to measure the voltage & current?
3. How to limit/reduce the amplified power so that it is not excessive or will not damage my motor?
4. I'm trying to make the motor turn both cw and ccw, but after programming and implementing this amplifier, it only turns cw. How can I make it turn in both direction? Do I need to use the circuit as in page 8 "Servo Motor Control"? If yes, how can I set the amplifier gain? Is it by changing the 1M resistor only?

Sorry if my questions sound stupid/ very basic.
Thanks

[Buechsenschuetz · Accepted solution]

1. This is a power amplifier, using the usual configuration of a non-inverting op-amp amplifier. The gain is set by the ratio of the 1k resistor and the 20k resistor. Let's call the 1k resistor R1 and the 20k resistor R2, then the gain G = (R1+R2)/R1. You should keep R1 at the recommended value (1k), increasing it might affect the offset voltage, i.e. the voltage present at the output when the input is 0V.

2. See above.

3. This circuit has short-circuit protection only and can deliver up to 3amps. But it will run with a wide range of supply voltages. The maximum output voltage will be close to the supply voltage. Only method to limit output power I can see is limiting the supply voltage such that the maximum output current does not damage your motor.

4. I am not very familiar with servo motors but I think to make them turn the other way round you have to reverse the supply voltage of the motor. To get a negative output voltage of the circuit (with reference to GND, since the output load is referenced to GND), you have to feed a negative input voltage. With an input voltage going positive only (with respect to GND), the motor will always receive a positive voltage. (The circuit on page 8 works somewhat different, it uses the difference between the input voltages of both circuits as input signal).

To shift an "unipolar" (i.e. going positive only) input voltage to one which goes from a negative value to a positive value, you probably need some level shifting circuitry. Usually this is done with an analog adder, adding a reference voltage to the input voltage. Since the LM675 basically is an op-amp with very high output current, it is probably possible to add this circuitry to the basic circuit, using a reference voltage source. 

[nnnnnnnn]

Hi!

Thanks for your reply.

May I know how to implement the an analog adder/ level shifting circuitry?

Thanks

[Buechsenschuetz]

You can find op amp circuits for adding/subtracting voltages in any textbook about op amps (and I think it might be useful for you to learn a bit about op amp circuitry). You will also need a reference voltage circuit, there are lots of fixed and adjustable voltage references available.

[nnnnnnnn]

Thanks a lot