power amplifier again- pls help me

[nza0104]

Hi!

I'm currently trying to amplify the power so that the power from my digital to analog converter can turn/ drive my motor using LM675 as in the first page of this datasheet:
https://www.electro-tech-online.com/custompdfs/2010/12/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 to turn in both cw and ccw directions, but after implementing this amplifier　as in page 1 of the datasheet, it only turns cw. How can I make it turn in both direction? I have a dual power supply.　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

 

[MikeMl]

Hi!

I'm currently trying to amplify the power so that the power from my digital to analog converter can turn/ drive my motor using LM675 as in the first page of this datasheet:
https://www.electro-tech-online.com/custompdfs/2010/12/LM675-1.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?

No, it is by changing the voltage divider ratio by changing either the 20K or the 1K, or both.

2. How can I know/ verify what is the power amplifier gain without having to measure the voltage & current?

The voltage gain as drawn in the data sheet is (1+20K/1K), or 21.
The power gain is very hard to figure because the motor is such a complex load.

3. How to limit/reduce the amplified power so that it is not excessive or will not damage my motor?

If you set the power supply voltages Vcc and Vee to be 2 to 3V higher than what the motor is rated for, then you cannot hurt the motor. The voltage gain should be set to Vmotormax/Vout(DAC)max. So if you motor will take 15V, and the DAC puts out +-2.5V, then the gain should be set to 15/2.5 = 6, which means a divider ratio of 6-1. The supply rails should be set to +18V and -18V.

4. I'm trying to make the motor to turn in both cw and ccw directions, but after implementing this amplifier　as in page 1 of the datasheet, it only turns cw. How can I make it turn in both direction? I have a dual power supply.　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...

The reason that the motor only turns one way is likely that you need a bipolar DAC which puts out -2.5V (full CCW) to +2.5V (full CW), with 0V being stopped. -5V to +5V would work, too. If you are stuck with a monopolar DAC (0 to 5V), you will have to offset that by -2.5V before it goes into the power amp...

 

[nza0104]

Hi MikeMl!

Thank u very much for your reply

The supply rails should be set to +18V and -18V.

May I know what is meant by supply rails? Is it the range of values for VCC and VEE in the figure in Page 1 of the datasheet? How do you get the values +18V and -18V?

The reason that the motor only turns one way is likely that you need a bipolar DAC which puts out -2.5V (full CCW) to +2.5V (full CW), with 0V being stopped. -5V to +5V would work, too. If you are stuck with a monopolar DAC (0 to 5V), you will have to offset that by -2.5V before it goes into the power amp...

4a) May I know how can I set the offset by -2.5V?
4b) I'm using the DAC as in **broken link removed** . One of my motors needs 6V and 740mA and another one needs 5V and 200mA. The DAC can supply up to 10V and 5mA only. And my Vin from the DAC is a square wave signal with amplitude +/- 5V and frequency of 1 rad/s.
If I connect the Vin and GND of the DAC directly to terminals of one of my motors without the amplifier, the motor can move in both cw and ccw directions, but it's turning very weakly/ slowly. But if I insert the power amplifier, the motor turns faster & 'stronger' but it only turns in one direction. Is there something wrong with my connection? I only connect the Vin from DAC to to pin 1 of LM675, motor terminals to pin 4 of LM675 and the power supply's ground, and I do not connect the DAC's ground to anywhere.
4c)Is there anyway of changing monopolar DAC to bipolar DAc besides using the offset?

Thanks

 

[MikeMl]

...May I know what is meant by supply rails? Is it the range of values for VCC and VEE in the figure in Page 1 of the datasheet? How do you get the values +18V and -18V?

Yes. Also note that to get VCC and VEE requires that there are two supplies connected in series, with the "center tap" connected to the ground symbol. Look at the graph "Output Voltage Swing vs Supply Voltage" graph on page 4 of the data sheet. If you know the max motor voltage, you can read the required VCC and VEE off this graph.

4a) May I know how can I set the offset by -2.5V?

Add an opamp stage, or sum an offset into the power amp itself. If you set up you DAC properly, you don't need to. (see below)

4b) I'm using the DAC as in **broken link removed** . One of my motors needs 6V and 740mA and another one needs 5V and 200mA. The DAC can supply up to 10V and 5mA only. And my Vin from the DAC is a square wave signal with amplitude +/- 5V and frequency of 1 rad/s.

That means you just want the motor to run full speed one way, then full speed the other? Why are you using a 20bit DAC? A single digital bit would make the motor do that????

That DAC can be configured to put out +-5V full-scale. That is how it should be configured to feed into the power amp. RTFM to figure out how to configure the DAC.

If your DAC puts out +-5V, for the 5V motor, the powerAmp gain should be 1, and 1.2 for the 6V motor. Based on the data sheet, that means the Vcc/Vee supplies should be ~+-8V.

If I connect the Vin and GND of the DAC directly to terminals of one of my motors without the amplifier, the motor can move in both cw and ccw directions, but it's turning very weakly/ slowly. But if I insert the power amplifier, the motor turns faster & 'stronger' but it only turns in one direction. Is there something wrong with my connection? I only connect the Vin from DAC to to pin 1 of LM675, motor terminals to pin 4 of LM675 and the power supply's ground, and I do not connect the DAC's ground to anywhere.

The DAC's ground pin MUST BE CONNECTED to the power amplifier's ground!!!!!

4c)Is there anyway of changing monopolar DAC to bipolar DAc besides using the offset?...

Yes, RTFM for the DAC.

 

[nza0104]

thanks a lot