Power amplifier- please help me

[fasdfas]

Hello,

I'm using a PCI 6251 together with SCB 68. I would like to control 2 motors, one is 6V and 740mA and another one is 5V 200mA. As far as I know SCB 68 can only give up to 10V and 5mA and the motor manufacturers do not provide any motor drives. May I know how can I drive this motors using SCB 68? Do I need a power amplifier? Can you recommend a specific power amplifier or any other alternatives? Soon I will also need to control the motors using PID and other control algorithms.

I've tried to construct the power amplifier as in the Fig. 2 in the attachment: Vin is a 12V voltage supply and RL is a 1K ohm resistor, but it does not work. Instead of amplifying the power, the voltage and current across RL is almost zero. However the voltage across the capacitors is large. Or does circuit this only work for AC?

What should I do?  Any help is highly appreciated. Thank you very much.

[Henrik_Volkers]

Yes it's designed for AC 🙂

Replace C1, C5 with a wire and make Rf about 6k and the motor should run... at least once 😆  if you don't have any protection for your amplifier agains voltage spikes from you inductive motor. google snubber.

Next bad thing might be the power loss.. and that you maybe don't have the full range. This circuit is developed with a AC output in mind and runs with a DC bias V/2

The usual way is using a FET (and protection circuitry!) and your timer outputs  and use PWM to control the motors.

[nnnnnnnn]

Thank you so much for your reply.

How to implement the power amplifier using FET? Is there any other way of controlling the motor without using PWM? Sorry, I'm new in this.

Thanks

[Henrik_Volkers]

Do the motors need to change direction?

Here is a datasheet with a bidirectional drive

The FET is just used as a switch, for a small load (<500mA) the BS170 is fine. For bigger ones look at IRF or ask you local dealer.  Look at the LV examples for PWM output.

One  R of 10k from the timer(PWM) output to the gate, a RC snubber (4.7ohm+200nF) , powersupply and you are done 🙂

[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 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

[Henrik_Volkers]

Here is the schematic from the datasheet:

1) You can change the gain by changing the ratio of the 20k/1k feedback.Vout = Vin * 20k/1k = 20*Vin

Don't change it to less than a gain of 10 without additional compensation (probably start to swing), but you can use a 22k (or 10k) potentiometer at the input . The wiper goes to the + input of the amp.

2) by using the the formula noted in 1) multiplied with the ratio of your input divider (pot) if used. Or by using a voltmeter and measure the input and output ratio.

3) by choosing the right power supply voltage 😉  . Or adding a current limiter or ....

4) by using a bipolar powersupply (+ and - Vee to GND) . If you don't have or don't want it, use a servo motor control, but I wouldn't use the one in the datasheet .... sorry, no time for searching another one.

[nnnnnnnn]

Hi!

Thank u very much for your reply

If I connect the Vin and ground of my 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.
I'm using the DAC as in http://sine.ni.com/ds/app/doc/p/id/ds-22/lang/en . 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.

Thanks

[Henrik_Volkers]

You will need a dual powersupply, mean +8V from Vcc to GND and -8V from Vee to GND .

If you only have a single powersupply but the output is not referenced to GND (no connection between - output and EARTH/GND/housing ) you can use one LM675  to create a buffered GND at half the powersupply voltage like mentioned in the datasheet page 2. Like that you will need 3 power OPs to drive your two motors with a 16V single power supply.

Your DAC output will also need a GND reference!

Before blowing something up...  draw your design and post it here or show it someone knowing analog (real 😉 ) electronics.

[nnnnnnnn]

Yes, I'm using a dual power supply.

I am only trying to amplify the power of one of the motors at the moment. My circut is as in the attachment.

The input, Vin is a +- 5V square wave signal with 1rad/s frequency.

If I connect the motor directly to the analog output and ground of SCB68, it will turn both in directions, but very slowly.   

But if I insert the amplifier as in the attachment, it turns quickly, but only in one direction. Is there any mistake in my connection?

Thanks

[Henrik_Volkers]

Sorry, I don't open .doc  from the internet 😕

png ?