nza0104 Posted December 3, 2010 Report Share Posted December 3, 2010 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:http://www.national.com/ds/LM/LM675.pdfI 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 3, 2010 Report Share Posted December 3, 2010 how can I:1. Set the gain of the power amplifier? Is it by changing the 20k resistor only?The 20k is the best one to change. But changing the 1k resistor also changes the gain.2. How can I know/ verify what is the power amplifier gain without having to measure the voltage & current?The formula for the voltage gain of a non-inverting opamp is the ratio of the feedback resistors plus 1. so the ratio of 20k and 1k is 20 plus 1= a gain of 21.3. How to limit/reduce the amplified power so that it is not excessive or will not damage my motor?It limits the max current to typically 3A to 4A depending on its supply voltage and its output current.It makes a very poor motor driver because it is not a switch, it is a linear amplifier. It has low output current and a high voltage loss when it is used as a switch.Are you using the power opamp to vary the motor's speed? Then the power opamp will overheat. Motor speed controllers use Pulse-Width-Modulation so there is only a small amount of heat.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.Its output is positive when its input voltage is positive. Its output is negative when its input voltage is negative. 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?I don't think your motor is used as a servo that moves then stops when it reaches a certain position.A transistor circuit can control motor direction. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 3, 2010 Report Share Posted December 3, 2010 If the opamp is used as the output stage of a pulse width modulated signal, then the motor will be controlled by pulse width modulation. However, since that opamp can drive lower impedance loads, it might be possible to simply add a push pull amplifier. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 3, 2010 Report Share Posted December 3, 2010 If the opamp is used as the output stage of a pulse width modulated signal, then the motor will be controlled by pulse width modulation. However, since that opamp can drive lower impedance loads, it might be possible to simply add a push pull amplifier.But the power opamp is a push pull amplifier with a lot of loss (a Mosfet H-bridge should be used instead). Quote Link to comment Share on other sites More sharing options...
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