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nza0104
nza0104 • Jan 25, 2011

another power amplifier- lm675 questions

Hi!

I'm constructing the "Non-inverting amplifier" in page 1 of https://www.national.com/ds/LM/LM675.pdf. In my circuit I replace the 20k ohm resistor with a 1k ohm resistor since I actually need a voltage gain of 2.
1. If the motor is set to turn forward, the voltage gain is correct, but if I set the motor to turn backward/ reverse, I do not get the correct voltage gain. What is my mistake? From my observation, if I increase the current from my power supply (I use +-8V), it will also affect the voltage gain. What is the correct current that I should set at my power supply unit?
2. I have two motors whose power needed to be amplified. I am using 2 units of LM675. I have only one set of power supply unit, so I just connect the first LM675's pin5 to the second LM675's pin5 and then to the VCC. Then I also connect the first LM675's pin3 to the second LM675's pin3 and then to the VCC. Is this right? I do not get the correct voltage gain with this connection. What is the correct way of connecting the amplifiers, since I only have one set of power supply unit?
3. What is the effect of 0.1mircroF capacitors at VCC and VEE? Is it ok not to use any capacitors at all at VCC and VEE?

Thanks.
freak16
freak16 • Jan 25, 2011
Hello Nza!
Welcome to CE.
I saw your queries and tried to solve them by best of my knowledge. I am starting from last.
3) 0.1 imicro f capacitor are used as to obtain dc supply free of ripples so they are nothing more than a coupling capacitor which allow high frequency components to go inside ground.
so you can avoid them.
2)you are giving positive vcc at both the pins 3 and 5??
Do one thing take two batteries and connect the positive terminal of one to the negative of other

+vcc ------|1 ------|1---- -vcc
and give this negative terminal to pin 3 and positive to pin 5.
let me know if it work or not.
3)how do you reverese your connection?
nza0104
nza0104 • Jan 27, 2011
hi!
thanks.
i gave +8V to pin 5 and -8V to pin 3, using the same connection as u mentioned, but i use 2 power supply units instead of batteries. but the voltage gain was not correct if the motor turns in reverse direction.
i make the motor turn in reverse direction by giving negative voltage at Vin on pin 1.

thanks again
freak16
freak16 • Jan 27, 2011
okay i got the problem..
dont give negative Vin at pin 1..
just the reverse the motor connections and change nothing else .
it will surely work

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