PWM-controlled dual PSU

I am trying to build a bench variable power supply. I would like to have it digitally controlled via PWM (using an Arduino). I made tests using something like this: microcontroller - How does this power supply circuit work? (MCU + LM317) - Electrical Engineering Stack Exchange

In my current setup, I have R4 = 0 ohm. The input of the opamp is from a voltage divider instead of PWM for those tests. I am having strange results. When the input is 0V, the output of the opamp is -6.5 V (the opamp is supplied with +/-16.6 V). Since the output is 0V, it means that R5 drops 6.5V instead of the 1.25 V it should drop.

Do you have any idea on how to make this design work and also why does the opamp outputs -6.5V instead of 0 (the op amp is a NE5532, which I had on hand)
 
Interesting. im doing something similar with PIC 4550 USB interface i got the microcontroller USB port to recognise the windows host and i completed first part of the firmware with .NET c# host software see this.. I'm reading from 0 to 5-volts in this case and its 4- channel using a standards 10-bit ADC

Here is a preview
[IMGDEAD]http://img18.imageshack.us/img18/459/333qe.jpg[/IMGDEAD]
 
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I am trying to build a bench variable power supply. I would like to have it digitally controlled via PWM (using an Arduino). I made tests using something like this: microcontroller - How does this power supply circuit work? (MCU + LM317) - Electrical Engineering Stack Exchange

In my current setup, I have R4 = 0 ohm. The input of the opamp is from a voltage divider instead of PWM for those tests. I am having strange results. When the input is 0V, the output of the opamp is -6.5 V (the opamp is supplied with +/-16.6 V). Since the output is 0V, it means that R5 drops 6.5V instead of the 1.25 V it should drop.

Do you have any idea on how to make this design work and also why does the opamp outputs -6.5V instead of 0 (the op amp is a NE5532, which I had on hand)

by the way the max voltage for a NE5532 is
VCC+ Supply voltage min=-5 max=15 V
VCC– Supply voltage min=–5 max=–15 V

i think you damaged the device by supplying to much voltage. read the datasheet.

http://www.ti.com/lit/ds/symlink/ne5532.pdf
 
You maybe right. I will bring down the voltage and retry. I am surprised that a mere 1.6 V would break it, since the datasheet states that +/- 22V as an absolute maximum.

I started by using a LM318 which allows a higher supply. But I forgot to lower the voltage for the NE5532.

I may burn more opamps as I experiment :D
 
In fact, in this configuration, the maximum input voltage would be 5 volts, since the PWM signal comes from an Arduino. The intent is to use a voltage divider where R6 = 2 x R7, bringing the output to 15 V when the PWM is 100% duty cycle (or more than 15V, I haven't decided yet).

In my original post, I forgot to mention that the op amp negative supply was connected to -16V. Do you recommend using the ground instead?

I plan to use another op amp for the negative output with an inverted output.

I fix the op amps voltages from other regulators.