Hello all,
I am trying to resurrect an old PC AT 200W power supply. It's a Morex MXC 200P.
The problem being the output voltages are a bit low. The 5V line measured 4.5V and the 12V line is about 11V.
I couldn't find a circuit diagram on line but was able to identify the mode of control by examining the traces.
Voltage is regulated primarily by the error amplifier inputs on pin 1 and 2. Pin1 receives feedback from the 5v and 12v outputs, while pin2 receives the 5v reference voltage from pin 14 (through 2x 4.7k resistor divider to 2.5V). When the pin 1 voltage is above 2.5V, switching is paused.
I notice the pin 14 reference voltage is not steady, it fluctuates from 4.1 V to 5V.
I thought the reference voltage should be kept pretty much constant internally at 5V inside the TL494 and should not fluctuate like it does. It would make sense that when the reference voltage is below 5V, the output voltages will also down regulate below the supposed 5v and 12v. Does it smell like a defective TL494?
To prove the point, I even cut the pin 14 right off the chip, and supplied a regulated 5v to the board where the pin 14 attached before the cut. Sadly the chip shut down, with pin 1 measured 0.6v and pin 2 at 2.5V.
I have 2 questions for your opinion.
Firstly, does the pin 14 do more than just outputting a regulated 5V, that the internal of the chip will not work if it is not connected to the external circuit?
Secondly, the fact that the reference voltage from pin 14 is more than 15% off the supposed 5V, does it smell like a bad TL494?
Thanks for reading this.
I am trying to resurrect an old PC AT 200W power supply. It's a Morex MXC 200P.
The problem being the output voltages are a bit low. The 5V line measured 4.5V and the 12V line is about 11V.
I couldn't find a circuit diagram on line but was able to identify the mode of control by examining the traces.
Voltage is regulated primarily by the error amplifier inputs on pin 1 and 2. Pin1 receives feedback from the 5v and 12v outputs, while pin2 receives the 5v reference voltage from pin 14 (through 2x 4.7k resistor divider to 2.5V). When the pin 1 voltage is above 2.5V, switching is paused.
I notice the pin 14 reference voltage is not steady, it fluctuates from 4.1 V to 5V.
I thought the reference voltage should be kept pretty much constant internally at 5V inside the TL494 and should not fluctuate like it does. It would make sense that when the reference voltage is below 5V, the output voltages will also down regulate below the supposed 5v and 12v. Does it smell like a defective TL494?
To prove the point, I even cut the pin 14 right off the chip, and supplied a regulated 5v to the board where the pin 14 attached before the cut. Sadly the chip shut down, with pin 1 measured 0.6v and pin 2 at 2.5V.
I have 2 questions for your opinion.
Firstly, does the pin 14 do more than just outputting a regulated 5V, that the internal of the chip will not work if it is not connected to the external circuit?
Secondly, the fact that the reference voltage from pin 14 is more than 15% off the supposed 5V, does it smell like a bad TL494?
Thanks for reading this.