VVV
Advanced Member level 5
ei33 flyback 3842
Here is a better way to get the P/S working. Connect the transistor and the pot as shown. Adjust the pot so that the wiper is connected to GND. This will ensure the voltage at pin 1 is about 0.6 volts and the duty-cycle is at minimum. With most UC3842s it will be actually 0%, so there will not be any drive pulses.
Start up the IC like I said earlier, using a lab supply, set initially to 18V. When the IC starts up (monitor pin 8 to see when it goes to 5V or monitor the oscillator on pin 4 to see when it oscillates), reduce the supply voltage to 12V.
Now slowly adjust the pot until you have just some narrow drive pulses to the MOSFET. Record the drain waveform and post it.
By adjusting the pot you effectively adjust the output voltage. Go slowly and see what happens. Stop if the voltage goes much above 14V.
Monitor the bias voltage, too, and see how it changes and how high it gets.
If the loop is alive and working properly, at some point you will see that the voltage no longer increases as you adjust the pot, because the loop has taken over and it's now regulating.
This should be the safest method to power up your supply without risking anything.
Caution, though: the pot is on the mains side, so be careful. An isolation transformer would be highly recommended to power the primary.
Let me know how it goes and try to post waveforms of the voltages on all the windings (before the diodes). This is really important.
Here is a better way to get the P/S working. Connect the transistor and the pot as shown. Adjust the pot so that the wiper is connected to GND. This will ensure the voltage at pin 1 is about 0.6 volts and the duty-cycle is at minimum. With most UC3842s it will be actually 0%, so there will not be any drive pulses.
Start up the IC like I said earlier, using a lab supply, set initially to 18V. When the IC starts up (monitor pin 8 to see when it goes to 5V or monitor the oscillator on pin 4 to see when it oscillates), reduce the supply voltage to 12V.
Now slowly adjust the pot until you have just some narrow drive pulses to the MOSFET. Record the drain waveform and post it.
By adjusting the pot you effectively adjust the output voltage. Go slowly and see what happens. Stop if the voltage goes much above 14V.
Monitor the bias voltage, too, and see how it changes and how high it gets.
If the loop is alive and working properly, at some point you will see that the voltage no longer increases as you adjust the pot, because the loop has taken over and it's now regulating.
This should be the safest method to power up your supply without risking anything.
Caution, though: the pot is on the mains side, so be careful. An isolation transformer would be highly recommended to power the primary.
Let me know how it goes and try to post waveforms of the voltages on all the windings (before the diodes). This is really important.
