3842 problem

[5282]

i have problem with 3842 ic
 


i use 150k resistor for r1 the voltage on c3 is only=2.8v 

i don't connect the mosfet to 3842

[T3sl4co1l]

...Yes?

Do you understand the datasheet? Have you read the datasheet?

This circuit has problems. I have never seen R3 used before. There is usually a capacitor from pin 3 to ground, pin 8 to ground, and an R+C across the '431.  As shown, it will oscillate horribly (once the MOSFET is connected and all, that is).

Tim

[Andy Watson]

What have you done with pin 5 on the 3842 ?

[prawncrackers]

i burn 3842 ICs when i dont connect them to the mosfet AND the current sense resistor(R6 in your schematic) so definitely check that out.

you can try a series incandescent bulb(around 100w mains voltage) to limit the current to your circuit if youre afraid something might pop.

you also forgot your pin 5 in the schematic; this goes to ground right?
also you might want to try and omit R3. experiment only when your very basic circuit functions then work your way up for the modifications.

[5282]

thank all
i not use  R3 in my board
yes i already connect pin 5 to GND
there is low votlage in 3842 vcc=2.8v with 150kohm
now the mosfet is useless because 3842 not work with 2.8v

[T3sl4co1l]

...I guess that's a no, then...

The startup circuitry will keep turning itself on and off, holding VCC at a low average voltage.  It's still peaking high enough to turn on the MOSFET.  If you do not have an oscilloscope, you will not be able to see this.

Without a MOSFET connected, there is no power from the AUX winding, and the controller will never remain on.

I don't recommend building SMPS without an oscilloscope, anyway, because you will never have a way to tell whether it's working well or not.

Tim

[Seekonk]

Two things to try:  Greatly increase the value of C3.  Reverse terminals 3&4.

[prawncrackers]

also did you check the orientation of transformer windings? im thinking this is a flyback converter.

in any case you might want to try using an auxiliary power supply to power the 3842. i sometimes use one when making converters, just a 12-15v wallwart/charger to pin7.

[Seekonk]

A LED indicator on the output may give an indication if the oscillator ever starts working when you don't have a scope. As said before, powering the chip independently is the best step