| Electronics > Repair |
| BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed) |
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| MathWizard:
On U146, pins 3 and 4 are high voltage power in, then after a winding pin2 goes to the MOSFET drain. And there's an auxiliary winding between p5 and p6 on GND. Some of the energy that gets stored in the main winding, get transfered into this little winding, and then typically around 10-15V is created and rectified by D115, and smoothed by 100uF/25V near by. I'm thinking this is Forward Converter topology, but I don't remember the details enough to explain it all, but when the average current in the primary is increasing, energy gets stored building up a magnetic field. And then when the averge current in the inductor starts decreasing, and the field starts decreasing, then you get an induced voltage on the output side and aux windings, and current flows up from ground, and out the top of the windings. |
| inse:
Read the UC3844‘s datasheet, it explains what the chip can drive. The windings can be measured with a multimeter, the polarity only tested when desoldered (or without the transistor). Feed a suitable (50kHz) signal in the primary and check the phase angle of the secondary voltage. |
| max.wwwang:
Diode D111 is in a metal casing, with marking "V5 319" visible. What type of diode is it? Zener? |
| max.wwwang:
--- Quote from: MathWizard on December 20, 2024, 11:58:47 pm ---On U146, pins 3 and 4 are high voltage power in, --- End quote --- Correct, rectified hi V from mains. --- Quote from: MathWizard on December 20, 2024, 11:58:47 pm ---... then after a winding pin2 goes to the MOSFET drain. --- End quote --- Correct. So if pin 2 and pin 3 or 4 is one windings, then its on/off will be switched by the MOSFET, used to generate induced voltage on the secondaries. --- Quote from: MathWizard on December 20, 2024, 11:58:47 pm ---And there's an auxiliary winding between p5 and p6 on GND. Some of the energy that gets stored in the main winding, get transfered into this little winding, and then typically around 10-15V is created and rectified by D115, and smoothed by 100uF/25V near by. --- End quote --- This is where the difficulties come from. I've had a closer look at the poles of the transformer trying to deduce the internal wiring. I use numbers to indicate the sizes of the wires connected to the pins, the smaller the number, the thicker the wire. And I use "2x" to indicate there is two wires to one pin (of same size) On the primary side: Pin 2: 1 Pin 3: 1 Pin 4: 3 (2x) Pin 5: 2 Pin 6: 2 So pin 2 and 3 has to be of one winding, so are pin 5 and 6. What's puzzling is that how can pin3 be connected to both ends of the same winding?! On the secondary side: Pin 7: 1 Pin 8: 1 Pin 9: 2, 3 Pin 10: 2 Pin 11: 1, 2(? not very sure) Pin 12: 1 This is more difficult: One possibility is that pin3 7/8 are one winding, so are pins 11/23, and pins 9/10. But again, where is the single size 3 wire for?! (Unless the size 2 wire on pin 11 is actually size 3, going to pin 9.) There is no way that one end of the winding of the primary side goes to the secondary side, that will defeat the isolation. And I checked with DMM. Absolutely both sides are isolated. It looks like this weird thing – attached. |
| max.wwwang:
--- Quote from: inse on December 21, 2024, 04:14:36 am ---Read the UC3844‘s datasheet, it explains what the chip can drive. The windings can be measured with a multimeter, the polarity only tested when desoldered (or without the transistor). Feed a suitable (50kHz) signal in the primary and check the phase angle of the secondary voltage. --- End quote --- Yes, taking U146 off helps. But I'm not prepared to do that due to the likelihood of ruining the transformer and the board. |
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