# Read value of SMD diode (mini-melf)

Discussion in 'Datasheets, Manuals and Component Identification' started by SebaX75, Feb 9, 2012.

1. ### Rleo6965

585
9
Jan 22, 2012
That value could be the resistance of resistor in paralle with ZD1. But this confirm that there's no shorted component on gate of Power Fet.

ZD1 was not shorted . ZD1 will read 0V if shorted both forward and reverse reading.
ZD1 2.4V might be okay. It depend on Power FET Gate threshold voltage or Vgs. ZD1 was to protect gate of Power FET from over voltage.

Maybe we should check again the resistance of smd resistor 114 and 114X2. I'm not sure if 114X2 were 2 pcs.114 in parallel or equivalent to 55K. If its in parallel, 55k + 110K=165K. Please check resistance of smd resistor that connect between ZD1 and primary filter capacitor. A very high value will cause too low or 1.6V reading on gate of power fet and not able to conduct or start oscillation of inverter circuit.

Last edited: Dec 10, 2012
2. ### SebaX75

45
0
Jan 10, 2012
The resistors are in series, but a strange thing (IMO) happens: measuring the resistance between point 1 and 2 of my schematics, reading starts at about more than 2Mohm and down slowly until 370kohm, then drops suddenly picking up 30kohm or less and going up to about 357kohm... is it possible that the FET or the transistor go into conduction state, avtivated by the voltage accumulated by a capacitor?
Real resistors value, in order from 1 to 2, are: 109, 109.6, 109.2, 10.95 and 18.10 (parallel of ZD1).
On primary filter capacitor there are 302V, so theoretically the voltage across ZD1 should be higher than 1.6 V...
Considering removing the zener diodes, at the points where they are mounted, you should have about 15V where is mounted ZD1 and about 24V where is mounted ZD4.
Always starting from 1 to 2, the voltages are: 302V, 202V, 104V, 6.88V and then 1.66V (and this is not motivated).

Thanks,
Sebastian

Last edited: Dec 10, 2012
3. ### Rleo6965

585
9
Jan 22, 2012
If you look close on your schematic diagram. Points 1 and 2 were connected in large value 3 pcs. capacitor in parallel connection. This is the reason that inaccurate reading or charging and discharging of capacitor to your multimeter.

Try to replace ZD1 w/ 5,1V or 6V. Be sure that you use about 0.5A fuse to avoid damages to power fet. Power on unit and observe dc output.

4. ### SebaX75

45
0
Jan 10, 2012
I don't have diode with value you have told... I must order them.
I've tested another identical SMD diode (I've bought five of them) and the value was 1,96V (source 12V and series resistor 10kohm).
Can I use 4 diode (4148) in direct polarization to have 2,4V? Must I replace fuse in this case too?

Thanks,
Sebastian

5. ### Rleo6965

585
9
Jan 22, 2012
Solder 3 pcs of ZD1 in series on a wrap w/ insulator like masking tape. Place 2 wire and solder it to location of ZD1. Polarity of zener diode and wiring must be correct. . This will triple the zener voltage value.

4148 was a fast recovery diode. We can't use it as zener diode.

6. ### SebaX75

45
0
Jan 10, 2012
Excuse my late in reply, but I'm away from home quite often...
I've checked the datasheet of STK0460F and it has a Vgs min of 2V and max of 4...
So I've tested with a 200mA fuse and with 2 zener diodes in series (2,96V)... always no output, so I've tested with 3 zener diodes and it's very strange, now I've only 1,65V and welds and connections appear to be correct.
Suggestions?

Thanks,
Sebastian

7. ### Rleo6965

585
9
Jan 22, 2012
It seems ZD1 was not defective. Some components was pulling down the gate voltage to 1.65V which FET can't conduct. Try to check components connected to gate of FET for possible short or change value. Your schematic for the gate of FET was not complete.

8. ### SebaX75

45
0
Jan 10, 2012
Hi,
I've found a zener diode with a real value at 4,81V... so I've replaced the diode in series, but when I've powered it the fuse is blown.
I've tested the fet without dismount it with an ohm meter, but no short circuit are persent.
In the next day I'll recheck schematics.

Thanks,
Sebastian

9. ### Rleo6965

585
9
Jan 22, 2012
It's possible that FET started the inverter circuit . Test fuse capacity was not enough for the inverter circuit that cause it to blow. Since the FET was not damaged. Replaced the small capacity fuse half the capacity of the original capacity of fuse. What's the original fuse of your power supply?

But test your power supply without load.

10. ### SebaX75

45
0
Jan 10, 2012
Hi,
logically all tests was done without load.
The blown fuse was a 0,2A and orginal fuse is 2A; I've replaced with 0,8A and now it's working without load, but when I connect a resistor to get about 0,5A, the voltage drop at 0,6V... could be the different transistor?

Thanks,
Sebastian

11. ### Rleo6965

585
9
Jan 22, 2012
0.6V was minimal. What was your load? Increase fuse to 1 amp. and place your actual load. Hope your load was not defective.

If the fuse blow and the fuse glass was not burnt or black. Then possible that fuse capacity was still enough. Replace with 2 amp. fuse and hope it work. If blown fuse glass color was black. You still have problem. It's the sign of defective power fet. or short in your load.

Good luck.

12. ### SebaX75

45
0
Jan 10, 2012
I've done and error writing... not 0,6V, but 0,06V.
Fuse is ok, no short circuit... without load output is 9,27V.
Yesterday I've used a resistor as load, I've used a resistor of 18ohm to have about 0,5A, but when I've connected it the voltage has dropped to 0,06V.
Now I've tested a bit more with 100ohm, 47ohm and 37ohm... the output voltage was 9,26V, 8,92V and 0,05V... the conclusion is that the max output current is less than 20mA.
Could be this problem due to transistor replacement? I've used an smd transistor 1P.

Thanks,
Sebastian

13. ### Rleo6965

585
9
Jan 22, 2012
Where was 1P smd transistor connected? I suggest that you complete drawing the TL431 voltage reference circuit. This circuit feedback signal to inverter if voltage slightly drop from its normal dc output.. Inverter circuit will therefore correct it output voltage.

This is the reason dc output drops to 0V because inverter did not receive feedback signal voltage from TL431.

14. ### SebaX75

45
0
Jan 10, 2012
Hi,
I've recovered 1P from old power supply... the TL431 is present on my schematic, it's the last component in low voltage section. On cathode I've about 8V and on reference I've 2,4V. When I connect the load all voltage go to 0.
What voltages should I expect on the transistor with load and not?

Thanks,
Sebastian

15. ### Rleo6965

585
9
Jan 22, 2012
Feedback circuit not function if dc output goes to 0V with load. Check again TL431 , Opto isolator and other components that connects to Gate of Power FET. It's better that you draw that feedback circuit for proper analysis.

16. ### SebaX75

45
0
Jan 10, 2012
Ok, I'll try to redraw the complete schematic with feedback circuit... how can I check if TL431 or optoisolator work in the correct manner?

Thanks,
Sebastian

17. ### Rleo6965

585
9
Jan 22, 2012
Show me first correct drawing and I'll try to explain.

18. ### SebaX75

45
0
Jan 10, 2012
Hi,
I've done a new schematic draw and I've examined another time all the PCB...
Can you told me if schematic logic is ok? If your answer is yes I may have found a problem with a track that is broken.

Thanks,
Sebastian

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19. ### SebaX75

45
0
Jan 10, 2012
Ok, now it's working, the problem was the cutted track.
Thanks for all the help and teachings, very useful to better understand how the switching power supply works... now I can move on to the next "level", a power supply with output voltage lower than expected, but without load, and it's not a capacitor problem (already replaced).

Very thanks,
Sebastian

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