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Read value of SMD diode (mini-melf)

SebaX75

Jan 10, 2012
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Hi, I've worked on a photo to trace all track (in black) and wrote down component on component side (green, but some component I don't know what they are)... if this is enough to make the diagnosis would be perfect, otherwise I will try to draw the diagram.
If you want a better or bigger photo I can upload it somewhere else.

Thanks,
Sebastian
 

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henrypereira

Nov 7, 2012
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Hi, I've worked on a photo to trace all track (in black) and wrote down component on component side (green, but some component I don't know what they are)... if this is enough to make the diagnosis would be perfect, otherwise I will try to draw the diagram.
If you want a better or bigger photo I can upload it somewhere else.

Thanks,
Sebastian

can you signal what transistor you are looking for please?
 

SebaX75

Jan 10, 2012
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In the past I've done a short circuit on the zone ZD1 and ZD4 during measure, so I suppose that the transistor Q2 is burned (about mid horizontal and 1/3 in height)...

Thanks,
Sebastian
 

henrypereira

Nov 7, 2012
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In the past I've done a short circuit on the zone ZD1 and ZD4 during measure, so I suppose that the transistor Q2 is burned (about mid horizontal and 1/3 in height)...

Thanks,
Sebastian

have you tested the transistot it self ?
try to take it off from the ciscuit and test out of the ciscuit becaise sometimes it makes you think it is on short if tested in the circuit board ,but its an npn with the base in the midle
 

SebaX75

Jan 10, 2012
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Hi,
I've unsoldered it and tested, it's not working, not a short circuit, but is as it have no polarity... diode test between pin 1-2 and 2-1 give the same value.
Are you sure that it is a NPN? All NPN are good? Some ideas about Ic, Vcb, Vce?

Thanks,
Sebastian
 

Rleo6965

Jan 22, 2012
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Tried draw a schematic diagram basing on your posted board. I found out that some traces or drawing were wrong.

I think your power supply diagram would be similar to this 15W power supply.
 

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SebaX75

Jan 10, 2012
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Hi,
I've rechecked draw and probably I've found the error (and it's possible that there are some deficiencies signed in red... I don't understand what could be under the HF transformer); I've drawn transformer in a wrong position, windings should be moved to the left of a weld.
I've see your schematics, but it seems different... you have have fewer parts, diodes "in contrast" are on the control circuit, mine seems that are "after" the mosfet and only two component with 3 pins: transistor and kp142.
In my power adapter there are one transistor (SMD), a precision shunt regulator (TL431 in the high part of photo) and a N-channel MOSFET (FQPF5N60C at about mid of photo in the bottom)... the control circuit seems to be more complicated and with that blown transistor.

Thanks,
Sebastian
 

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Rleo6965

Jan 22, 2012
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I've googled that 15 watts switching power supply to show that the power transformer winding circuit was similar to your defective power supply. It's only for your guide.

I'll try again to draw your schematic again in my free time.. It seems the primary winding and feedback winding for oscillation were correctly drawn.

But you must draw your schematic for comparison.
 
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SebaX75

Jan 10, 2012
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Ok, I'll try to draw schematic from my photo... consider only that I'm very slow... I drew the picture only because I was faster.

Thanks,
Sebastian
 

SebaX75

Jan 10, 2012
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Hi,
I've found other errors or missing tracks, on low voltage section... this is the new picture.
I'm trying to get schematics from the photo... I hope to post it for the weekend.

Thanks,
Sebastian
 

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SebaX75

Jan 10, 2012
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Ok,
I draw the circuit diagram, if I haven't made ​​any mistakes, it is in the attached image.

Bye,
Sebastian
 

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Rleo6965

Jan 22, 2012
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Here's another simple power supply diagram. This will guide to draw correct connections of your diagram. Use ohmmeter to confirm connection. Your making progress.:)


The ac signal feedback voltage lower primary winding will drive power transistor to oscillation. Probably 30khz or higher. The current limit transistor monitor the voltage drop from the 10 ohm emitter resistor. So the small transistor will shutoff power transistor for overcurrent. I still wonder that TL431 have no opto isolator.

In your drawing of TL431 connection. It must always be connected to +dc output . so it can monitor voltage variation and send back to power transistor to adjust its output.

Compare this diagram with your drawing. It's similar.
 

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SebaX75

Jan 10, 2012
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I've tried to replace the transistor with another NPN, but it still does not work.
I'm sure that no optoisolator is present.
During checking with the ohmmeter I do not understand the functions of the points marked in red, and then identified with question marks, no continuity between this point...
Where I can start to check because it still does not work?

Thanks,
Sebastian
 

Rleo6965

Jan 22, 2012
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I draw red lines in your schematic. This is the possible connection. I can't confirm correctly without the actual board for tracing.

Warning. Don't touch big filter capacitor after powering off power supply. 300V charge voltage might still exist. Especially if inverter circuit did not work. Don't discharge capacitor using wire or screw driver. Use the ac plug of your soldering iron. The soldering iron heater will act as shunt resistor.

Let's try to troubleshoot your board.
1. Replace original fuse w/ smaller capacity to protect power fet.
2. Be sure R1 10 ohm resistor was not open.
3, Remove NPN Q2 from the circuit for testing purpose only.
4. Place your red probe multimeter set in HZ in Testpoint1. Power On power supply. We need to see more than 20khz in this point. Even momentary only. Without this feedback signal your Power supply will not work.
5. If no signal. Turn off power. Don't touch with your hand conductor on pcb. 300V charge voltage might still exist on filter capacitor. Set your multimeter to DC voltage and place red probe on D of Power FET. Use alligator clip for safety. A charge voltage will be read. Apply power again on power supply. You must measure more than 250V DC in Drain of FET. Check fuse if no voltage in Drain.

Let me know the result.
 

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davenn

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Sep 5, 2009
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I've tried to replace the transistor with another NPN, but it still does not work.
I'm sure that no optoisolator is present.
During checking with the ohmmeter I do not understand the functions of the points marked in red, and then identified with question marks, no continuity between this point...
Where I can start to check because it still does not work?

Thanks,
Sebastian

the box you have shown with the ? is most likely the opto-coupler (isolator)
this provides feedback from the secondary side to the primary side to produce a regulated DC output voltage

Dave
 

SebaX75

Jan 10, 2012
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Very good work davenn, I've unsoldered some components to watch under the transformer, and it's the optoisolator.
Thanks for troubleshoting guide Rleo6965, in the next days I'll try the steps you wrote... I've already worked with switching adapter, so I know the danger of the capacitor on the primary circuit.
In the next days I'll post the results.

Thanks,
Sebastian
 

davenn

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Very good work davenn, I've unsoldered some components to watch under the transformer, and it's the optoisolator.
Thanks for troubleshoting guide Rleo6965, in the next days I'll try the steps you wrote... I've already worked with switching adapter, so I know the danger of the capacitor on the primary circuit.
In the next days I'll post the results.

Thanks,
Sebastian

thats cool :)

keep up the good work and post the results :)

D
 

SebaX75

Jan 10, 2012
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Hi,
I've checked the power supply...
No short circuit or components shorted on high voltage stage, so fuse is good.
Resistor on source is good, less than 1 ohm.
I've replaced the transistor with another NPN (not desoldered).
On Testpoint1 (ZD1) no frequency is present, only DC at 1,6V.
On ZD4 are present about 6,8V.
On drain there are 298V.
Other suggestions?

Thanks,
Sebastian
 
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Rleo6965

Jan 22, 2012
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It seems 1.6V was not enough to switch on power fet to conduct or start the inverter. I think ZD1 was the one your inquiring in the first place.

Turn off power and test the following component. No need to remove component from board. Using Ohmmeter, test resistance across ZD1. Then set multimeter to Diode test. Test again voltage drop reading across ZD1. Forward test should read 0.5V and reverse reading should be higher voltage drop.

I suspect that board problem was on oscillator feedback circuit.
 

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SebaX75

Jan 10, 2012
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Hi,
DZ1 was the diode that I replaced... I've ordered it from Farnell and it should be an SMD zener diode of 2.4 V (http://it.farnell.com/nxp/bzv55-c2v4/diodo-zener-2-4-v-500mw/dp/1097193?Ntt=1097193).
The Ohmmeter probably don't put the diode in conduction state... I can read, in forward and reverse polarization, a value that start from 17kohm and go up until about 18kohm (resistor value).
With Diode test, I read in forward 0,711V and in reverse 1,627V.
It could be that Farnell gave me the wrong diodes? But no one zener dioda has a working voltage of 1,6V.

Thanks,
Sebastian
 
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