Need help identifying a diode

[budm]

Re: Need help identifying a diode

To test zener diode you will need Variable power supply that can go from 0~50VDC (for this case) and a 1K resistor in series with the diode, you will hook up the resistor-diode combo in reverse bias mode (Positive of the power supply to resistor leg, the other end of the resistor leg to the CATHODE of the Diode, the ANODE of the Diode to the Negative terminal of the power supply), the you connect the DMM to measure the Voltage across the diode and you turn up the output Voltage of the power supply to see when the Voltage across the diode stops climbing.
Or if you have one of this type of power supply:
https://www.youtube.com/watch?v=5OcoF0nkgng

[budm]

Re: Need help identifying a diode

PDF of the virtual PCBA.

[fzabkar]

Re: Need help identifying a diode

D1 is connected between the power pins of the IC. Therefore it must be a zener diode. The IC's recommended Vcc is between 12V and 30V, although it has a 16V start-up threshold. Therefore I would expect that D1 would be in the 20V range.

I would now examine the markings of D1 and compare them against D3. AIUI, D3 is most probably a 30V zener.

BTW, T1 is a 2SK1088, which is an N-channel MOSFET, not "P-CH" as indicated in the virtual PCB. I suspect that the error is due to the confusion resulting from the (annoying) negative voltages.

[budm]

Re: Need help identifying a diode

Thank for catching that, it is N-CH with the negative supply connect to the Source pin, I have to keep reminding myself that the circuit is positive GND.

[fzabkar]

Re: Need help identifying a diode

Here is a circuit diagram that hopefully matches the virtual PCB:

                                        PCB-Gnd
                                         ===
                                         |
                                         |
  +-----------------------------------------+------+---------------------+------+----------+---+---+
  |                     |   |           |   |     |  |  |
  |                     |   |           |   |     |  |  |
  |                     |   |           |   |     |  |  - D4
  |                     |   |           |   |     |  |  ^
  |                     |   |           |   |     |  |  |
  |                     |   |           |   |     |  |  |
  |                  100R .-.  .-. 220R        |   |     |  |  |
  |                  R19 | |  | | R20       D2 | C14 |+   C15 |+ | .-.
  |                    | |  | |          -   ===    === --- | | R21
  |                    '-'  '-'          ^   ---    --- --- | | 11R
  |                     |   |           |   |     |  | '-'
  |                     |   |           |   |  L4   |  |  |
  |                   T2 >|  |   /        |   |  ___  |  |  |  J7
  |                  2SA988 |----+----|<|--------------)------+----UUU---+---+---+----o
  |                     /|    /         |   |       C16 | -28Vout
  |                     |      D3        |   |         |
  |                     |     30V ?       |   |         |
  |                    .-.    Zener       |   C| L3       |
  |                    | |R18            |   C| toroid     V
  |                    | |2K2            |   C|         z
  |                 C2   '-'              |   |         |
  |                  ||  |              |   |         |
  |                +---||----+----+            |   |         |
  |                |  ||  |  |            |   |         o
  |               .-.    .-.  |            |   |         J6
  |              R13| |  R16| |  |            +------+         -5V
  |               | |  1K5| |  |               |
  |               '-'    '-'  |               |
  |                |     |  |               |
  |                |     |  |               |
  |                |    ===  |               |
  |     M51977P       |    ICGnd |               |
  |                |8       |7               |
  |    .------------------------------------------------.          |
  .-.    |           F/B      DET     |     R12   |
  | |R7   |                        |2     ___   | D 
  | |    |                      Vout|----+----|___|--||-+
  '-'    |                        |  |     G ||<- T1
  |    |      E                  |  .-.     ||-+ 2SK1088
  .-.    |      m O                 |  | | R10     |
  | |620R  |      i n       T   I      |  | |       | S
  | |R8  1|      t /   T    -   C C C   |  '-'       |
  '-'  +--| Collector t O O -    O   G L L   |4  |       |
  |   | |      e f V O  C  F  C n M M  VF|----+       |
  +---+-+--| Vcc    r f P N  F  F  T d + -   |  |       |
  |  |+ 16'------------------------------------------------'  |       |
  z ===       3 |5 | 6| 9| 10| 11| 12|13|15|14|      |       |
  A ---        | | | .-. | .-. |+ | | |--+   C9 |       |
 D1 |  | C2       | | | | | --- | | === | | --- |    ---       |
  |  |        | | | | | --- | | --- | | --- |    ---       |
  |  |        | | | '-' |C4'-' |C7| | |C8|    |       |
  |  |        | | | |R4 |  |R5 | | | | |    |       |
  |  |        | | | |  |  |  | | | | |    |       |
  +---+----------------+--+--+--+---+---+---+--+--+--+--)--------+--------------+
                         |    |            |
                        ===    |            |
                        ICGnd   |          R9.-.
                             |         0.2R| |
                             |           | |
                             |         R6  '-'
                             |         ___  |
                       F1     +----------------|___|--+
                  J2    _       ___    ___     |
                 -48Vin o---o \_/o---+----|___|--+---UUU-----+----+
                           |   R3  | L2    |  |
                         C1 |      |    C5 |  | C6
                          ---   C3 ---     --- ---
                          ---     ===     === ===
                           |     +|     +|  +|
                           |      |      |  |
                           +-----------+-----------+----+
                                 |
                                ===
                                PCB_Gnd

[bigbrew25]

Re: Need help identifying a diode

Thanks guys!! I am putting in a request to get a bigger variable power supply to test this diode. If it is 30v, do you think this one will work?

http://www.mouser.com/ProductDetail/...Ym0iUvVaVA0%3d

[fzabkar]

Re: Need help identifying a diode

Pin 7 of the IC is an input to the internal error amp. When the IC is regulating properly, the voltage on this pin should equal the reference voltage, namely 2.5V. Therefore the current in R16 and R18 should be 1.7mA (= 2.5V / 1.5K). This means that the voltage drop across R19 should be 0.17V (= 100R x 1.7mA), and the voltage at the base of T2 should be about -0.8V (= 0.17 + 0.6). Therefore the current through R20 and the zener should be around 4mA (= 0.8V / 220R), and the dissipation in the zener should be around 120mW (= 4mA x 30V).

ISTM that a 500mW zener should be more than enough for this circuit. That said, I would examine all the related components, particularly T1. If T1 were to develop a D-S short circuit, then D3, T2, R19, R20, and R9 would be destroyed.

[bigbrew25]

Re: Need help identifying a diode

Thank you everyone!!!! The Mouser link that i posted was the correct diode. You guys are awesome!

[budm]

Re: Need help identifying a diode

So it is working now?
One bad thing about buck converter is that if the MOSFET failed shorted it will dump 48V to the output.

[stj]

Re: Need help identifying a diode

i posted s schem of a zener tester some time back.
it could generate over 200v @5ma

https://www.badcaps.net/forum/showpost.php?p=463545