Was done enjoying "Friday Afternoon"      

  at  WIN Enterprises Inc.     
17.4.1 Power Supply, quickly and simple.

  On the one hand, I needed a simple power supply and quickly. On the other hand, I already had an old power supply, which I recently burned second time. Combining these two things, and a simple DC-DC converter, I solved this problem.
  The main board is of interest only as the history of Japanese electronics from the 60's. But two germanium transistors - go directly to another project.

  Replacement - existing converters from different sources. Several of them are in the photo. The top one is almost impossible to kill, the rest have a usual quality.

  All I need to know - voltage on the transformer. Two windings give 9 and 15 volts, which in sum, after rectifier, gives about 33 volts on the output capacitor. Most cheap converters withstand this input voltage. My choice is "Buck DC-DC, based on Chinese version of LM2596",  just because I have it. In accordance with datasheet it is good up to Vin=+40VDC.

   Modification ends by replacing the trimmer (on photo it is already removed) with a variable resistor (10k), which is attached to the front panel.


    No one is going to make it perfect, so base
board along with capacitors and rectifier is taken and cut from broken ATX power supply. A voltage regulator LM7812 and MOSFET transistor (fan control), are taken from there as well.
   Just want to say that these converters don't work without additional cooling (heat sink or fan), if you want to get output more than 0.5 Amps.
    Standoffs let put it all together like a sandwich pack.

Two things are needed additionally. Hi power diode, in the reverse direction, parallel to the outgoing terminals and two capacitors (~100uF & 0,1u), in the same place. To run front 'milliamp meter' (1mA, 100Ohms), with two scales (30V & 1,5A), two resistors (30k & 0,066Ohms) are needed. The last resistor is easy to make from one wire (~70 cm.) from a network cable.
    If you expect to get a good power supply, this way, then this will not work. And it's easy to see if you connect the oscilloscope to the output of the power supply. The 1,5Amps load will make this clear.

   Ripple of output voltage more than you can imagine. There are many reasons for this, but there are main: The conversion frequency is too low (50 kHz instead of 150 kHz according to datasheet), lack of an output filter and additional capacitor over variable resistor.
   At the moment this can not be called a 'Power Supply'. But as 'Battery Charger' it will work well.

   A simple modification that make it better.
... .. .
As usual, the page is not done yet.

17.04.2017  SKootS


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