Friday, March 29, 2013

Power Suppy Refactoring

In this previous post I had the chance to present and tear down my old custom-made power supply. It worked flawlessly for about 10 years. I decided to buy a new linear lab power supply because I needed something better with more features such as constant current operation and digital display of both voltage and current.

After a while I realized it was a pitty to leave this old PSU at a corner considering that the transformer was of very good quality. A second unit would certainly come in handy for certain projects requiring dual power sources. Even though I used this power supply for quite a long time in many projects, its design had some flaws. For instance:

  • Electronics undersized for the transformer output power: the transformer could deliver above 8 Amps with 10% voltage drop in the secondary winding, however the circuitry could not regulate much more than 3 amps without entering overcurrent protection (probably the LM338 own overtemperature protection kicking in);
  • Only one of the two secondary windings being used. Both the output power and power used by support electronics would be sourced from the same transformer tap;
  • Poorly layed out board with power rails not being given proper size for the amps rating of the transformer/regulator;
  • Small heatsink for the maximim potential dissipation of the regulator: in a low voltage, high current setting, like for example 1.6 Volts at 5 Amps, taking into account that the input voltage cannot be varied (always the same transformer tap connected to the regulator), given the 24 Volts of input, it translates to a drop of 22.4 Volts. (24-1.6). Multiplying by the 5 Amps of the load, we get about 112 Watts of dissipation on the regulator. Well this is more power than a heatsink such as the one in the picture below is capable of dissipating, so taking into account the LM338 overtemperature protection we can never reach the 5 Amps at 1.6 Volts.
  • Unnecessary filter module on the AC input (extracted from a switching power supply, not needed for linear PSUs);
  • Mains voltage wires in poor contition. Power switch (carrying mains voltage) close to the low voltage DC part;
  • Low quality measurmenent instruments;
  • Cumbersome current shunt for measuring DC Amps;
  • Chassis not looking very solid.

Having identified all these problems I decided that I had to rebuild practically everything from scratch to obtain a proper PSU. Opened an account at Mouser, and started ordering all the parts I would need to complete this rebuild.

In this project the goal is to make the power supply adjustable both in current and voltage. The aim is to deliver up to 8 Amps from 0 to 20 Volts with the new circuitry. 

It will be based on the popular LM723 device, and for the power transistor I will use 4 x 2N3055. The two secondary windings of the transformer will be used, where one will be dedicated to power the LM723, the fan, the voltage and ammeter displays, while another of the windings will be used for the actual output voltage. This prevents  fluctuations in the load current from interfering with the operation of the support components.

I am currently waiting for all the parts to arrive. In the meantime I was able to advance with most of the mechanical aspects. The heatsink I salvaged from a broken LED light. I cut about 12.5 cm from this structure, yelding what I believe it will do a good job moving away the heat from the 2N3055 transistors, the driver transistor and the main bridge rectifier:

Attached to the heatsink is the fan which already existed in the previous version of the power supply.

In intend to have all of the main functions in the front panel, which include the power switch, the digital voltmeter and ammeter, knobs for coarse and fine adjustment of both voltage and current, and the binding posts for the DC output and ground:

The panel itself will be a galvanized metal sheet I salvaged from an old VCR. The basic shape is already cut, it only lacks the holes for the instruments, buttons and knobs:

Mostl likely later posts will be dedicated to advances of this project. After the remaining components arrive and as time allows, I expect to give this constructional project some progress, and perhaps finish it within a weekend or two.


Rui Martins said...

Looks like it will be a great rebuild :)

If you need to cut the front panel to spec drawing, just say so, and I can cut that for you.

Creation Factory said...

Hi Rui, that could be interesting :) I still don't know the exact size (only the outer frame) of the LED displays, but once these arrive I will have more accurate info. How do you want the spec drawing? Is there any tool you recommend for drawing it? Regarding the material, will a similar sheet of galvanized foil be handled by your machine (I haven't measured but has probably around 0.2 mm of thickness)?