Where applicable, click on the picture for enlargements. Above you see the photo of my finished model. Looks good, performance is excellent, and I am very happy with it. Notice however that the panel meters are shown non-modified, meaning that I added and modified the stripes for the two settings after the picture was taken.

I purchased a brandnew transformer, model 165S25 manufactured by Hammond. It is of the regular kind. You can get a 'low-profile', horizontal type which mounts a bit lower. Whatever model you have or buy make sure it fits your case including the large capacitor.
The 165S25 transformer has 5 wires, the primary side has two black wires which are connected to your 115 vac. The secondary has 3 wires, in my case two green and one green/yellow. The green/yellow is not used. Isolated it with some heat shrink and tie it up, see Fig. 10. I never cut the wire off. The two green wires go to your Bridge Rectifier. It is probably marked 'AC' or '~'. Make sure the transformer makes good ground with the chassis, which in my case meant removing the paint. I then use a file to take the varnish off one bottom corner of the transformer. When you finish mounting the transformer into the case, take a multimeter or continuity tester and make sure the chassis of the transformer makes good connection with the chassis of the power supply.
The fuse is a 3.15A slow-blow type to prevent it blows when you switch on the power supply with a bit of load. This transformer is a real heavy one and weighs several pounds. If you're planning to use it with heavy loads, I would suggest to get one which can provide 12-15Amps at a desired voltage, but, the 10-amp transformer listed in the parts list will work. The difference is that the 10 amp transformer gets pretty hot if you use it at the maximum current. If you would have a 12-15 amp type it will only get a little warm. In my case, I do use the 10amp setting often but not for extended periods. A couple hours at best, and the three cpu-fans are going continuously.
It is probably best to mount the Bridge Rectifier, fuse holder, and power cord first before bolting down the transformer. As you can see in Fig. 11, I used a mounting bracket for the large capacitor, this bracket is also mounted on the back panel so don't forget to drill the holes first. I used a 115vac receptacle, fuse, and on/off switch combo which I had laying around and the whole thing is mounted on the back panel.
Crimp spades onto the two secondary green wires and mount the transformer. Connect those wires to the Bridge Rectifier as mentioned earlier. Install the large capacitor and wire up to the '+' and '-' of the Bridge Rectifier. From this point on all other '+' and '-' connections are taken from the large capacitor terminals. Use thick wire.
As mentioned earllier, instead of a Bridge Rectifier you could use four seperate 'stud-mount' diodes and make your own bridge. The two anti-rattle capacitors, C1 and C2, should be mounted directly onto the transformer or the bridge rectifier for best performance. Power diode D3 is a very vital component in this power supply and so was choosen a bit over-rated to make sure it will perform satisfactory under all circumstances and temperature changes. You probably already know that a diode is temperature sensitive which is most noticable in the 0.7 volt range. Since the same D3 also has a job of current limiting it is best to make sure this diode does not get too hot. So, we really want a solid diode of 20 amps minimum.
In case of a short circuit, there is at least 2.5 amps of current going through each power transistor, and that is a lot at about 60 to 70 watts of dissipating energy. That is why the 2N3772 power transistors come in which can dissipate 350 watts or so. And so, as a matter of speaking, at 30 volts we could well assured short out the output jacks and fry an egg on the output power transistors.

Now lets have a look at L1 & L2. L1, C9, L2, C10 are soldered as close as possible to the output jacks. We even have to cut the solder leads as short as possible. C10 is soldered directly on to the '+' and '-' output jacks. C9 is soldered parallel over the L1/C10/L2 network. The two thick wires coming from the printed circuit board are soldered onto each leg of C9. Oh yeah, I mention again that the PCB has to be mounted isolated from the case. The common ground connection is connected to the '-' jack via L2. This is a little bit of tinkering but can be done. We are working here with 10 amps so worth all the efforts.

I call the ferrite beads "pig snouts" because that's what it looks like to me, but hey, call it whatever you want. You need to make two of them. On the circuit diagram they are indicated as L1 and L2. One or two turns of thick magnet wire will do the trick.

By now you must be anxious to try everything out. Well, be patient, we're almost done. We still have to adjust the eight trim potentiometers on the pcb. And you really need to sit down for it and carefully take your time. Look for a time and place when you can do this quietly and undisturbed. The last thing you want is to open this heavy power supply up again and re-adjust it because of initial sloppy adjustments. So, take your time, go slow, and verify each adjustment until you're satisfied.
The adjustments are done in a special sequence and if you keep yourself to this procedure then I doubt you would encounter any problems. Okay then, here goes it.

Adjustment procedures:
FIRST check for correct wiring from and to pcb, jacks, meters, and coolrib. Very important.
Before starting the adjustments, familiarize yourself with the trim pots on the printed circuit board and the potentiometers on the front panel. The two on the front panel are R3 and R12, the others are on the pcb. I mention this to avoid confusion while doing the adjustments. If you wish, mark all the pots ahead of time by writing the 'R' numbers on a piece of scotch tape or something. It will help a lot!
Important: make sure to 'zero' the panel meters with the little plastic screw attached to the needle movement unit.

Open up the connection of the thick wire between the pcb and the positive of C3 and insert a small fuse of a couple hundred milliAmps. If you don't have a small fuse handy then you can also use a 1/2 watt 10-ohm resistor or something similar. Do NOT plug in the power supply yet! Turn all trim pots to the left (counter-clock-wise) all the way. Set the two potentiometers on the front panel about halfway. Set the two switches on the front panel (1/10A, 6/30V) to the low settings, meaning the current switch on 1 amp, and the volt switch on 6 volt.
Take your digital multimeter and secure its minus (black) lead on the minus output jack. Plug in the power supply and switch the power. If all is well and there is no smoke, the main fuse and small temporary fuse on C3 remain okay, we can continue.
Put the plus (red) multimeter probe on topside of potentiometer R3. This is the position closest to the minus of C4 on the pcb. You have to measure there a voltage of precisely 6 volts.

If needed, this voltage can be adjusted by turning the R2 pot. Turning the potentiometer (R3) on the frontpanel will move the panelmeter but NOT the multimeter. If you move the red probe to the '+' output jack and you should find a variable voltage (via R3) between 0.7 and 6 Volt.

Take your time! Don't worry about the current meter at this time, it probably will not move at all because there is no current. All you do at this time is adjusting the low-voltage scale.
Leave the multimeter probes connected to the '+' and '-' output jacks and switch the Volt-switch (S2a) on the frontpanel to the 30 Volt position. You will see that the voltage makes a big jump upwards. We adjust R3 all the way to the right (clockwise) and adjust trimmer potentiometer R23 until your multimeter shows 30 volts. We now adjust R26 until the panel meter shows the same, 30 volts. Switch back to the 6-volt position and adjust the panel meter to 6-volt full-scale with R24. If you're done with this and you are satisfied, then have beer on me for a job well done. You are half way finished!

Switch off the power, unplug the powercord. Drain the charge from C3 first and then remove the temporary fuse between the positive of C3 and the pcb and re-connect the wire to start adjustments on the current settings.
Switch the panel meters to the 6 volt and 1 amp positions and turn current-limiter R12 on the front panel all the way to the left (counter-clock-wise). Set the volt meter on the frontpanel to 4 volt with R3. Select a setting on your multimeter of 100 or 300 milli-amps dc. Take the red probe and insert a resistor of 39 ohms between the red probe and the '+' of the output jack. You will notice that current flows through that resistor. The panel meter also shows a bit of current and at the same time the needle of the panel-voltmeter falls back a little to about 2 volts or even lower. If that is the case you know your current limiter is working properly and you can continue with the adjustment procedures.
Remove the 39 ohm resistor. Switch your multimeter to the highest current setting (preferably 10A) and connect it directly to the '+' and '-' output jacks. The meter should show no more current than with the 39-ohm resistor, even less this time. Carefully open up R12 (front panel) clockwise until you see increased current on both multimeter and panel meter. A good multimeter will go to at least 10 amps, but I guess the job can be done with 2 or 3 amps also. On the other hand it actually would be better to borrow a good multimeter from a friend or rental shop if you don't have one yourself.
Okay, on with it. Open R12, slowly, as far as possible and note the current reading. REMEMBER you are still in the 1A/6V setting! If there are no problems the current reading probably shows 1/2 amp or something in that area. Let it sit in that condition for awhile and observe the temperature of the large cool rib. It should warm up a little bit. If all is okay and still no smoke you can safely assume that the circuitry works correctly.

So far so good. The following adjustments have to be done in the correct sequence. Switch the power supply OFF. Set the panel switch to the 10-Amp setting and also the multimeter to as high a current setting as possible. Turn R12 all the way to the left, the multimeter still connected to the '+' and '-' output jacks on the front panel. Turn the power supply ON. You should notice almost no current at all. The setting of the 'Volt' potentiometer does not matter much at this time so don't worry about it. Carefully adjust R12 to a high as possible value and stop when it shows about 5 amps on the multimeter. Adjust the panel meter with R20 until it shows the same value as the multimeter. When you're done the panel meter should show half way the 10-A scale. Just make sure that your multimeter can handle 10 amps. If not, then don't exceed that value with R12 or you blow up your multimeter.

Turn R12 again all the way to the left and flick the switch on the front panel to the 1-A setting. Adjust R12 all the way to the right and with R18 adjust the value of the multimeter with the value of the panel meter until they're equal.
In the mean time the coolrib is getting quite hot during all the adjustments in the 10-A settings. But that is done now. You have now adjusted six of the eight trimmer pots and so still two to go.
Remove the multimeter. Turn both potentiometers on the frontpanel (R3/R12) all the way to the left (0 position). Return the switches to the 1-A and 6-V settings. Short out the output jacks on the frontpanel with a piece of wire. Turn R12 all the way to the right and adjust R14 until the 'current' panel meter indicates precisely 1-amp (full scale).
That done, turn R12 back all the way to the left and place the current switch in the 10-A setting. Adjust the full scale of the panel meter with R16 until it shows exactly 10 amps. At this setting the cool rib heats up quickly so keep an eye on the temperature. You are done. Finished. I'll bet you are smiling now. After all, you now have an analog piece of equipment equal or better then the commercial unit and for a fraction of the cost.

Inside the enclosure I keep a little plastic box which contains the schematic diagram and some spare parts just in case I need it in the future. The parts I use are the 723 IC, the zener diodes, darlington, and one 2N3772 power transistor. Why? Well, just because everything is so-called short-circuit-protected it does not mean it can't happen, for example by a power surge or lightning. Murphy is always on the look-out. But on a safe note, this power supply is almost indestructible and you really have to abuse it to blow a fuse. Also, who knows, 20 years from now the uA723, 2N6388, or the 2N3772, is no longer available. See bottom of this article for a suggested spare-parts list.

Now, what can you do with this power supply? Anything you want. Charge regular NiCad or Lead-Acid batteries, run all kinds of motors, styro-foam cutter, etc. It is limited only by your imagination.

Parts List:
Resistors
1/4 Watt, Carbon, 5% (or better), unless otherwise indicated

R1 = 470 ohm, 1/2 watt, yellow-purple-brown
R2 = 2 K, trimmer pot
R3 = 5 K, potentiometer, linear
R4 = 560 ohm
R5 = 47 ohm, yellow-purple-black
R6 = 0.1 ohm, ww, 5%, 1-watt (see text)
R7 = 0.1 ohm, ww, 5%, 1-watt (see text)
R8 = 0.1 ohm, ww, 5%, 1-watt (see text)
R9 = 0.1 ohm, ww, 5%, 1-watt (see text)
R10 = 0.33 ohm, ww, 5%, 10-watt
R11 = 0.33 ohm, ww, 5%, 10-watt
R12 = 470 ohm, potentiometer, linear
R13 = 820 ohm, gray-red-brown
R14 = 500 ohm, trimmer pot
R15 = 150 ohm, brown-green-brown
R16 = 100 ohm, trim pot
R17 = See Text
R18 = See Text
R19 = See Text
R20 = See Text
R21 = 5K, metalfilm, 1%
R22 = 820 ohm, gray-red-brown
R23 = 500 ohm, trimmer pot
R24 = See Text (non-variable: 25K trim pot)
R25 = See Text (omit for non-variable voltage)
R26 = See Text
R27 = See Text
R28 = 3.3K, 1/2 watt, 5%, orange-orange-red
R29 = 3.3K, 1/2 watt, 5%, orange-orange-red
R30 = 3.3K, 1/2 watt, 5%, orange-orange-red
R31 = 3.3K, 1/2 watt, 5%, orange-orange-red

Capacitors
C1 = 3.3 nF, ceramic
C2 = 3.3 nF, ceramic
C3 = 15000 µF or higher, 45V+, electrolytic
C4 = 1000 µF, 63V, electrolytic
 C5 = 4.7 µF, 25V, tantalum, or high-quality electrolytic
C6 = 4.7 µF, 25V, tantalum, or high-quality electrolytic
C7 = 470pF, ceramic
C8 = 10 µF, 63V, electrolytic
C9 = 1 µF, foil type. see text
C10 = 22 nF, ceramic

Semiconductors
D1 = 1N4004, 1N4005, 1N4007, BY127, etc.
D2 = 1N4148, BAX13, BAX16, etc.
D3 = 1N4389 power diode, 20A+ (see text)
D5,D6,D7,D8 = Leds, any type (see text)
ZD1 = 1N4754A, Zenerdiode, 1 watt, 39V
ZD2 = 1N4636A, Zener, 250mW, 6.2 or 6.8V
Q1 = TIP140, MJ2501, BD267A, 2N6388, etc.
Q2,Q3,Q4,Q5 = Transistor, 2N3772/NTE181
L1,L2 = Ferrite Bead (click here for more info)
IC1 = MC723, µA723, LM723
BR1 = Bridge Rectifier (see text)

Miscellaneous
M1 - Panelmeter, see text
M2 - Panelmeter, see text
T1 - Transformer, Hammond 165S25 (25V/10A), or similar
F1 = Fuse, 3.15A, slow-blow
S1 = Toggle switch, ON-OFF, DPDT, sub-mini
S2a-b = Toggle switch, ON-OFF-ON, DPDT
S3a-b = Toggle switch, ON-OFF-ON, DPDT
S4 = For use with one panelmeter: ON-OFF-ON, DPDT
Fuseholder, very large coolrib for the 4 power transistors, Q1, and D3 (isolated), wire, solder, 2 knobs, instrument case, power cord, etc. The meter scales are re-scaled with rub-on lettering.


Possible Component Substitutes:
 D1 = 1N4004, 1N4005, 1N4007, BY127, NTE116, NTE125
 D2 = 1N4148, BAX13, BAX16, NTE519
 D3 = Possible types: MUR2510, MUR3010CT, NTE6246, NTE6247, etc.
ZD1 = 1N4754A, NTE5086A, ECG5086A
ZD2 = 1N4736A, NTE5071A
 Q1 = BD267A, TIP140, MJ2501, NTE263, NTE270
 Q2 = NTE181
IC1 = µA723, LM723, NE723, NTE923D

Last Minute changes and other info:
  • At the right you see the AC stuff I used. Came of another defective unit. It contains the on/off switch, fuse, and receptacle. Works just beautiful. Click on the picture for an enlarged view. The cool rib is a little smaller in width then the enclosure so was just a nice opportunity to get a more professional look. Click on the picture for an enlarged view.

  • Important changes: The total resistance (0.1 ohm) for R6,R7,R8,R9 includes the wire-length to the transistors. Adjust the resistance value accordingly for each. C7 is changed in value to 470pF!

    - If you decide to use the older 'metal-can' version of the 723, the pin out is shown at the left.

  • For the Led's I personally choose green for the low scales (0-6V/0-1A) and red for the high scales (1-30V/1-10A). But hey, use whatever preference you have. High-brightnes types is what I'm using, but again, use whatever you like.

  • The bridge rectifier is one with a metal part attached for mounting on a coolrib. You can use 4 seperate stud-mount diodes of 75V/12-15A minimum. They MUST be mounted isolated on a coolrib.

  • Powerdiode D3: I used an older "stud-mount" type of 35Amps because I had it available and it has to go onto the coolrib assembly. But use whatever you have laying arround, just keep in mind it needs to be cooled and needs to be a minimum of 20A.

  • Don't forget to mount R10/R11 a little bit away from the PCB. I used 1/4" (5mm) ceramic stand-offs.

  • L1 and L2: Why are they there and what exactly do they do? They eliminate the switching noise that could otherwise possibly interfere with other equipment (like a frequency counter). If you can't obtain the regular ferrite bead type, you can use the toroid (from an old pc power supply) that filters the +5V. Works good.

  • Capacitor C3: Big sucker, but needed. Mine is a computer-grade 22,000µF at 50V. Came from a powersupply out of an old 'floor-model' tape-drive. You can buy them new but you pay the price, around $35.00 CAN in my area. Keep in mind, if you are thinking of combining two or more capacitors that the their working voltage must be the same and that you may need a larger enclosure.

  • Panel meters: I decided to stick with the analog panel meters. I like to see exactly what i'm doing and in this project they are just as accurate. The 'Volts' panelmeter will most likely be a 100-millivolt type with 30 or 60 scale stripes. The 'Amps' meter will probably be a 1-mA type with 50 or 100 stripes on the scale. The internal resistance (Ri) of the meters is not at all important since anything with a build-in resistor or resistance wire will be removed anyways.

  • Cooling Fans: After some experimenting I decided to increase the 2 cpu fans with one more making the total 3. I also decided to exchange the cpu fans, depicted in the photograph, for a different type which is a bit larger and has more fins. The whole cooling circuit with the fans now work like a charm. I will likely modify the fan circuit and use a more common op-amp such as the 741, and create a printed circuit board and parts layout for ease of use. Most types of thermistors will work so don't worry too much. You just may have to play with the series resistance a bit. No big deal.

  • Optional Led (yellow 3mm) and 1K8 resistor for the automatic fan control. Added this later after the front panel was already finished. Although you may be able to hear the fans when they kick in, I prefer a visual indicator as well. Secondly, I like bells and whistles...(grin).

    Suggested Spare Parts List, Optional:
    (Mount it preferable somewhere inside or outside the enclosure).
    1 - Copy of Schematic Diagram
    1 - LM723 (no cmos)
    1 - 2N3772 (NTE181)
    1 - 2N6388 (NTE263)
    1 - 1N4754A
    1 - 1N4636A
    1 - 1N4148
    1 - 1N4005 (1N4007)

    Put a copy of the schematic diagram in a sealable plastic bag and mount it somewhere underneath the power supply case.

    Copyright and Credits:
    Re-posting or taking graphics in any way or form from my website, without my written consent, is expressily prohibited by international copyright © laws and enforced by International Law Enforcement.
    Back to Circuits Page or Continue to Part 4 (construction photos)

    Copyright © Tony van Roon, VA3AVR
    Last Updated: January 5, 2006