DIY 0-30V Regulated Power Supply

Introduction

So over the weekend I saw a commercial 0-30V 3A regulated power supply with digital output, current & voltage control. The only drawback was it costs RM499 which is well over my budged. So I decided why not build my own from whatever i could scrap and save some money. Firstly I didn't need current control and i needed something within 1A. Searching the interweb turned out many designs but they were all complex and most didn't come with eagle schematics, then I remembered that last year i purchased a schematic book from Jalan Pasar which had a schematic for a 0-30V AC/DC Regulated Power Supply able to supply up to 1A, the schematic looked easy enough plus the parts were relatively inexpensive.

Bill of Materials

Qty	Item	@	Cost (RM)	Note
1	PCB	2	2
1	Metal Case	19	19	Dimension 5 x 3.5 x 6
1	Transformer	25	25	15-0-15 24VA 1.6A
1	Bridge Rectifier	-		Supports up to 2A Salvaged
1	Heatsink	-		Salvaged
1	LM317T	3.5	3.5	0-35V @ 1.5A
1	330 Ohm	0.1	0.1	1/4 W
1	100 uF eCap	0.2	0.2	50V
1	1000 uF eCap	0.5	0.5	50V Salvaged
2	0.1 uF (104)	0.1	0.2	Ceramic NP
1	10K Pot	1	1
4	T-Block	1	4	2 pin
1	Pot Knob	1.5	1.5
1	1.5A Fuse + Holder	1.20	1.20
1	AC Switch	-		Salvaged
1	0-50V Meter	14	14
1	Power Cord	-	-	Salvaged
4	PCB Plastic Stand	0.3	1.2
2	ETB 1303	1.5	3	Red & Black
2	Wire Hooks	2.2	4.4

		Total	80.8

Designing The Schematics

D-I-Y Kit schematic from book purchased at Jalan Pasar.

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Notice that the schematic recommends using a 100 ohm resistor for R1 and a 5/10K Pot for VR. I decided to follow this and my supply hit the max even before the nob hit 1/4 of a turn, had to replace R1 with 330R to fix that problem.

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Schematic on Eagle

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Making Improvements & Fixes

- Adding Protection Diode

Quote from National Semiconductor data-sheet for LM317T (Page 8)

"When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 10μF capacitors have low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is enough energy to damage parts of the IC. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and the rate of decrease of VIN. In the LM117, this discharge path is through a large junction that is able to sustain 15A surge with no problem. This is not true of other types of positive regulators. For output capacitors of 25μF or less, there is no need to use diodes."

- Adding Bridge Rectifier

Adding a bridge rectifier makes it easier as there is less routing on the PCB. The bridge rectifier chosen could sustain up to 2A.

- Adding Plastic Stands

Plastic Stands would mean that my PCB would not be touching the housing. This is needed if your using a metal housing.

- Adding T-Blocks

Adding T-Blocks meant that it would be easier to replace/remove wires if there was an upgrade in the future.

- Replacing 100R with 330R

Using an LM317 Calculator from http://www.whatcircuits.com/lm317-calculator-v2/ i found that when using a 330R was more suited for a 10K Pot in order for the power supply to reach its max while having the knob turned to almost max (Using a 100R would mean turning the pot 1/4 to reach the max output of the power supply).