Dual Polarity Power Supply

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Dual Polarity Power Supply

WARNING:
This project involves working with line voltages (typically 120 or 240VAC) and therefore should only be built by those comfortable working with potentially lethal voltages/currents. The circuitry confined to the circuit board is not particularly dangerous, however connecting it through the requisite step-down transformer presents a risk of severe electrical shock. I can assume no responsibility for anyone injured in attempting to build this power supply. A thorough understanding of electrical safety practices and a healthy dose of respect will help keep you building electronics for a long time!

This document describes how to construct a dual polarity linear power supply, which can be configured for any positive or negative voltage between 1.2-35V. A power supply is the fundamental building block of all but the simplest of electronic devices. It converts the alternating current (AC) from our wall outlets into direct current (DC) at some specified voltage. Because so many electronic devices need DC to function properly, a versatile power supply is the perfect addition to a hobbyist's collection of test equipment. The supply is based on National Semiconductor's LM317 and LM337 variable voltage regulators, which have become very popular because they provide impressive regulation characteristics and relatively high power output from a small TO-220 package. These regulators are specified to deliver up to 1.5A (output voltage dependent) if provided adequate heatsinking, which is sufficient for testing or powering a wide variety of everyday circuits.

There are two versions of the power supply, yet both are fundamentally the same. The first is a compact fixed-voltage supply measuring 2.9" x 2.2", which is well suited for applications requiring small size and modest current capacity such as active crossover networks and other op-amp based projects. The second is a larger (4.6" x 2.25") variable supply sporting more robust heatsinking, and the regulators are positioned such that they may be mounted to a large chassis heatsink for even greater heat dissipation. And since this version of the supply is variable, it is ideal for experimenting. The regulators are fully short-circuit and thermally protected, so there's no risk of destroying the supply if you hook something up incorrectly or accidentally short the outputs together.

Circuit Description

The circuit is fairly standard of a linear supply, and the function of each part can be described as follows:

TX1 is a center-tapped transformer which converts the line voltage from 120 or 240VAC to a lower value which can be managed by the filter circuitry. See note 1 for more details.
Diodes D1-D4 rectify the AC from the transformer output into a pulsing DC waveform.
Capacitors C1a and C2a are large electrolytics which perform bulk filtering duties of the pulsing DC waveform into a raw DC voltage. C1b and C2b are small film capacitors bypassing the electrolytics to improve transient response and filter high frequency line noise.
U1 and U2 are the complementary adjustable voltage regulators LM317 (+) & LM337 (-). The output voltage may be programmed easily with two external resistors.
Resistors R1-R4 set the regulated output voltage. The LM317 & 337 regulators develop 1.25V across R1 and R3, respectively. This reference voltage causes a constant current to flow through the voltage-setting resistors R2 and R4 which generates an output voltage approximately equal to the following relationship:

Capacitors C3 and C4 improve the regulators' ripple rejection from 65dB to 80dB (typical) by preventing ripple voltage from being amplified at the output of the regulator.
Capacitors C5a and C6a are electrolytics that stiffen the output voltage and reduce output impedance. Capacitors C5b and C6b are small film bypass caps (similar to those at the regulator input) to filter any high frequency noise present at the output. The size of the four bypass capacitors is not particularly critical--the most important quality being they have reasonably low impedance at high frequencies, which is true of virtually all modern polyester, polypropylene, polystyrene, or mylar film capacitors.
Finally, D5 and D6 protect the regulator from damage in the event the input is shorted. Lacking these, the output capacitance will attempt to "dump" its charge instantaneously through low impedance paths internal to the regulators. The diodes bypass the regulator and allow the charge to safely dissipate.

48VCT (24-0-24) transformer is about the practical upper limit of commonly available models. This limit is set by the maximum input voltage of the regulators, and also by the bulk filter capacitors which are rated at 35VDC. (Astute observers will note that these are floating regulators, so they can withstand considerably higher input voltages as long as the input-output differential does not exceed 40V). Select a transformer which will output a slightly higher peak AC voltage than the DC voltage you expect to obtain from the power supply. Recall that peak AC voltage is 1.414 times higher than the RMS value specified by the transformer.

How about an example? Let's say you want to get +15VDC from the power supply. You will probably want to use a 24VCT (12-0-12) transformer, which will supply a peak voltage of 12(1.414)=16.97V to the the rectifiers. The slight overhead (~2V) will account for the voltage drop across the diodes and regulator dropout. However, the greater the output load, the more voltage overhead required due to increased regulator dropout. The idea is to select a transformer whose peak output voltage is slightly higher than the desired regulated voltage (enough to compensate for diode drop and regulator dropout), all of which will minimize heat dissipation and generally improve the life of the power supply. Similarly, if you are building the variable supply and expect to utilize the full range of voltage variability, you will probably want to use a 48VCT transformer.

Table 1. Parts list for variable power suply

Ref. Des.	Part Type	Value	Capacity	Form	Lead Spacing
`TX1`	`transformer`	`48VCT max`			`n/a`
`SW1`	`switch`	`SPST`			`n/a`
`F1`	`fuse`	`2A slow`			`n/a`
`D1 - D6`	`1N4003 diode`		`1A 200PIV`	`A`	`400mils` `(10mm)`
`C1a & C2a`	`cap polarized` `electrolytic`	`2200uF`	`50V`	`R`	`300mils` `(7.5mm)`
`C1b & C2b`	`cap film`	`0.1-1.0uF`	`50V min`	`R`	`590;490;390mils` `(15;12.5;10mm)`
`U1`	`LM317 volt reg`	^+1.2-+37V	`1.5A`		`TO-220 package`
`U2`	`LM337 volt reg`	^-37--1.2V	`1.5A`		`TO-220 package`
`C3-C4`	`cap polarized` `electrolytic`	`10uF`	`50V`	`R`	`80mils` `(2mm)`
`R1 & R3`	`resistor` `metal film`	`120ohm`	`1/2W`	`A`	`500mils` `(12.7mm)`
`R2 & R4`	`potentiometer` `linear trim`	`4.7k`	`1/2W`		`400 x 500mils` `(10 x 12.7mm)`
`C5a & C6a`	`cap polarized` `electrolytic`	`470uF`	`50V`	`R`	`200mils` `(5mm)`
`C5b & C6b`	`cap film`	`0.1-1.0uF`	`50V min`	`R`	`740;620;500mils` `(18.8;15.7;12.7mm)`
`HS1 - HS2`	`heat sink`				`1000mil` `(25.4mm)`

Notes on parts selection include those above, plus:

Layout of PCB enables regulators to be commonly mounted to a heavy chassis heatsink to improve power dissipation. If using this configuration, you must isolate the devices from the heatsink with mica insulators & nylon screws because the metal tab of each regulator is internally connected to its output pin.
Suitable potentiometer:
Suitable extruded aluminum heat sinks.

Ferric Chloride

Green-black solution in water with a mild acid-like odor. May cause eye and skin burns. May be harmful if swallowed. Ingestion may cause rapid heartbeat, low blood pressure, shock, and possible coma. May cause kidney and liver damage. Inhalation of vapor may cause severe respiratory tract irritation. Substance has caused adverse reproductive effects in animals.

Target Organs: Kidneys, liver, cardiovascular system.

Adverse Health Effects

If in Eyes: May cause eye burns. Contact produces irritation, tearing, and burning pain.

If on Skin: May be absorbed through the skin. May cause severe irritation and possible burns.

If Swallowed: May cause irritation of the digestive tract. May cause liver and kidney damage. Causes severe pain, nausea, vomiting, diarrhea, and shock. May cause severe irritation of the mouth and throat. May cause low blood pressure, rapid heartbeat, skin discoloration, and possible coma.

If Inhaled: Vapor may cause severe respiratory tract irritation.

Prolonged or repeated exposure may cause adverse reproductive effects.
Repeated exposure may increase an increased body load of iron.

FIRST AID MEASURES:

Eyes: Immediately flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower lids. Get medical aid immediately. Do NOT allow victim to rub or keep eyes closed.

Skin: Immediately flush skin with plenty of soap and water for at least 15 minutes while removing contaminated clothing and shoes. Get medical aid if irritation develops or persists.

Ingestion: If victim is conscious and alert, give 2-4 cupfuls of milk or water. Get medical aid immediately.

Inhalation: Remove from exposure to fresh air immediately. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical aid.

Notes to Physician:

Treat symptomatically and supportively.
The use of an iron chelator should be determined only by qualified medical personnel.

Handling:

Wash thoroughly after handling.
Remove contaminated clothing and wash before reuse.
Use with adequate ventilation.
Do not get in eyes, on skin, or on clothing.
Do not ingest or inhale.

Storage:

Do not store in direct sunlight.
Store in a cool, dry, well-ventilated area away from incompatible substances.

OSHA Vacated PELs:

Ferric Chloride: No OSHA Vacated PELs are listed for this chemical.

Physical Properties:

Physical State: Liquid
Appearance: Green-black solution.
Odor: Mild acid-like.
pH: 2.0 (0.1M sol.)
Vapor Pressure: Not available.
Vapor Density: Not available.
Evaporation Rate: Not available.
Viscosity: Not available.
Boiling Point: 212 degrees F
Freezing/Melting Point: Not available.
Decomposition Temperature: Not available.
Solubility: Soluble in water.
Specific Gravity/Density: 1.4 (water=1)
Molecular Formula: FeCl3
Molecular Weight: 162.206