Adjustable LM317/LM337 1.5A Supply
Article by Ray Wilson
This is an advanced project and should only be built if you have previous experience safely constructing a line powered power supply. A transformer with a center tapped 24VAC to 26VAC secondary capable of supplying 1 to 2 amps is required for this supply. I do not show a transformer, switch or fuse in the schematic or list a transformer in the parts list. The builder is responsible for knowing the correct transformer to obtain that meets the electrical safety requirements in their locale and how to safely wire it to the PCB correctly. Consult your electronics savvy professor, teacher or friend for guidance if you want to learn more about making line powered supplies.

***** Line voltage is lethal. Always insure that no one can come into contact with it when used in your project. *****

PLEASE NOTE EXAMPLE! A center tapped 24 VAC transformer has 24 VAC on the two outer secondary terminals and 12 volts between either outer secondary terminal and the secondary's center tap.

When you rectify AC voltage with a diode into a storage cap you generally end up with 1.414 x VAC of the AC source. Thus when picking a transformer for your project keep that in mind. Also if you want to get 15 VDC from a voltage regulator (or -15V for that matter) you need to put in at least 18 VDC (or -18 VDC) to give the regulator the breathing room it needs to function best. So if your project is going to use +/-12V a 24 VAC CENTER TAPPED transformer will do fine. However, If you plan to use +/-15V you'll want to use at a minimum a 25.6 VAC CENTER TAPPED transformer and no higher than a 36 VAC CENTER TAPPED transformer.

This is a great bipolar power supply for your modular synthesizer project. This LM317/LM337 based supply boasts 1.5A capability and adjustable output so you can supply your projects with anything between +/-9 and +/-15 volts.

As you can see below, you will need to heat sink the regulators if you plan to draw the maximum power this supply can deliver. I would even suggest larger heat sinks than those shown. I made these from aluminum 'L' channel and if they start to get too warm as I add modules I will increase the size of them to dissipate more heat.

Also, single turn trimmers will work but multi-turn will allow you to set the voltage with more accuracy.

I list 10,000 uF caps in the parts list for the main supply smoothing caps and these will give you excellent supply smoothing but if you find caps that fit .4" spacing that are larger or smaller in capacity (3300 uF to 15,000 uF) they will still work for lower currents so don't get too hung up trying to find 10,000 uF electrolytics. I also list tantalum for some caps for best results but aluminum electros will again work as well. The more current you plan to draw from the supply the larger you should make the main smoothing caps.

Adjustable LM317/LM337 1.5A Supply Schematic

Not a whole lot to describe really. This board is based directly on the best practice circuit for these chips described in the National Semiconductor's data sheets for the LM317 and LM337. The 2K trimmers are used to adjust the output voltages independently. Adjust the output to the desired voltage while monitoring each with a good DVM and you are ready to go.


Adjustable LM317/LM337 1.5A Supply Part Designators


Adjustable LM317/LM337 1.5A Supply Part Values


Adjustable LM317/LM337 1.5A Supply PCB Bottom Coppper


Adjustable LM317/LM337 1.5A Supply PCB Top Coppper


Adjustable LM317/LM337 1.5A Supply PCB Silk Screen


Adjustable LM317/LM337 1.5A Supply Project Parts List

Qty. Description Value Designators
1  Bridge Rectifier  KBU401 (or equivalent)  BR1  
1  LM317T Adj. Pos. Voltage Regulator  LM317T  U1  
1  LM337T Adj. Neg. Voltage Regulator  LM337T  U2  
2  Resistor 1/4 Watt 1%  1K  R3, R7  
2  Resistor 1/4 Watt 1%  240 ohm  R1, R5  
2  Resistor 1/4 Watt 1%  4.7K  R2, R6  
2  Trim Pot (multi-turn preferrable)  2K  R4, R8  
4  1N4002  1N4002  CR2, CR1, CR3, CR4  
2  Capacitor Ceramic  .1uF at least 35V  C2, C8  
4   Electrolytic Capacitor (35V or 25V) 0.4" (10mm) lead spacing  

Caps of this style fit the board. Enter the part# in the search at Mouser.com

Mouser part# 598-SLPX153M025C7P3 (15,000uF 25V)
Mouser part# 598-SLPX103M035E3P3 (10,000uF 35V)
Mouser part# 598-SLP153M025C4P3 (15,000uF 25V)
Mouser part# 598-SLP103M035C7P3 (10,000uF 35V)

 

10000 to 15000uF
25V or 35V  
C4, C5, C6, C7  
2  Tantalum Electrolytic Capacitor  10uF 35V  C3, C10  
2  Tantalum Electrolytic Capacitor  1uF 35V  C1, C9