Lets make a High Current power supply with LM317 Voltage Regulator which can supply upto 5 Amp. LM317 is one of the oldest, most used and wonderful linear voltage regulators. I personally love this IC (yes, it’s not a transistor) a lot, not because it server it purpose well, but because it’s cheap, easy and never disappoints you while making a regulated DC power supply. In this blog we will make a High Current power supply with LM317 Voltage Regulator and a couple of Power Transistors
I am an IT guy, as you know. Electronics is my hobby, and as a hobbyist I need custom made power supplies more often than others. It’s because, I can’t afford a lab bench power supply worth several hundred dollars. So I prefer to make my own power supply as and when required. And that’s where the problem started appearing. Yes, surprisingly enough, Electronics experts prefer to buy a DC Power supply module than making your own. Trust me, that’s what I gathered as experience while knocking the door of several Electronics forums and Bulletin Boards. By then I was already upset by the callousness of the Electronics Blogs for their several hundred pages of untested, not working and intentionally mis-made circuits. NONE of them works seamlessly – sorry to say!
And suddenly you ask your a question on how to make a simple high current power supply with LM317 on StackExchange, and they come up with bagful of suggestions to use a Switch Mode Regulator. Yaak! I hate it. It’s complex, you often need inductors, high power rated ones are rarely available, Ferrite Transformers – virtually impossible to find one in local market (unlike from Chinese sellers on Ebay or AliExpress). Remember, Switch Mode Power Supply is NEVER your first choice when working on a hobby project or college project. It you take half of your entire project’s time to make the power supply itself. You need a quick, easy and cheap solution. And LM317 serves the purpose more perfectly than anyone else. I will tell you why experts on the Electronics forum always recommends using SMPS. They are not geeks, they are agent of Industries. And SMPS is the only solution if you are making a commercial product in the Electronics Industry. They are no-doubt power efficient. But who cares about power efficiency for a College project. It’s overestimation, mate!
Ok, that’s enough of the introduction. Let’s come to point. In this blog post I am going to share a FULLY WORKING, TESTED High Current DC Power Supply Circuit. I have created the circuit on a Bread Board today and tested. It worked just perfectly. Since my 2N3055 Transistor was hanging on the wires without proper heatsink, I couldn’t test it with 5 amp current. However, given the ratings of 2N3055 is well above 5 Amp, this circuit should work just fine at 5 Amps. I have tested upto 2.5 Amps and everything was perfect; I didn’t notice severe voltage drop at 2.5 Amp load which is awesome.
This circuit is not original, it’s copied from the Datasheet of LM317 (Section 8.3.12 High-Current Adjustable Regulator Circuit). However, I have done little adjustments to the circuit to make sure that you do not get stuck with unavailability of components. For example, it uses a 500 Ohm resistor. Do not worry, 500 Ohm is not mandatory, you can simply use a 470 Ohm in its place. When 470 Ohm is really easy to find, 500 Ohm is very very hard to find. If you are a hobbyist and do not have much knowledge on each and every components of a circuit and their functions, these values will often bother you. Any author, not intended to confuse the reader should NEVER use such components. I wonder why the datasheet of LM317 is not as perfect as the IC itself 🙂 !
LM317 based Regulated High Current DC Power Supply Circuit Diagram
Let’s see the Circuit Diagram first. To see a larger version of the image click on the circuit diagram below.
Nothing hectic here, a small explanation of the working principle of the circuit. LM317 does its usual job here. With the help of voltage divider created by the resistor R5 and series resistors R4, R7 voltage at the Adj pin of LM317 is controlled. This helps LM317 regulate the output voltage and keep it constant at the desired level. R4 is a potentiometer (preset if on board) which helps you to adjust the output voltage. The 2N3055 transistor (Q1) along with TIP2955 (Q2) in the schematic allow higher currents to flow at the output which is beyond the capability of LM317 (1.5 Amps). In combination, LM317 controls the output voltage and the NPN-PNP Transistor pair allows most of the current to flow from Input to Output. Makes sense? Let me know in comments.
Following is the list of components. Almost none of the Electronic Blogger care to write component alternatives while publishing a schematic. That seemed the most fishy part. I am not doing that mistake 🙂
|Component ID||Part Number||Alternatives|
|IC U1||LM317 TO-220 Package||LM338 will work too (not verified)|
|Transistor Q1||2N3055 Metallic TO-3 Package||TIP3055, TIP41 (Large Heatsink), TIP73|
|Transistor Q1||TIP2955 TO-247 Package||MJE2955, TIP42, BD140|
|Resistor R1||4.7K Ohm 0.5 Watt||5K Ohm|
|Resistor R2||22 Ohm 1 Watt||18 Ohm|
|Resistor R3||470 Ohm 0.5 Watt||500 Ohm|
|Resistor R4||4.7K Ohm Potentiometer||5K Potentiometer, 4.7K Preset|
|Resistor R5||120 Ohm 0.5 Watt||150 Ohm, 220 Ohm will work too, regulated voltage range will vary|
|Resistor R7||2.2K Ohm 0.5 Watt||1.8K Ohm|
|Capacitor C1||4700uF, 50V Electrolytic||2 x 2200uF, 50V parallel will work too|
|Capacitor C2||10uF, 50V Electrolytic||22uF, 50V|
|Capacitor C3||47uF, 50V Electrolytic||100uF, 50V|
|Rectifier BR1||GBU808||GBPC606 or any 6 Amp Bridge Rectifier|
|Diode D1||1N4007||1N4001, 1N4002 or 1N5402|
|Transformer||12-0-12 Center Tapped 5 Amp Step Down Transformer. Leave center tap unused, use the 2 end terminals||0-18 or 0-24 Transformer of 5 Amp or smaller current limit (120 or less VA Trafo)|
|Heatsinks||Large TO-3 Heatsink + LM317 Heatsink|
You can buy power supply components online from Jujubuy.com or Ebay.in.
Assembling the Circuit
I must say, it’s not safe to test the circuit on a breadboard at higher current. Breadboard connectors and jumper wires are not adequate for 5 Ampere current. So, assemble the circuit on a Dot Vero or Transistor Vero Board. Optionally, if you are familiar with the process of PCB Fabrication you can use the following PCB Layout for printing the circuit on a Copper Clad board and assemble the circuit on PCB.
LM317 Based High Current Power Supply PCB Layout
- JP1: Solder a thick wire between the 2 holes of JP1.
- 2N3055: Solder 3 thick wires on the holes designated for 2N3055. Solder the Transistor to the end of the 3 wires off the PCB. 2N3055 must be mounted on a large TO-3 compatible Heat Sink and can’t be soldered on the PCB. So, it has to be connected to the PCB using wires.
- 4.7K POT: Solder 3 wires on the holes designated for 4.7K Pot. Solder the Potentiometer to the end of the 3 wires off the PCB. It is connected to the PCB using wires, which allows you to attach the potentiometer on the front panel of power supply cabinet.
- I/P and OP: Use 2 screw terminals or solder thick wire on holes designated for AC input and regulated DC output. The I/P is directly connected to the 2 end terminals of CT Transformer.
Testing the Regulated Power Supply
Ideally in order to test such a voltage regulated high current power supply you must attach a load to the output. However, keeping the output terminals open, doesn’t harm the circuit itself. However, if you try to measure output voltage with a Multimeter, you might get undesired results. Output of the circuit is across C3 Capacitor. Attach a 2.2K Ohm 1 Watt resistor across the capacitor and measure voltage or current of the output. The lesser the load resistance is, the higher the current reading would be. This circuit is not well protected from short circuit. Hence measuring current on close circuit without a suitable load is never recommended. It will take the circuit current to absolute maximum limit of the transformer (often above 5 Amp for a 5 Amp Trafo) and would damage several parts of the circuit.
I am very keen to see your responses and suggestions. Please let me know if you need help to make this High Current Power Supply. I will be glad to help.
Credits: Thanks for help www.electronicspoint.com
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