How to design a 1.2V 15A power supply from 12V/9V input? LM317 or alternatives?

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#1 21668604 18 Jun 2013 10:46

JP JP

Anonymous

Post #1
21668604 18 Jun 2013 10:46

I want to make a circuit which receives 12V or 9V input (from a computer charger e.g.) and give 1.2 V (like a Nihm battery) at 15A. I was told I could use LM317 to regulate voltage but i need more information. Any help? thanks.

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#2 21668605 18 Jun 2013 11:20

David Adams David Adams

Anonymous

Post #2
21668605 18 Jun 2013 11:20

My first question is where are you going to get a source to deliver that kind of current at that low of a voltage.

Regulating the voltage is not that easy either as something has to absorb the voltage loss at that current. The LM 317 can act as the driver for the voltage regulator but it cannot be the voltage regulator delivering 15 amps as they are only rated for 1 to 1.5 amps.

Suppose you had a 12 volt source that can give you 15 amps - then the regulator brings it down to 1.2 volts - so the regulator has to absorb 10.8 volts at 15 amps which gives you a power level of 162 watts that has to be dissipated as heat. By the way this is a classical design and not that of a switching power supply.

Maybe Colin can come up with the switching power supply version.
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#3 21668606 18 Jun 2013 11:37

JP JP

Anonymous

Post #3
21668606 18 Jun 2013 11:37

Assuming I have a charger with 12V @ 1A, is it possible to raise A to 15A?
#4 21668607 18 Jun 2013 12:26

Chuck Sydlo Chuck Sydlo

Anonymous

Post #4
21668607 18 Jun 2013 12:26

Analogy- Your trying to fill a garden hose from a squirt gun!
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#5 21668608 18 Jun 2013 12:29

Chuck Sydlo Chuck Sydlo

Anonymous

Post #5
21668608 18 Jun 2013 12:29

NO......12v@1amp=12 watts
12v@15 amps=180 watts

Where are the additional 168 watts coming from?
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#6 21668609 18 Jun 2013 12:30

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #6
21668609 18 Jun 2013 12:30

A common and efficient way to convert 12 volts to 1.2 volts at 15 amps is using a "buck" converter.

An example of this is the Linear Technology LTM4627 uModule. Given an efficiency of 85% the 12 volt supply will need to supply 1.5 amps.

http://www.linear.com/product/LTM4627
#7 21668610 18 Jun 2013 12:47

JP JP

Anonymous

Post #7
21668610 18 Jun 2013 12:47

So if I have an AC 230V current how can get 1.2V and 15A ?
#8 21668611 18 Jun 2013 13:57

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #8
21668611 18 Jun 2013 13:57

That's even harder ... first you need to reduce the voltage. This is commonly done with a transformer, which can be expensive. The good news is that transformers are pretty efficient. AC can be converted to DC with a bridge rectifier - though at 15A it would be really big and expensive, lots of capacitors to filter or average the output, and then a good switching regulator design that can output 15A - most switching regulator designs output 1 to 3A - you'd need lots of external parts. If it was easy everybody'd do it.
#9 21668612 18 Jun 2013 17:30

Chuck Sydlo Chuck Sydlo

Anonymous

Post #9
21668612 18 Jun 2013 17:30

Sry, I missed the decimal point! Still not possible just less impossible. 1.2 vault x 15 amps is 18 watts so still 6 watts short. and that's assuming 100% efficiency which doesn't happen.
#10 21668613 18 Jun 2013 17:34

Chuck Sydlo Chuck Sydlo

Anonymous

Post #10
21668613 18 Jun 2013 17:34

Anyone else curious to know what in the world would require 1.2 volts DC at 15 amps? I suspect there are simpler ways to do the job!
#11 21668614 18 Jun 2013 18:53

Rodney Green Rodney Green

Anonymous

Post #11
21668614 18 Jun 2013 18:53

I have a feeling that you have been given a bum steer here. An efficient switch mode power supply can easily give a much highrer output current ( at a lower voltage of course) than the input current. May I suggest you look at switch mode power supplies on the web as a first step. I have built a low voltae AC supply capable of tens of amps from a 12 volt DC source with an oscillator and IRF50 MOS FETS. If you need to use a linear regulator as the discussion sems to indicate, then a low voltage square wave generated from the 12 volts as mentioned above has almost no lo loss. This could act as a pre regulated input for your linear regulator.. Hope this helps.
#12 21668615 18 Jun 2013 19:00

stephen Van Buskirk stephen Van Buskirk

Anonymous

Post #12
21668615 18 Jun 2013 19:00

Of course it can increase the current at a lower voltage, but you can't make watts out of thin air. 1.2v @ 15 A is 18 W. No linear regulator that I know can handle anywhere near 15A and switching power supply that supplies 15A is no piece of cake either. I'm also not sure why you would need a linear regulator with a 15A switching regulator as a pre-stage.
#13 21668616 19 Jun 2013 05:26

Jimmy Babida Jimmy Babida

Anonymous

Post #13
21668616 19 Jun 2013 05:26

The LM317 is adjustable voltage regulator that can handled a maximum current of 1 ampere. In order to handled a 15 amperes current you need a current boost transistor. The 2N3055 transistor is commonly used as current boost. For 15 ampere you need at least 4 pcs 2N3055. Provided that the circuit receives a 12V or 9V at 20 amperes. The output of LM317 is directly coupled to the base of the 4 pcs 2N3055. The collectors is directly connected to the input which is 12V or 9V. The emitters are connected to 0.15 ohms 10 watts wirewound resistors before goes to output. Good luck
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Topic summary

✨ Designing a 1.2V 15A power supply from a 12V or 9V input presents significant challenges. The LM317 linear regulator alone cannot handle 15A, as it is rated for about 1 to 1.5A maximum. To achieve 15A output with an LM317, multiple power transistors such as 2N3055 can be used as current boosters, requiring at least four in parallel, along with appropriate resistors and a sufficiently powerful input source capable of delivering over 20A at 9-12V. However, linear regulation at this current and voltage drop results in substantial power dissipation (e.g., 162W at 12V to 1.2V, 15A), causing excessive heat and inefficiency. A more practical and efficient solution is a switching regulator, specifically a buck converter, which can step down voltage with higher efficiency and lower heat dissipation. An example is the Linear Technology LTM4627 uModule, which can deliver 1.2V at 15A with about 85% efficiency, requiring only about 1.5A input current at 12V. For AC mains input (230V), a transformer is needed to step down voltage before rectification and regulation, but such high current DC supplies require large, costly components and careful design. Overall, switching power supplies are recommended over linear regulators for this application due to efficiency and thermal management considerations.
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FAQ

TL;DR: 1.2 V at 15 A equals 18 W, and “you can’t make watts out of thin air.” Use a high-current buck converter, not an LM317 alone. [Elektroda, stephen Van Buskirk, post #21668615]

Why it matters: This FAQ helps makers choose a safe, efficient path to 1.2 V/15 A from 12 V or 230 V AC without burning 162 W as heat.

Quick Facts

LM317 is rated about 1–1.5 A; a 12 V→1.2 V linear drop at 15 A would dissipate ~162 W. [Elektroda, David Adams, post #21668605]
A buck converter (e.g., LTM4627) at 85% efficiency needs ~1.5 A from 12 V to deliver 1.2 V/15 A. [Elektroda, DAVID CUTHBERT, post #21668609]
Power in must cover power out: 12 V×1 A=12 W cannot deliver 1.2 V×15 A=18 W. [Elektroda, Chuck Sydlo, post #21668608]
From 230 V AC, you’ll need a transformer, bridge rectifier, large capacitors, then a robust switching stage. [Elektroda, stephen Van Buskirk, post #21668611]
LM317 with pass transistors: proposal used four 2N3055 devices and ~20 A input with emitter resistors. [Elektroda, Jimmy Babida, post #21668616]

Can an LM317 directly supply 15 A at 1.2 V?

No. LM317 is around 1–1.5 A. Dropping 12 V to 1.2 V at 15 A wastes about 162 W as heat, which is impractical. [Elektroda, David Adams, post #21668605]

Can I increase 12 V, 1 A to 1.2 V, 15 A by regulation?

No. 1.2 V at 15 A is 18 W, exceeding the 12 W available. As one expert notes, “you can’t make watts out of thin air.” [Elektroda, stephen Van Buskirk, post #21668615]

What is a buck converter, and is it suitable here?

A buck converter steps voltage down while increasing current, conserving power minus losses. It’s the efficient way to get 1.2 V at 15 A from 12 V. [Elektroda, DAVID CUTHBERT, post #21668609]

How much 12 V input current do I need for 1.2 V/15 A?

With an 85% efficient buck, expect about 1.5 A from 12 V. That aligns with module guidance for this operating point. [Elektroda, DAVID CUTHBERT, post #21668609]

Is a linear regulator chain a good idea for 1.2 V/15 A?

No. A linear drop from 12 V to 1.2 V at 15 A dumps roughly 162 W as heat, demanding massive heatsinking and wasting power. [Elektroda, David Adams, post #21668605]

How do I get 1.2 V/15 A from 230 V AC safely?

Step down with a transformer, rectify with a bridge, add large filter capacitors, then use a high-current switching stage. Size everything for 15 A. [Elektroda, stephen Van Buskirk, post #21668611]

Could an LM317 drive pass transistors to reach 15 A?

One proposal used LM317 driving four 2N3055 pass transistors with 0.15 Ω emitter resistors and ~20 A input. Thermal design remains critical. [Elektroda, Jimmy Babida, post #21668616]

Why not use the LM317 as the main 15 A regulator?

It can act as a driver, not the 15 A element. LM317’s current rating and the heat at low output voltage make it unsuitable alone. [Elektroda, David Adams, post #21668605]

What’s the practical difficulty level of a 15 A switching supply?

It is non-trivial. High-current switchers need careful layout, magnetics, and protection. “No piece of cake,” as one expert said. [Elektroda, stephen Van Buskirk, post #21668615]

What do I need from the mains side for 15 A DC output?

A transformer rated for the VA, a rectifier that handles surge and current, large capacitors, and a capable buck stage are required. [Elektroda, stephen Van Buskirk, post #21668611]

Any quick analogy for why 12 V/1 A can’t feed 1.2 V/15 A?

Think squirt gun versus garden hose. The source power is too low to sustain the higher-current load, regardless of regulator type. [Elektroda, Chuck Sydlo, post #21668607]

What if my application truly needs 1.2 V at 15 A?

State the load details. Contributors noted there may be simpler solutions depending on the device, wiring, or cell configuration. [Elektroda, Chuck Sydlo, post #21668613]

How do I plan a 12 V → 1.2 V/15 A buck design?

Select a 15 A-capable buck module (e.g., LTM4627 class). 2. Size the 12 V source for ~1.5 A at 85% efficiency. 3. Validate thermals and wiring. [Elektroda, DAVID CUTHBERT, post #21668609]

What’s a common failure or edge case to watch for?

Under-sizing a linear stage risks thermal runaway. At 12 V in, a 1.2 V/15 A linear drop means about 162 W to dissipate. [Elektroda, David Adams, post #21668605]

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