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]
