FAQ
TL;DR: Building a 1.2–37 V, 10 A lab PSU hinges on two numbers: BD911’s 90 W max dissipation [Elektroda, stallion, post #432457]; and, as one builder warns, “you need a veeeeery large heat sink” [Elektroda, Ptolek, post #432322]
Why it matters: undersizing thermal or transformer parts makes the LM317-Darlington design fail or short-out. Ideal for hobbyists upgrading LM317 boards to double-digit current safely.
Quick Facts
• LM317 input limit: 40 V (LM317T) / 57 V (LM317HVT) [Texas Inst., DS]
• Safe dropout: 3 V between Vin and Vout at 1 A load [TI, DS]
• Emitter resistors 0.33–0.68 Ω set ≈6–10 A range [Elektroda, krzysztof723, post #15163048]
• Recommended transformer: 2 × 14 VAC, 300–400 W, ≥12 A secondary [Elektroda, krzysztof723, post #15163048]
• Single BD911 fails above 90 W; use ≥4×KD502 or BD249C in parallel with 0.33 Ω sharing resistors [Elektroda, stallion, post #432457]
How much current can an LM317 really deliver?
Alone, the LM317 tops out at 1.5 A [TI, DS]. By adding a Darlington pass stage (e.g., BD911 + 4×KD502) and 0.33–0.47 Ω emitter resistors, builders on the forum report stable 8–10 A output [Elektroda, laciaty1981, post #14788228]
Why does the voltage sag when I draw 10 A?
A single series transistor adds its VBE drop and internal resistance. At 10 A that drop can reach 0.6–0.7 V, so output falls unless feedback is taken after the pass devices [Elektroda, zipp, post #434478]
What transformer size stops the BD911 from overheating?
Keep Vin–Vout ≈ 3–6 V under load. A 2×14 VAC, 300 W toroid limits dissipation to ~60 W across four pass transistors at 12 V/10 A [Elektroda, krzysztof723, post #15163048]
Which heat sink should I pick for four BD249C transistors?
Use at least 0.4 K/W thermal resistance with a 80 mm fan. Forum builds show 200 × 100 × 40 mm finned blocks keeping devices below 70 °C at 10 A [Elektroda, kotbury, post #17978882]
How do I set the current-limit range?
- Start with 0.47 Ω, 5 W resistors in each emitter.
- Load the PSU at 5 A and turn the 470 Ω pot until limiting begins.
- For lower minimum current, raise resistors to 0.56 Ω; for higher maximum, drop to 0.33 Ω. [Elektroda, krzysztof723, post #15163048]
What happens if I short the output?
With foldback limiting active, current collapses to 1–2 A while pass transistors drop full Vin. Without a 10 A fuse, a single BD911 can blow instantly because "at 1.2 V/10 A it would dissipate about 300 W" [Elektroda, stallion, post #432457]
Can I swap the D22-20-04 protection diode for something else?
Yes. Any 20 A, ≥200 V silicon rectifier (e.g., 6A10, P600M) works. It just blocks reverse battery energy [Elektroda, kotbury, post #20266310]
Is a soft-start circuit mandatory?
Recommended but not mandatory. Without it, inrush into 30 000 µF filter caps can trip mains breakers; builders reused microwave-oven NTC soft-start boards successfully [Elektroda, Anonymous, post #17222918]
Will the design run from a 24 V switching supply instead of a transformer?
Yes—feed the 24 VDC into the LM317 input after removing the bridge and bulk cap. Keep Vin under 40 V and ensure the SMPS can source 12 A continuous [Elektroda, krzysztof723, post #16251955]
Edge case: what fails first at minimum 1.2 V, 10 A output?
Thermal stress. With 28 V across pass devices, total dissipation exceeds 280 W. Even six KD502s on a fan-cooled sink hit 120 °C in seconds [calculated; stallion data #432457].
Can I substitute MOSFETs for the Darlington stage?
Not drop-in. MOSFETs need gate drive > Vout and add reverse-polarity issues. You’d redesign the limiter around source resistors and an op-amp; the thread’s bipolar feedback points won’t bias properly.
Why does the limit LED light only above 4 V output?
The BD140 indicator transistor needs about 0.6 V across the 0.33 Ω resistors to turn on; below 4 V, foldback keeps sense voltage too low [Elektroda, mario8423, post #15580857]
Best practice for grounding the board?
Run a single, wide trace from filter-cap negative straight to the output jack. Tie signal grounds (LM317 adjust node, sense resistors) near that star point to avoid 50 mV regulation wobble [Elektroda, krzysztof723, post #14554493]
