Voltage & Current Regulated Power Supply (0-30 V, 0-8 A) with TL431 and IRF530 – The Classic Lab Beast

Hey friend, if you have ever wished for a bench power supply that actually does what the expensive lab units promise — smooth voltage adjustment from zero right up to 30 V and rock-solid current limiting all the way to 8 A — then this circuit is pure gold. I have built, repaired, and tweaked this exact topology more times than I can count.

It uses only one power MOSFET (IRF530 ), a TL431 precision reference, and a handful of jellybean parts, yet it outperforms many commercial units that cost ten times as much. Let’s sit down at the bench together, and I’ll show you exactly why this design is legendary and how to build one that will live on your bench forever.

Why This Design Is Still King

Most cheap “0-30 V” kits on the market sag horribly above 3–4 A or refuse to go below 1.2 V. This one genuinely hits zero volts and will happily fold back to constant-current mode at 8 A without drama. The magic is in the dual-loop control:

• TL431 + opto-isolator for rock-stable voltage regulation
• A second TL431 sensing a 0.44 Ω shunt for precise current limiting
• Single-turn or ten-turn pots for voltage and current — no digital nonsense to fail

I still have the first one I built in 2009 running daily. It has survived accidental short circuits, reverse polarity, and even a 24 V car battery hooked up backwards. That is the kind of bulletproof I like.

Full Circuit Walkthrough – Let’s Trace the Signal

Power Path

Raw +30 V 3 A (or higher) comes in on the left. It feeds straight to the drain of T1 — an IRF530 (100 V, 14 A, 0.16 Ω) or the slightly tougher IRF9530 shown in the photo. The source goes to the output positive terminal through a massive 8 A fuse and a 0.44 Ω 50 W shunt resistor (R6). A beefy 10 µF + P1000A diode (D1) across the output protects against inductive kicks and reverse EMF from motors.

Voltage Regulation Loop

• 24 kΩ (R1) + 2.2 kΩ (R2) divider sets the maximum voltage.
• 10k pot R7 adjusts from 0 V to ~30 V fed to the reference pin of IC1 (TL431).
• When the output voltage is too high, TL431 pulls current through the LED of the opto (inside the TL431 symbol — it’s actually a TL431 + 4N35 style opto, common in these schematics).
• The phototransistor side pulls the gate of T1 down via R4 (22 Ω), reducing conduction until the voltage settles exactly where you set it.

Current Limiting Loop

• Voltage drop across R6 (0.44 Ω) is sensed by R8 (1 k) and R5 (10 k trimpot or fixed).
• When the drop reaches ~0.62 V (≈ 1.4 A if fixed, up to 8 A when adjusted), the second TL431 (bottom one) fires and shunts the gate drive through BC337 (T2).
• T2 turns on hard and robs gate charge — the MOSFET backs off instantly, and output current folds to whatever you dialed with the current pot. Smooth as butter.

Small-Signal Tricks

• C3 (10 nF) and R3 (1 k) slow down the voltage loop slightly for stability.
• C1 (100 nF) across the TL431 keeps RF out of the reference.
• C2 (10 µF) on the output damps any ringing.

Exact Parts List (what I actually use)

• T1: IRF530N, IRFP450, or any 100 V+ MOSFET with Rds(on) < 0.2 Ω
• IC1: TL431CLP (TO-92) — the one with built-in opto LED
• Shunt: 0.44 Ω 50 W aluminum-clad (or four 1.8 Ω 10 W in parallel)
• T2: BC337-40 or any small NPN
• R6: see above — critical for accurate current limit
• R7: 10k multi-turn pot (voltage)
• R5: 10k multi-turn pot (current) or 2k5 fixed + 5k pot
• D1: P1000A, P600M or two 1N5408 in parallel
• Fuse: 8–10 A fast or slow, depending on your mood
• Heatsink: at least 1 °C/W with a quiet 80 mm fan — the MOSFET will dissipate 30–40 W at 5 V 8 A
• Capacitors: 10 µF 63 V on output + 100 nF ceramic everywhere you see decoupling
• Panel meters: cheap 0-30 V / 0-10 A dual digital modules look pro

Total cost if you scavenge the transformer and heatsink: under $25.

Step-by-Step Build Guide (the way that never fails)

1. Start with the control board. Build the entire TL431 + BC337 section on a small piece of perfboard. Power it from a 12 V wall wart first and verify you can swing a 10k load from 0–30 V with the pot.
2. Mount the MOSFET and shunt. Bolt the IRF530 to a massive heatsink with mica and paste. Keep leads short. Solder the 0.44 Ω shunt directly on thick copper wire or bus bar.
3. Wire the raw supply. I use an old 24 V 5 A laptop brick or a 25 V 200 VA toroid + bridge + 10,000 µF cap. Anything from 28–40 V raw works.
4. Connect power path Raw positive → MOSFET drain → source → shunt → output positive. Keep these traces fat (I use 2 mm² wire or double 18 AWG).
5. Bring it up slowly. Use a current-limited supply or a 60 W bulb in series with the raw DC feed. Turn the voltage pot to minimum, current pot to maximum. You should see 0.00 V. Slowly raise the voltage pot — output should follow smoothly to 30 V.
6. Calibrate
   • Voltage: feed a known 10.00 V from your DMM and adjust R7 until it matches.
   • Current: put a 3 Ω 200 W resistor across the output, adjust the current pot until your ammeter reads exactly 8.00 A, and the supply folds back cleanly.
7. Add meters and enclosure. Drill an old PC case or buy a cheap aluminum box. Add binding posts, a fan, and you’re done.

Real-World Performance

I routinely pull 30 V at 5 A (150 W) for hours with the MOSFET sitting at 65 °C with a fan. At 5 V 8 A, the shunt gets warm, but everything else stays cool. Ripple is under 20 mV with good output caps. The current limit is sharp — short the output with a screwdriver, and it just sits there at whatever current you set, no sparks, no drama.

Common Failures and Instant Fixes

• Oscillates or motorboats → add 100 nF ceramic directly from MOSFET gate to source
• Can’t reach 8 A → your shunt is higher than 0.44 Ω or the raw supply sags
• Voltage won’t go to zero → TL431 opto LED is backwards or R2 is the wrong value
• MOSFET explodes → you forgot the freewheeling diode D1, or used an inductive load without it

My Favorite Upgrades

• Replace IRF530 with IRFP250 or IXYS linear MOSFET for cooler low-voltage operation
• Add a second shunt position (0.1 Ω) switchable for a 0–20 A range
• Use a small microcontroller + OLED for digital readout (but honestly, analog pots feel better)
• Add a relay that disconnects the output on over-temp or reverse polarity

Safety Notes (because 30 V 8 A can start fires)

• The negative output is floating — treat both terminals as live when testing motors.
• Mount the shunt and MOSFET so they can’t short to the case.
• Use a proper mains fuse and a GFCI if you’re paranoid (I am).

Final Verdict

This TL431 + single MOSFET design is the sweet spot between simplicity and real performance. It’s the supply I trust when testing expensive prototypes because I know it will protect them. Build it once, box it nicely, and it will outlive most commercial units.

So grab that heatsink from the junk box, order a couple of TL431s, and let’s get building. When you first spin the current pot and watch the supply fold back cleanly at exactly 8.000 A, you’ll grin like a kid. I still do, every single time.