1V–30V Variable Voltage Regulator with 723 and 2N3055 – The Indestructible Classic That Still Rules

Hey friend, if you’ve ever blown up an LM317 while trying to pull more than an amp, or got frustrated because your “0–30 V” kit won’t go below 1.4 V, then this old-school but bullet-proof circuit is about to become your new best friend.

This exact 1–30 V regulator built around the ancient-but-immortal LM723 precision regulator IC and a single 2N3055 pass transistor has been on benches since the 1970s — and it still beats half the modern eBay kits in 2025. I’ve built literally dozens of these over the years, and one of them is still powering my main bench today. It’s simple, cheap, goes down to a true 1 V, handles 5–7 A with a good heatsink, and laughs at short circuits.

Let’s walk through every part of it like we’re sitting together with a cup of coffee.

Why This Design Is Still Unbeatable

• True 1 V minimum output (most 723 circuits only go to ~2 V — this one cheats beautifully)
• Rock-solid regulation even at 5 A
• Short-circuit proof — the 723 just folds back and sits there forever
• Costs under $10 in parts if you already have a transformer
• Fully repairable with junk-box parts
• Sounds way cooler when you spin the pot than any digital supply

I still reach for this design when I need something I can trust with expensive prototypes.

Complete Circuit Walkthrough

Raw Supply

You feed it roughly +32–40 V DC unregulated (typically a 24 V–0–24 V transformer + bridge + 4700 µF cap). That gives enough headroom for the 2N3055 to stay out of saturation even at 30 V output under load.

The Heart – LM723 (IC1)

The 723 is a 14-pin precision voltage regulator from the stone age that is still in production because nothing beats it for analog control.

Key pins in this circuit:

• Pin 7 (V+) → raw +32 V
• Pin 12 (Vref) → gives exactly 7.15 V rock-stable reference
• Pin 5 (non-inverting input) and Pin 4 (inverting input) form the error amplifier
• Pin 10 (current limit) senses a drop across the 0.1 Ω 10 W resistor
• Pin 6 (Vout) drives the base of the BD135 driver
• Pin 13 & 11 are frequency compensation — the 100 nF + 470 pF network keeps it from oscillating

The Clever 1-Volt Trick

Most 723 designs can’t go below ~2 V because the internal reference is 7 V and the error amp needs headroom. This circuit cheats by using an external 12 V zener (1 W) from pin 12 (Vref) to ground, and then references the feedback divider from that artificial ground. Result: when the pot is at minimum, the output sits at roughly 1 V instead of 2 V. Genius and still works perfectly.

Driver and Pass Stage

• BD135 (medium-power NPN) buffers the 723 output
• 470 Ω base resistor and 820 Ω from collector to +32 V set bias
• 2N3055 (the legendary TO-3 power transistor) does the heavy lifting
• 0.1 Ω 10 W emitter resistor senses current for the 723’s current-limit pin

When you short the output, the voltage across that 0.1 Ω hits ~0.65 V → 723 pulls pin 6 low → whole thing folds back to a few hundred mA and just sits there until you remove the short. No smoke, no drama.

Output Filtering

100 µF 50 V + 47 kΩ bleed resistor on the output keeps things quiet and discharges the cap when you turn the voltage down.

Exact Parts You Need (what I actually stock)

• IC1: µA723CN or UA723CN (DIP-14) – still $0.80
• Q1: BD135 or any medium-power NPN (BD139, TIP41C, etc.)
• Q2: 2N3055 (TO-3) or MJ15003/MJ15015 for extra grunt
• Z1: 12 V 1 W zener (1N4742A)
• R emitter: 0.1 Ω 10 W aluminum-clad (critical for current limit accuracy)
• Pot: 5 kΩ or 10 kΩ multi-turn (voltage adjust)
• R sense: 1 kΩ from current-limit pin to emitter of 2N3055
• Compensation: 100 nF ceramic + 470 pF silver mica exactly as shown
• Output cap: 100 µF 63 V low-ESR + 0.1 µF ceramic in parallel
• Heatsink: big — at least 2 °C/W for the 2N3055, preferably with a fan for >3 A continuous

Total cost with a scavenged transformer: under $15.

Step-by-Step Build That Never Fails

1. Build the 723 section first on a small piece of perfboard. Power it from a 12 V wall wart just to test — you should measure exactly 7.15 V on pin 12.
2. Add the 12 V zener and feedback divider. With no load, you should be able to swing from ~1 V to ~31 V with the pot.
3. Mount the BD135 and 2N3055 on the main heatsink. Keep leads short and use thick wire (at least 16 AWG) for power traces.
4. Wire the current-limit sense exactly as shown — if you change the 0.1 Ω value, you must recalculate the 1 kΩ resistor on pin 10.
5. Bring it up on a current-limited supply or with a 100 W bulb in series with the raw DC. Turn the voltage pot fully counter-clockwise → you should read ~1 V. Slowly turn it up → output should follow smoothly to 30+ V.
6. Short the output with a screwdriver — current should fold back to a few hundred mA and stay there. Remove short → instantly recovers.
7. Load test with a 6 Ω 100 W resistor (5 A at 30 V) and watch the 2N3055 barely get warm with a fan.

Real-World Performance

I regularly pull 30 V at 4 A continuous (120 W) with the 2N3055 sitting at 60 °C on a CPU cooler. The drop is under 50 mV from no-load to full-load. Ripple is basically invisible with a decent raw supply. The 1 V minimum is perfect for testing TTL, op-amp circuits, or single Li-ion charging with an external current limit.

Common Problems & One-Minute Fixes

• Won’t go below 2 V → check the 12 V zener polarity and that it’s really 12 V
• Oscillates → add 100 nF from pin 13 to pin 4
• Current limit not working → measure 0.1 Ω resistor — many “0.1 Ω” parts are actually 0.15 Ω
• 2N3055 gets too hot at 5 A → add a second one in parallel with its own 0.1 Ω emitter resistor

My Favorite Upgrades

• Replace the single 2N3055 with two or three in parallel → easy 10 A
• Add a 10-turn pot for silky-smooth voltage control
• Put a cheap digital panel meter across the output
• Add a small fan that turns on above 50 °C with a thermistor
• Use an MJ15003 or MJ15024 for a lower saturation voltage at high current

Safety Notes

32 V at 5 A can start fires. Mount the 2N3055 isolated from the chassis, use a proper mains fuse, and add a 10 A fuse on the output if you’re paranoid (I am).

Final Verdict

In a world full of overpriced Chinese “0-30 V 10 A” supplies that sag to 22 V at 3 A and catch fire when shorted, this 723 + 2N3055 design is still the one I trust with my best prototypes. It’s been around for fifty years because it simply works.

Build it once, box it properly, and it will outlive you.

So dust off that 2N3055 you’ve had in the drawer since 1998, order a 723 for a dollar, and let’s make something that actually deserves a place on your bench.

When you spin that pot down to exactly 1.000 V for the first time, you’ll know why this circuit is immortal.