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[Solved] LM338 Regulated Power Supply Diagram: 1.25V-30V, 0-3A - Resistor Selection & Adjustment Help

Daniel.1990 12582 19
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How do I choose the resistor values and adjust an LM338 bench power supply to get 1.25–30 V and 0–3 A with coarse and fine control?

You will not get true 0 V or proper current limiting from 0 A with this LM338-based circuit unless you keep the negative auxiliary supply; that negative voltage is what enables both zero-output adjustment and current regulation at low output voltages [#16782259] [#16782260] [#16777671] The LM338’s internal protection only protects the regulator itself, so if you add an external pass transistor, that transistor is not protected by the built-in protection [#16781699] The R2 = 150 Ω part is the current-adjust potentiometer [#16782561] For coarse/fine voltage adjustment, add the second potentiometer in series with the main 2.2 kΩ pot, and make the fine-adjust pot about 10% of the coarse one [#16783409] Another suggested wiring is to place the 2k2 pot in parallel with the fixed resistor and connect each wiper to ADJ through its own resistor, but that version reduces the useful precise-regulation range at high current [#16783627] A different, simpler path suggested in the thread was to use the LT1083 application circuit with a pre-regulator from the datasheet [#16782276]
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  • #1 16776638
    Daniel.1990
    Level 6  
    Posts: 14
    Rate: 3
    Hello.
    I want to enrich my workshop with a regulated power supply. I found a pattern and did a little bit of it. could anyone check if i made it correctly and help in choosing resistors?
    The power supply is to have parameters 1.25V-30V and 0-3A, I also care about coarse and accurate adjustment of these parameters. I am beginner, that's why I count on your help. If this is a bad department, please move.
    Regards.

    LM338 Regulated Power Supply Diagram: 1.25V-30V, 0-3A - Resistor Selection & Adjustment Helpzasila..PNG (56.95 kB)You must be logged in to download this attachment.
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    #3 16777671
    _jta_
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    It will not be regulated from 0 and from about 0.66A; current limitation will not work at low voltage (below about 1.4V). I don't know if the fan has a chance to work sensibly ... And what's the deal with BC557 and BD136? Maybe look for something better, because it will be nothing but trouble.
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  • #4 16777740
    Daniel.1990
    Level 6  
    Posts: 14
    Rate: 3
    Thanks for the advice. I will look for a different pattern and present it here so that someone with more experience can analyze it. As for this layout with BD557. It was supposed to be an electronic fuse but after analyzing I gave up this idea and apparently forgot to remove it from the schematic.

    As for buying a ready power supply, I'd rather avoid it. And there is always the satisfaction of doing something yourself.
  • #5 16781213
    Daniel.1990
    Level 6  
    Posts: 14
    Rate: 3
    Hello.

    I would ask you to check this pattern for correctness. I also want to ask more experienced colleagues what I need to change in it to achieve current regulation from 0A and how to make the current limitation work from about 0V?
    The next question is how to add short-circuit protection to this scheme and signaling a possible short circuit with an LED.
    As for controlling the fan with an NTC thermistor, the system was tested by me and works decently. I hope for a quick response.
    LM338 Regulated Power Supply Diagram: 1.25V-30V, 0-3A - Resistor Selection & Adjustment HelpZASILACZ S..MAT.PNG (55.79 kB)You must be logged in to download this attachment.
    Regards!
    daniel
  • #6 16781501
    _jta_
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    what do I need to change in it to achieve current regulation from 0A and how to make the current limitation work from about 0V?

    On the LM338K it is difficult - the matter was discussed for the LM317, read about the charger on the LM317.

    The next question is how to add short-circuit protection to this scheme and signaling a possible short circuit with an LED.

    I have the impression that LM338 (like LM317 and many others) has such protection built in.
  • #7 16781552
    Anonymous
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  • #8 16781699
    _jta_
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    The protection does not work (or rather, it works not quite as you would like: it protects the LM338, but the transistor does not) in a system with an external power transistor, and it is not here.
  • #9 16781852
    Anonymous
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  • #10 16782031
    Daniel.1990
    Level 6  
    Posts: 14
    Rate: 3
    Bieda z nędzą wrote:
    And I am surprised by these four potentiometers in the voltage control circuit. I would already understand two, coarse and fine regulation, but four?


    As for four potentiometers. These two are for adjustment
    Current and two to voltage. At least, that was the result of the original pattern.
    And back to the pattern. Will this meet my expectations?

    LM338 Regulated Power Supply Diagram: 1.25V-30V, 0-3A - Resistor Selection & Adjustment HelpZasilacz_2...878742.png (166.38 kB)You must be logged in to download this attachment.
  • #11 16782067
    Anonymous
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  • #12 16782077
    Daniel.1990
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    Bieda z nędzą wrote:
    Yes, of course it will, because as you can see, you like a lot of overclocked circuits. By the way, this power supply is very dangerous for powered systems, due to the auxiliary negative voltage power supply.


    I just don't trust simple systems. I prefer to use it as more recombined but be sure that it makes sense. One more question, can I remove this negative guide somehow? Is it needed for the proper functioning of this power supply
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    #13 16782259
    _jta_
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    This negative voltage enables: (1) current regulation also working at low output voltages; and (2) output voltage adjustment from 0.
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    #14 16782260
    Anonymous
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    #15 16782276
    Freddy
    Level 43  
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    In the LT1083 catalog card you have on page 13 a very nice and cool system with a pre-regulator.
    It's a little complicated, but it's worth doing.
  • #16 16782484
    Daniel.1990
    Level 6  
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    One thing remains. R2 150 is a potentiometer with which we adjust the current value? Is my reasoning wrong?
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    #17 16782561
    Anonymous
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  • #18 16783203
    Daniel.1990
    Level 6  
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    How can I add a second potentiometer to the R2 2.2k potentiometer to get a coarse and fine adjustment? In the case of R2 150 I don't have such a problem.
    Regards
    daniel
  • #19 16783409
    Anonymous
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    #20 16783627
    _jta_
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    You can give 2k2 in parallel, connecting the ends of both potentiometers with each other, but then the sliders of both need to be connected to the ADJ pin through resistors, e.g. the one from the R2 potentiometer = 150R through the 100R resistor, and the one from 2k2 through 2k. I considered another option, the 2k2 potentiometer in parallel to the 120R resistor (only then it can replace it with a larger one, the 150R would be just right), from a slider through a 2k resistor connection to ADJ, and the ADJ would not be connected directly to ground, but via a 100R resistor - but in this version, precise regulation at high current would have a small range (at maximum zero), which makes little sense.
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Topic summary

✨ The discussion revolves around the construction of a regulated power supply using the LM338, targeting output parameters of 1.25V-30V and 0-3A. The original poster seeks assistance in verifying their schematic and selecting appropriate resistors, particularly for achieving current regulation from 0A. Responses highlight challenges with current limitation at low voltages and suggest alternatives like the L200 for better performance. Concerns about the use of external transistors and the necessity of negative voltage for proper operation are raised. The conversation also touches on potentiometer configurations for voltage and current adjustments, with suggestions for adding fine adjustment capabilities. Overall, the community provides insights into circuit design, component selection, and safety considerations.
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FAQ

TL;DR: Building an LM338 bench PSU? Note this expert warning: “current limitation will not work at low voltage (below about 1.4 V).” That single fact shapes safe resistor choices, 0 V output goals, and current‑limit behavior. [Elektroda, jta, post #16777671] Why it matters: You’ll avoid smoked loads and design a supply that actually hits 0 V and regulates current where it counts. This FAQ is for DIYers tuning 1.25–30 V, 0–3 A LM338 supplies who want reliable coarse/fine adjustment and protection.

Quick Facts

Can I get true 0 V output from an LM338 supply?

Yes, but not from a single positive rail alone. You need a small auxiliary negative rail. It lets the adjust node go below ground so the output reaches 0 V. It also helps the current limiter work at very low outputs. “This negative voltage enables output voltage adjustment from 0.” [Elektroda, jta, post #16782259]

Why does current limiting fail near 0–1.4 V?

The LM338’s internal circuitry needs headroom to sense and limit current. Below about 1.4 V at the output, the limiter behavior degrades. Designers often add a negative rail so sensing and control have margin at low voltages. Quote: “current limitation will not work at low voltage (below about 1.4 V).” [Elektroda, jta, post #16777671]

Which potentiometer sets current in the shared schematic?

In the discussed layout, the R2 ≈150 Ω potentiometer adjusts the current limit. Keep its wiper leads short and add a small series resistor to prevent hard zero. That stabilizes adjustment and protects the wiper. [Elektroda, Anonymous, post #16782561]

How do I add coarse and fine voltage knobs?

Place a second potentiometer in series with the main voltage pot. Size the fine pot to about 10% of the main pot’s value. This gives smooth trimming without making the range twitchy. Keep both pots’ end terminals solidly tied to the same nodes. [Elektroda, Anonymous, post #16783409]

Is there another way to wire dual pots on LM338?

You can put the two pots in parallel, then route each wiper to ADJ through different resistors (e.g., 100 Ω from the current pot, 2 k from the 2k2 pot). This shapes ranges and improves stability. Avoid connecting ADJ directly without those series resistors. [Elektroda, jta, post #16783627]

Does LM338 have built‑in short‑circuit protection?

Yes. LM338 integrates current limiting and thermal shutdown to protect itself. Protection behavior changes if you add an external pass transistor; then the IC may survive while the transistor doesn’t. Keep that in mind during expansions. [Elektroda, jta, post #16781501]

Will built‑in protection still work with an external pass transistor?

No, not as you expect. The LM338 protects itself, but an external pass transistor can conduct destructive current during a fault. Add separate current‑sense and protection around the pass device if you extend output current. [Elektroda, Anonymous, post #16781552]

Are four adjustment pots necessary?

Not usually. Use one pair for voltage (coarse/fine) and one pair for current (coarse/fine). Extra pots complicate tuning without benefit. “I would already understand two, coarse and fine regulation, but four?” [Elektroda, Anonymous, post #16781852]

Is the auxiliary negative rail risky for connected devices?

It can be if miswired. The auxiliary negative supply can stress the load during faults or reference shifts. Implement proper grounding, output sensing, and protection to avoid injecting the negative rail into the DUT. Treat it with caution. [Elektroda, Anonymous, post #16782067]

How can I achieve current regulation starting at 0 A?

With LM338 alone, it’s hard. Designers use a small negative rail and suitable sense network, or choose an IC that natively supports 0 A regulation. L200 is a classic option when you need flexible current/voltage control. [Elektroda, Anonymous, post #16781552]

What pre‑regulator idea can cut heat and improve stability?

Consider the LT1083 datasheet’s pre‑regulator approach. It’s more complex but worth building for better thermal performance and dynamic response. Use it to drop most of the voltage before the main regulator stage. [Elektroda, Freddy, post #16782276]

My design won’t regulate from 0 V or low current—what’s wrong?

That’s expected with a basic LM338 setup. It won’t regulate from 0 V, and current limiting degrades below ~1.4 V. Add a negative rail or redesign the control loop to maintain headroom. Confirm your sense resistor wiring and reference points. [Elektroda, jta, post #16777671]

What resistor values help stabilize the ADJ node with two pots?

Route each wiper to ADJ through resistors (example: 100 Ω from the 150 Ω current pot, 2 k from the 2k2 voltage pot). This prevents interaction and keeps fine control linear. Avoid direct wiper‑to‑ADJ connections. [Elektroda, jta, post #16783627]

Do I have to buy a ready‑made supply instead of building?

Buying is simpler, but building teaches and customizes features. If you build, plan for safe low‑voltage behavior, heat, and protection. A commercial 30 V/5 A unit exists, but the OP chose to DIY for learning and satisfaction. [Elektroda, Daniel.1990, post #16777740]

What’s an easy 3‑step way to add fine voltage control?

  1. Insert a fine pot in series with the main voltage pot.
  2. Size it to ~10% of the main pot’s value.
  3. Keep wiring short; verify end‑to‑end resistance and range before power‑up. [Elektroda, Anonymous, post #16783409]

Any gotchas when testing below 1.5 V and above 2 A?

Yes. Expect weak current limiting below ~1.4 V unless you add a negative rail. Also note layout and cooling matter when sourcing multi‑amp loads. One post flags issues starting around 0.66 A if the scheme is not optimized. [Elektroda, jta, post #16777671]
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