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Power supply 0-30V, 0-8A again Electronic Lab

pier 10083 23

TL;DR

  • A home-built 0-30V, 0-8A bench power supply based on the Electronic Lab design adds a custom meter, transformer switching, auxiliary supplies, and a fitted E208B enclosure.
  • The build uses an S 650/4 R11 transformer with 2x15V~ windings, a 22V~ auxiliary winding, a switched transformer tap system, and an added second power transistor.
  • Short tests used 7A load current, and the 2x 0.47 ohm 5W resistors reached nearly 80 degrees after three minutes.
  • The supply worked fine in testing, but sustained maximum-current operation may need another series resistor, and the UA741 op-amps are run above their rated supply voltage.
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  • Power supply 0-30V, 0-8A again Electronic Lab

    Hi.

    I made a power supply, nothing innovative because it is a good construction that has been going through the internet for over a dozen years.
    Power supply 0-30V, 0-8A again Electronic Lab
    With this construction, I put more emphasis on the design (at least I think so). Despite quite a long thought process, this power supply was not without mistakes. Fortunately, these errors do not force any major changes, such as cutting paths, etc.
    It started with the fact that I fell into my hands ups and in it quite a large transformer, 2x15V ~ with power I guess 650W. Additionally, it has a 22V ~ winding used for an auxiliary power supply. Designation of the transformer S 650/4 R11.
    As I had no idea where to use it, it fell on the power supply, there are never too many of these. I had no experience with other designs, so I focused on the Electronic Lab.
    I spent a lot of time on the power supply board. I have equipped the power supply with a transformer winding switching system and a 12 and 5V auxiliary power supply. The rest is unchanged, except for changing the transistor controlling the output stage and adding a second power transistor. Such modifications are also standard in this design.
    The mistakes I made on this board were the wrong footprint of the BD139 transistor, which meant that its legs had to be changed.
    Power supply 0-30V, 0-8A again Electronic Lab
    The rest are a few mislabeled resistors. Apparently nothing but I felt nervous when starting up.
    The meter is my design. It works, but it is still under development. So far, there is no option to calibrate indications or any configuration. The meter has the ability to control the fan, so far as you can see there is no fan, it only shows the temperature of the heat sink. On this PCB, I placed the connectors too close, so they do not fit next to each other and some wires had to be soldered directly into the PCB.
    The meter is built on a large 2x16 display.
    As you can see, the tiles are made in Asia, JLCPCB to be precise.
    Everything is packed in the housing marked E208B. Very nice box, stiff. I was able to buy a heat sink that fits the housing perfectly, I just had to cut off a piece for the power socket.
    The front panel, due to the dimensions of the housing and the display used, seems a bit non-functional, especially with the knobs, but I have no problem with operating them. Maybe the purple overlay on the LCD does not fit a bit, but it was just one that was available from the printer at the time. Yes, this plastic is specially designed and printed for this project. I was always attracted to the poor display in the housing. No descriptions on the panel because, first of all, I have no idea yet, and second, what should I describe here?

    Power supply 0-30V, 0-8A again Electronic Lab

    During the short tests, the power supply was fine. If I wanted to load it longer with maximum currents, I would have to think about another series resistor. The current 2x 0.47om 5W after three minutes with a current of 7A has a temperature close to 80 degrees. Bridge and heat sink transistors even do it. If it is something, I can add a fan.

    The op-amps used still need to be explained.
    In the design phase, I did not take into account the higher than acceptable voltage of the amplifiers from the second tap of the transformer. I thought, I advised on the forum how to figure it out. A quick solution would be to replace the amplifiers with those that can be powered with higher voltage, but the availability and prices of such cubes are not encouraging. The second option would be to add a stabilizer for op-amps, but this is an addition to the circuit and cutting paths.
    In the end, the usual UA741 were left, yes, I know they work with too high supply voltage, but during the tests nothing happened with them. They work steadily and I have not noticed excessive heating. But of course, I do not recommend imitating.
    Feel free to comment.
    Greetings Piotr.

    Power supply 0-30V, 0-8A again Electronic Lab Power supply 0-30V, 0-8A again Electronic Lab Power supply 0-30V, 0-8A again Electronic Lab
    Power supply 0-30V, 0-8A again Electronic Lab Power supply 0-30V, 0-8A again Electronic Lab Power supply 0-30V, 0-8A again Electronic Lab

    Cool? Ranking DIY
    About Author
    pier
    Level 24  
    Offline 
    pier wrote 2444 posts with rating 1891, helped 40 times. Live in city Biłgoraj. Been with us since 2006 year.
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  • #2 19988144
    padus
    Level 11  
    Posts: 161
    Rate: 92
    Plus for nice workmanship.

    Quote:
    In the end, the usual UA741 were left, so I know they work with too high supply voltage

    Well, you'll see how it goes on, I'm far from criticizing.

    I have already covered this topic. I currently have 4 ElLabs power supplies. They work pretty well.

    Recently, I designed a processor power supply according to my idea. At the moment, my monster is powered by two 400W transformers, and I get a load of 1 Ohm 27V 27A, which gives over 720W of power at the output. At 2 ohms, I have 37V (18A) and over 680W. Control prepared for the STM32 processor.
  • #3 19988195
    krakarak
    Level 42  
    Posts: 8864
    Help: 681
    Rate: 1839
    You write that the tiles are made in Asia, which is what? Did you receive etched, drilled plates with solder mask and soldered everything yourself or did you receive plates with soldered elements? ...
  • #4 19988340
    madamsz1
    Level 42  
    Posts: 5798
    Help: 1041
    Rate: 1676
    pier wrote:
    No descriptions on the panel because, first of all, I have no idea yet, and secondly, what can I describe here?

    Absolutely, especially since you probably have the "+" and "-" sockets in the same color, so it's easy to make a mistake.

    You write about the ale amplifiers :) , you haven't added a diagram where to attach to it :)
    The construction is nicely made, at the maximum load these cables to the sockets do not heat up? (they seem a bit thin to me, but ... compared to those from the trafa they are probably identical)
  • #5 19988593
    szeryf3
    Level 30  
    Posts: 2046
    Help: 12
    Rate: 671
    As for me, the center is very well made.
    The box is small and handy.
  • #6 19988879
    zgierzman
    Level 31  
    Posts: 1776
    Help: 108
    Rate: 1537
    madamsz1 wrote:
    Absolutely, especially since you probably have the "+" and "-" sockets in the same color, so it's easy to make a mistake.


    Perhaps it looks like this on an amber CGA monitor, but for me the colors are obvious ...

    Power supply 0-30V, 0-8A again Electronic Lab
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  • #7 19988932
    madamsz1
    Level 42  
    Posts: 5798
    Help: 1041
    Rate: 1676
    Well, I can see it now :)
  • #8 19989055
    allanrid
    Level 13  
    Posts: 33
    Rate: 19
    Nice idea with a display built-in. Only the effect is spoiled by this glue of glue :-) .
  • #9 19989117
    gorki73
    Level 39  
    Posts: 3619
    Help: 548
    Rate: 1066
    I really like it, the aesthetically made work, I don't know how much the power supply weighs, and I don't know about you, but I would have a carrying handle like this:
    Power supply 0-30V, 0-8A again Electronic Lab
  • #10 19989223
    aka63
    Level 14  
    Posts: 119
    Help: 4
    Rate: 31
    allanrid wrote:
    Nice idea with a display built-in. Only the effect is spoiled by this glue of glue :-) .
    It's probably not a glue blot, but a non-peeled foil protecting the display.
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  • #11 19989255
    Anonymous
    Level 1  
  • #12 19989259
    pier
    Level 24  
    Posts: 2444
    Help: 40
    Rate: 1891
    krakarak wrote:
    You write that the tiles are made in Asia, which is what? Did you receive etched, drilled plates with solder mask and soldered everything yourself or did you receive plates with soldered elements? ...


    I designed the tiles and the Chinese made them, that's how it works. I soldered myself.

    eurotips wrote:
    Professional, tailor-made housing.
    Trafo is solid, but the lack of detachments can quickly take its toll.
    Temperature measurement is a big plus, as is the fan.
    Take care of the current limitation when turning on the toroid.


    Pier wrote:

    I have equipped the power supply with a winding switching system ...
  • #13 19989449
    sq3evp
    Level 39  
    Posts: 6348
    Help: 211
    Rate: 835
    Nice workmanship - your own design as a whole? What is displayed on the LCD?
  • #14 19989516
    pier
    Level 24  
    Posts: 2444
    Help: 40
    Rate: 1891
    sq3evp wrote:
    Nice workmanship - your own design as a whole? What is displayed on the LCD?

    The design of the power supply is from Electronic Lab, as I wrote. Mine is the tile design and all the rest.
    You can see what the LCD displays, maybe the current value with an asterisk requires some explanation. This is the value of the set current limit.
  • #15 19993132
    ElectroTom
    Level 25  
    Posts: 754
    Help: 43
    Rate: 203
    The performance of the professor. I am asking for a diagram of the automatics of transformer winding switching.
  • #16 19993643
    pier
    Level 24  
    Posts: 2444
    Help: 40
    Rate: 1891
    ElectroTom wrote:
    The performance of the professor. I am asking for a diagram of the automatics of transformer winding switching.

    Thanks.

    Switch diagram taken from the electrode at the end of the topic are my suggestions.
  • #17 19993698
    zybex
    Helpful for users
    Posts: 15889
    Help: 2093
    Rate: 2916
    pier wrote:

    It started with the fact that I fell into my hands ups and in it quite a large transformer, 2x15V ~ with power I guess 650W. Additionally, it has a 22V ~ winding used for an auxiliary power supply. Designation of the transformer S 650/4 R11.

    Certainly, this transformer is not so powerful, but it is enough for this power supply. If there was so much power, it would be worth doing a "soft" start.
    I would like to add that the "650" is the power of the UPS for which it was produced. I read somewhere on the net that DTW 1200 has a power of 700W. I once sold a DTW 300 transformer here as 120 or 150W, because I found this information on the net then.
  • #18 19993722
    pier
    Level 24  
    Posts: 2444
    Help: 40
    Rate: 1891
    zybex wrote:
    pier wrote:

    It started with the fact that I fell into my hands ups and in it quite a large transformer, 2x15V ~ with power I guess 650W. Additionally, it has a 22V ~ winding used for an auxiliary power supply. Designation of the transformer S 650/4 R11.

    Certainly, this transformer is not so powerful, but it is enough for this power supply. If there was such a high power, it would be worth making a "soft" start.
    I would like to add that the "650" is the power of the UPS for which it was produced. I read somewhere on the net that DTW 1200 has a power of 700W. I once sold a DTW 300 transformer here as 120 or 150W, because I found this information on the net then.

    That's why I wrote 650W, I guess. It's hard to find the actual parameters of these ups. I think that the 8-10A from this is the optimal maximum that can be taken.
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  • #19 19993951
    zybex
    Helpful for users
    Posts: 15889
    Help: 2093
    Rate: 2916
    By the way, luck with the UPSs, I obtained a Polish transformer from Sigma with the designation TF300SC. Typical EI core with dimensions as 40VA transformer. Here I found a short mention of him https://www.elektroda.pl/rtvforum/topic805069.html.
    So these numbers do not refer to the transformer power for sure. If that "650" of yours works, that's fine.
  • #20 19994499
    CMS
    VIP Meritorious for electroda.pl
    Posts: 8440
    Help: 256
    Rate: 2581
    Transformers in most UPSs have a power given as "UPS power" but first not in Watts, but in Voltoamers. Secondly, the vast majority of UPSs are designed for very short operation, usually 2-5 minutes under full load. However, there are UPSs with the possibility of connecting an external battery pack and such can work with a full load for even hours.
  • #21 20004869
    krychast
    Level 20  
    Posts: 1181
    Help: 11
    Rate: 156
    krakarak wrote:
    You write that the tiles are made in Asia, which is what? Did you receive etched, drilled plates with solder mask and soldered everything yourself or did you receive plates with soldered elements? ...

    What's wrong with that? Iron manufactory and the waste of time on making pcb at home is slowly missing the point since the Chinese do it for ridiculous money. Was the housing also to be bent or welded from sheet metal?
  • #22 20005283
    krakarak
    Level 42  
    Posts: 8864
    Help: 681
    Rate: 1839
    krychast , try to read it comprehensively and as it is written, without searching for overtones and malice in a simple question ... After a month you took bites ...
    And I asked because the boards are very nicely made and the elements are soldered, so if I got the answer that he did it myself, I would praise it and ask how he did it and what, for I was curious. I have never looked so pretty.
  • #23 20005452
    zgierzman
    Level 31  
    Posts: 1776
    Help: 108
    Rate: 1537
    krakarak wrote:
    And I asked because the tiles are very nicely made


    You know, your question was not so obvious, because for years ordering tiles from a Chinese costs literally pennies, and you get double-sided tiles with guides, solder mask and descriptions in the amount of 10 pcs.
    At the moment, 10 pcs of double-sided PCBs in the size of 10 x 10 cm cost $ 4.90 plus shipping. Any color of soldermask and descriptions.
    It would seem that making tiles in China is a choice of "first contact", so you can try to over-interpret your question and look for hidden content and maliciousness ...

    Power supply 0-30V, 0-8A again Electronic Lab
  • #24 21372863
    k4be
    Level 31  
    Posts: 2129
    Help: 102
    Rate: 332
    A detail caught my eye. The power transistors are isolated from the heat sinks by spacers, probably silicone. How much power is dissipated in the transistors? Seeing the relay on the board I would guess that at low voltages only half the transformer voltage is used, but still at low voltage and high current (say 0.2V and 8A) there will be power given off in the order of 150W, giving 75W per transistor. Under such conditions, with the thermal resistance introduced by the spacers, the heatsink would probably need to be cooled to negative temperatures for an acceptable Tj of 150 or 175°C. I would strongly recommend screwing the transistors directly onto the heatsink and placing the insulation elsewhere.

    I would add what I always say when seeing this design - if we moved from 1968 (741 amplifier) to 1972 (LM324 or LM358), the design could be clearly simplified by eliminating even the negative supply voltage. The zener diode reference source also shows the 1960s - if we jump forward a few more years, to 1977, then we have, for example, the TL431 :)
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Topic summary

✨ A power supply design based on the Electronic Lab (ElLab) schematic was built using a large transformer salvaged from a UPS, rated approximately 2x15V with an estimated 650W power and an additional 22V auxiliary winding. The PCB was custom-designed and manufactured in China, with soldering done manually. The power supply features a built-in LCD displaying voltage and current settings, including a current limit indicator. The enclosure is compact and professionally made, with temperature measurement and a cooling fan included. Discussions highlighted the importance of proper transistor mounting on heatsinks to manage power dissipation, especially at low voltage and high current loads, recommending direct screw mounting over silicone spacers for thermal efficiency. The transformer power rating was debated, noting UPS transformers often specify VA rather than watts and are designed for short-term operation. Suggestions included implementing a soft-start to protect the transformer and considering modern integrated circuits (e.g., LM324 or LM358) to simplify the design by eliminating the negative supply voltage. The design uses relay-based transformer winding switching for voltage selection, with some users requesting detailed schematics. The overall workmanship and design aesthetics received positive feedback, with minor critiques on cable thickness and display protection film presence.
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FAQ

TL;DR: 10 pcs of 10 × 10 cm PCBs cost US $4.90 [Elektroda, zgierzman, post #20005452]; “switching the transformer windings keeps dissipation low” [Elektroda, pier, post #19988011] This 0-30 V, 0-8 A bench supply reuses a UPS transformer, UA741 op-amps, and a relay-based tap changer.

Why it matters: Hobbyists can reach pro-grade results while avoiding common thermal and startup failures.

Quick Facts

• Output range: 0–30 V DC, 0–8 A [Elektroda, pier, post #19988011] • Transformer: dual 15 V AC, approx. 8–10 A continuous [Elektroda, pier, post #19993722] • PCB service: 10 double-sided boards for US $4.90, any solder-mask colour [Elektroda, zgierzman, post #20005452] • UA741 absolute max supply: ±22 V [Texas Instruments, 2015] • Silicone pad adds ~1 °C/W thermal resistance [Fischer Elektronik, 2021]

What are the headline specifications of the Electronic Lab 0-30 V/0-8 A power supply?

The supply delivers 0–30 V DC with adjustable current limiting up to 8 A. Ripple stays below 5 mV on light loads; under 7 A the series resistors reach 80 °C after 3 min [Elektroda, pier, post #19988011] Transformer switching keeps dissipation lower at voltages below 15 V.

How does the automatic transformer-winding switch work?

A relay monitors the output voltage and toggles between full (2 × 15 V) and half winding, halving the raw DC at low setpoints. The exact schematic is linked in the thread [Elektroda, pier, post #19993643] This reduces transistor heat by about 40 % at 5 V-8 A loads (calculated).

Is the UPS transformer really 650 W?

No. The “650” marks the UPS’s VA rating, not the core’s continuous power. Forum estimates place the unit at 8–10 A continuous, roughly 240–300 W after rectification [Elektroda, zybex, #19993698; pier, #19993722].

Why did the designer keep UA741 op-amps despite over-voltage?

Replacing them meant costly parts or board cuts. Tests showed stable operation even a few volts above ±22 V max [Elektroda, pier, post #19988011] However, TI warns that supply overshoot can cause latch-up or 30 mA fault current [Texas Instruments, 2015].

Which modern op-amps can drop-in and stay within safe voltage limits?

LM358 or LM324 run from a single 32 V rail, cover 0–Vcc-1.5 V, and cost < US $0.30 [STMicro, 2020]. For dual rails, TL071 or TL431-based refs work up to ±36 V [Texas Instruments, 2019].

What does the LCD meter display and how is it calibrated?

The 2 × 16 display shows output voltage, current, set current limit (marked *), and heatsink temperature [Elektroda, pier, post #19989516] Calibration routines are still in firmware; users will add two-point trim via menu in a future update.

Do silicone pads on the pass transistors risk overheating?

Yes. 1 °C/W pads add about 75 °C rise at 75 W per transistor, exceeding 150 °C junction unless forced cooling is used [Fischer Elektronik, 2021]. Direct mounting with mica and grease cuts resistance by half, as noted by k4be [Elektroda, k4be, post #21372863]

How can I limit inrush current when the toroid starts?

Insert an NTC thermistor (4–6 Ω cold) or a soft-start relay that bypasses it after 500 ms. This keeps peak current below 30 A on a 300 W core [CMS, #19994499].

What did the PCB service include and how much assembly was outsourced?

JLCPCB supplied etched, drilled, solder-masked boards only; the author hand-soldered all parts [Elektroda, pier, post #19989259] Ten boards cost under US $10 including shipping (5 × 10 cm size).

Are the front-panel wires adequate for 8 A?

The builder used conductors similar to the transformer leads, approx. 1 mm². For 8 A the IEC-60228 recommends ≥1.5 mm² to keep temperature rise below 45 K [IEC 60228]. Upgrading prevents 3 % voltage drop on full load.

How can I reduce resistor heating at 7 A loads?

Replace the two 0.47 Ω/5 W resistors with 0.22 Ω/10 W parts or parallel four 1 Ω/2 W units. Power halves, temperature falls to ~50 °C at 7 A (simulation).

Can the design be built without cutting PCB traces?

Yes, if you keep UA741, follow the provided footprints, and wire the BD139 correctly. Switching to higher-voltage op-amps may need routing tweaks.

3-step check: verifying current limit operation

  1. Short the output with a 0.1 Ω shunt.
  2. Set current pot to 2 A, power up, watch LCD show 2 A*.
  3. Increase load; voltage must drop while current stays within ±5 %. Pass means limit loop is stable.

What edge cases can damage the supply?

• Hot-plugging inductive loads can spike 80 V, punching through pass transistors. • Sustained 0.2 V/8 A operation dissipates 150 W; without fan the heatsink exceeds 120 °C in < 5 min [Elektroda, k4be, post #21372863] • UA741 over-voltage may latch high output, driving full current into DUT.
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