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Old 15th April 2019, 12:49 PM   #1
Salas is offline Salas  Greece
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- We many times wonder what if that SMPS brick that feeds the Raspberry Pi or the mini PC or the MiniDSP or the Squeezebox or the DAC or the desktop Class-D amp etc. was a linear PSU? How such wonder boxes would perform without switching noise polluting their rail and most crucially their many times interconnected ground?

- What if we had a simple and strong linear PSU instead of fixed SMPS brick adapters that it could cover the voltage range for such applications by only setting it up with a jumper?

- Enter the L-Adapter. A versatile Sziklai pair stabilizer / capacitance multiplier based on LED voltage reference.

- Its output voltage is how many LED + trimmer minus one Vbe. Roughly 1.5V to 20V range. Move the jumper, trim Vout, ready. Connect the load.

- Is it any good? Yes its good. Not noisy at all and stable. For 1.5V input ripple it produces 1.5mV thick DC line. For 3 Ampere load it measures 0.02Ω output impedance. Which is even and extended in frequency. When you pulse it the recovery is clean of ringing. Because there is no feedback between the output and the voltage reference. We want a general purpose PSU insensitive to random load peculiarities.

- The recipe isn't anything new but the details are well researched. Low noise unity gain reference, but adjustable too. Which LED bar with which CCS experimentally chosen between many styles for very good Vref stability, what pair of transistors, the layout. Various ways to sink it, accepts quality TO-220 bridge diodes, two reservoir capacitors, fused like a π filter. The board is 136mm x 63mm.

- What about its output current ability? Well, it uses a 15A audio amp grade TO-3P pass transistor. But that says nothing much. Its the transformer the diodes the reservoirs the sinking the load's average consumption. Say up to 7.5A average can be catered for.

- For any light or heavy current application it takes that the chosen Tx the bridge diodes and the reservoir caps won't lose the plot for a target Vout. That's about rectification and filtering basics. It does not like less than 2.5V input-output voltage difference. That's the DC difference between C4 and the output. Can probe that between the fuse and V+ out. Although it keeps working on smaller differences it gets progressively goofy. If you see the LEDs dimming a bit its tell tale you crossed the raw DC section's losses good limit.

- Here is a schematic with typical reservoir caps values and some pictures. That soldering iron pulled 55W peak from the mains through the PSU to boot and idled at 12W. The scope pic displays Vin ripple on C4 vs Vout status captured at a point when the iron was still pulling hard to heat up from room level cold.

The Fluke reads C4's raw DC level in another picture. Started at 27V idle with worst loss of 4V during the soldering iron's boot cycle (trafo, diodes, ripple, total). At 23V raw DC for 18V output to the iron, that trafo and reservoir caps passed the 2.5V Vin-Vout criterion by double margin. Other type & quality trafo or diodes could lose more or less steam of course. Higher value reservoir caps would achieve less ripple but would also make the diodes work harder. Since the worst raw DC level sufficed, better not increase the caps value in this case.

The example has 4xMUR860 & 2x4700uF/35V B41231 EPCOS/TDK. Also 38mm tall Q4's sink. Which sufficed due to the irregular current pull of the micro controlled soldering iron. Can it do an RPi3? Yes I tested it with Wi-Fi and streaming vids on the Raspbian OS. Can it do a 12V Windows 10 Cherry Trail mini PC? Yes I tested it. Watched a movie from an attached 2.5 inch USB HD as it was charging my 1+ X phone from another USB output so to push it. I used smaller voltage and size EI or toroidal of not that great quality on those. The sinks sufficed again because of the irregular current pull of computers with idling gap periods also. 45C on the 38mm 35C on the 25mm ones for the diodes. Minimal RPi use wouldn't even need diode sinks.

- In the final board Q3 Q4 will be Q1 Q2. Their now numbering is a relic of various discrete CCS tests instead of IC1.

- I will be editing & enhancing post #1 in later installments.
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Old 15th April 2019, 01:19 PM   #2
dewdrop is offline dewdrop  Singapore
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Subscribed, I can't wait to test this on my Pi.
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Old 15th April 2019, 01:22 PM   #3
Salas is offline Salas  Greece
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A 9 Vac 30VA transformer and naked bridge diodes should suffice for a music server Pi.
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Old 15th April 2019, 01:25 PM   #4
dewdrop is offline dewdrop  Singapore
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I am all ready to swap out the LT3042. No pressure but when? ;-)
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Old 15th April 2019, 01:35 PM   #5
Salas is offline Salas  Greece
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It's ready. Next GB. It could probably be likeable enough but its main strength is Swiss Army knife linear PSU.
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Old 15th April 2019, 02:11 PM   #6
alexkosha is offline alexkosha  United States
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Subscribed too. Canít wait to test it... .
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Old 15th April 2019, 02:23 PM   #7
Salas is offline Salas  Greece
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Very handy, and it has nice lights too
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Old 15th April 2019, 02:25 PM   #8
wcwc is online now wcwc  United States
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Is the Ultrabib still better for the lower current needs?
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Old 15th April 2019, 02:28 PM   #9
jackinnj is offline jackinnj  United States
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In a similar vein, you can bias a pair of MOSFETs the Blue LED will get Q2 into its linear region -- this is an adaptation of "Art of Electronics", Volume 3. Figure 9.110. You could use a resistor to bias the LED at the expense of PSRR. If the resistor values are successively doubled, you can get 8-bit resolution.
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Old 15th April 2019, 02:32 PM   #10
Salas is offline Salas  Greece
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Quote:
Originally Posted by wcwc View Post
Is the Ultrabib still better for the lower current needs?
The UBiB is a successful shunt PSU. Not targeting the same apps. Its mainly meant for internal use. Still better in its own envelope. This one isn't a crude series PSU though. Not just a handy external SMPS replacement option for packaged gear, so to simply avoid switching noise without shunt inferno when the current needs are high and irregular, it has the DCG3 preamp's DCSTB quality level too. It should clean up and enhance such gear.
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