L-Adapter

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S

shattered_dream

Member
Joined 2009
My Salas Adapter..... MVIMG_20200829_213903.jpg
 
K

Kiq8787

Member
Joined 2020
Salas said:
DVM will show no AC voltage when secondary instantaneous polarities are paralleled opposite. Over-current and heating will be happening already if testing with high energy i.e. with the primary on mains. Dim bulb tester is safest way to know paralleling is wrong in this case. Big glow tells you to reverse one secondary while current limiting opposes high current draw to happen. Dim Bulb Tester

To know safely before hand with generator and scope, feed 2.3VRMS 50Hz via 1k series resistor to the primary*, probe the secondaries, one for each scope's channel. When sinewaves align on the screen you know each wire on a probe's nose is in phase. Put a dot on them with a marker pen. When paralleling secondaries connect dotted cables together and non dotted cables together.

Example pictures from an R-Core of mine. Blue and violet proved in phase. Paralleled secondaries are the last picture. When I reversed wire colors mating, the yellow sinewave vanished and became a flat line. If colors were the same for each secondary pair I would have marked dots to distinguish.

*With that test signal each 10mV RMS scoped on secondaries will represent real life 1V RMS from unloaded secondaries when the transformer is for European Union mains. For USA mains transformer use 1.2V 60Hz signal.
Click to expand...

Hi Salas,

They sent me the correct r-core now. From your great instructions, I will check wire phase on secondaries. What I’m not sure, there are 2 sets of primary coils for 115V. How can I be sure both sets of primary coils are in phase when I tie them in parallel?

So this one has 2 primary windings 115V and 4 secondary windings for the 9V.

Thanks :)
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
Primaries should be reliably color coded. When having gen and scope you can also verify in reverse i.e. feeding signal to a secondary via 1k series resistor and scoping the primaries. 60Hz 0.9V should step up to 11.5V in this case.
 
I

InSides

Member
Joined 2004
Salas said:
Yes 500mA is not much. Means only 1.25W dissipation for 2.5V across Q2. Just have a thick enough case with ventilation holes placed in a way to aid air circulation through it. It should thermally support those relatively light running L-A's without drama.
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So here is (another) of my stupid questions. Found a suitable Conrad low-profile heatsink and I fit the Q2 via very short wires (4cm) to it so I can fit the L-Adapter in a 1U box.

Turns out that between the top plate (3mm thick aluminium, anodized) and the top of the heatsinks for the diodes and the capacitors, I only have around 3mm space (perhaps 2.5mm).

The top is riddled with cooling slots, and the aluminium is anodized, but should I be worried about such proximity?

Many thanks - waiting for the IEC inlet and I can feature the final build.
 
S

sumeshak

Member
Joined 2014
L Adapter for Rpi Zero

Hi Sir,
I am planning to make an L Adapter for powering Rpi Zero.
Assuming a max current of I_load = 500mA,
Capacitors = 4700uFx2
Diodes = MUR460

Transformer = 0-9V/1A (approx 10VA)

Substituting in the formula
Vpk-pk ripple= I_Load /2*f *C (use Volt Amp Hz Farad)
we get
Vpk-pk ripple= 0.53V



Should we add 100nF cap across the reservoir?
How about adding 10nF cap parallel to the ultra fast diodes?

Request your attention and comments.
Thank you!
sumesh

Salas said:
Yes they will be sufficient for the Rpi. Their combined ripple current ability is very capable. 2x4700uF are no slouch either especially for 1 Ampere.

Rpi is not a demanding load for the L-Adapter. For knowing more on rectification & filtering dynamics vs loading given the wider range of applications possible with this PSU read below:

Things are relevant to some factors. I was able to power a 55W peaking iron with just 2x4700uF for instance. But I had 18V good quality transformer rectified to 27VDC unloaded (that Tx was doing higher than its nominal). With 18VDC output setting in the L-A there was much Vin-Vout voltage space available to be partially eaten by the AC ripple voltage created when with not so high reservoir capacitance under high power demands.

What we look for is not to use too high capacitance when the loading targets cause acceptable ripple voltage, still allowing for no less than 2.5V Vin-Vout margin. So not to have great inrush currents and bridge diodes transient strain for no reason. Not too high Vin-Vout under load also. Because it creates more dissipation. But not to have high ripple voltage to be filtered by the circuit either. Say no more than 1.5V peak to peak in average. Its a juggling act if we want to sit at a most efficient point all things considered. Factors are many but we can roughly predict ripple voltage figures from just a few. The ballpark formula is:

Vpk-pk ripple= I_Load /2*f *C (use Volt Amp Hz Farad)

Don't forget f=50Hz or 60Hz (EUR USA). Also C=C3+C4 in our circuit. For example when C3+C4=20mF plug 0.02 Farad.

A true RMS multimeter can read ripple's AC RMS value when you don't have an oscilloscope to see peak to peak. Red probe on fuse's bottom end (the one near the main sink) and black on the 0V output. Multiply the reading by 3.3 for pk-pk because the waveform's shape is not a perfect sawtooth (x3.46). The actual Vpp result can even prove better than what the ballpark formula predicted.

When subtracting two diodes Vf loss and Vripple pk-pk for a given max load current and the reservoir capacitors used, we want our transfo to provide for all that plus 2.5V Vin-Vout DC left for the pass transistor to work well in worst case.

edit: Revised the post to more detail.
Click to expand...
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
Hi

You can surely use nF bypasses without fear of risk because they change nothing in the basic electrical parameters. But they may introduce resonances or slow downs. The subjective reports about L-A used as standard with Rpi are numerous in this thread and all positive.

More interesting procedure in my opinion would be use it in standard form for a few days to familiarize yourself subjectively and then try bypass caps to decide if they offer something to your liking.
 
S

sumeshak

Member
Joined 2014
Thank you Sir,
Request to verify the following component selection:
=============================
L Adapter for powering Rpi Zero.
Assuming a max current of I_load = 500mA,
Capacitors = 4700uFx2
Diodes = MUR460

Transformer = 0-9V/1A (approx 10VA)

Substituting in the formula
Vpk-pk ripple= I_Load /2*f *C (use Volt Amp Hz Farad)
we get
Vpk-pk ripple= 0.53V
================================
sumesh
 
M

michaelagiles

Member
Joined 2016
Troubleshooting re LA w/Rpi

Good Morning Gentlemen! (and Ladies?)
I built a LA to power an Rpi4 w/ Boss DAC. Originally the Rpi was connected to an USB , externally powered HDD and it worked well and sounded heavenly! The HDD packed in and I substituted a bu drive powered from the Rpi - temporarily !?.

Soo it started misbehaving - difficult to contact etc. Eventually failed and produces a large "Thump - Thump " through the system.


Output V is still 5.01vdc unloaded, but INCREASES to 5.15 vdc when loaded!
Any clues about where to start looking for the failure?


Thanks!
MG
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
See first if Vin - Vo voltage in the reg ever drops lower than acceptable (details in the pdf guide). Then what is the voltage at end side of output cable at the load in high current demand phases. Thicker/shorter wire maybe needed. Also if you got a scope see if the output rises due to some instability addition of AC waveform on the DC line i.e. oscillation. Could be a matter of application to an increased load than an outright failure.
 
C

CallMeMike

Member
Joined 2016
@michaelagiles

Not sure what a 'bu drive' might be... back-up drive...?... but I've had various issues with devices attached to RPi SBCs, devices which were fed 5VDC via the RPi's USB port (as well as data).

Almost without fail the problems were caused by the power draw of the device being greater than the RPi's USB power capability or the poor quality of the PSU (the claimed power being overrated) or a fault with the PSU.

For a quick test I always have around a simple but reliable PSU and an USB-to-USB cable modified for feeding DATA and 5VDC separately.
 
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