FS: 2 x 5V linear supply for Raspberry Pi

[smallangryboy]

2 x 30VA5VDC Kits

I purchased these from Paul at ReelAudio to power a Raspberry Pi and DAC. They are unused (untested). Paul is a meticulous engineer and the boards are made to a high standard. They are now surplus to requirements as I've gone a different route. They can deliver 5V @ 3A but have a variable adjustment so can be tweaked. Included are fuses and fuse holders, standoffs etc. basically everything that came bundled with them. They were over £200 for the pair.

Asking £150 + P&P

25122 25123 25124 25125 25126

[Yatsushiro]

PM Incoming...

[smallangryboy]

NOW SOLD

[Firebottle]

I know it is bad form to comment on a sales thread but these will not do 3A.

Look at the transformer rating, 2 secondaries of 1.25A = 2.5A in parallel but AC.
Take the transformer utilisation factor and you are looking at about 2A.

This is a very common mistake with Chinese psu's as well.

[smallangryboy]

In comms with Paul who made the boards for me:

Well here is a pic of 2 boards that will each deliver 5VDC @ 3Amps through to 15VDC @ 2Amps.
The mini blue pot allows the adjustment.
Obviously, the Torroidals and input/output wires are not yet fitted - but would be along with a kit of:
Mounting screws and pillars.
Heat sink fixing screws
2 x 20mm fuse holders with fuses.
2 x Earth lifter toggle switches.
Leds with Holders.
Wiring diagram of inputs (Mains) and output wires.

[Yatsushiro]

I know it is bad form to comment on a sales thread but these will not do 3A.

Look at the transformer rating, 2 secondaries of 1.25A = 2.5A in parallel but AC.
Take the transformer utilisation factor and you are looking at about 2A.

This is a very common mistake with Chinese psu's as well.


I may be being too simplistic here, but these are 30VA transformers, which ties up with the 2 x 1.25A @ 12v secondaries (2 x 12 x 1.25 = 30). Does it not follow that when delivering 5v, the maths suggests a theoretical 6A, and therefore a real 3A?

[Firebottle]

No you can't get 3A from parallel secondaries rated at only 2.5A.

If the transformer secondary is rated at 1.25A, as in this case, then 1.25A is what you can take from the transformer without it being overloaded. This is AC, alternating current, unfortunately you cannot equate this directly with the required current at DC, direct current.

There is a factor that comes into play called the TUF, transformer utilisation factor. When rectifying to DC there is also a loss associated with the rectifier topology.
In the case of a bridge rectifier these two factors are equal so the maximum DC current that can be taken is 81.2% of the transformer AC rating.

[Yatsushiro]

Thanks Alan.

As an aeronautical engineer I still find electronics a challenge...

Assuming all else being equal, if I were to use an alternative 30VA Vigortronix transformer (VTX-146-030-206, 2 x 6Vac, 2.5A), would that get me nearer to 3A?