Portsdown/Pluto powering related questions

[f5iwn]

Hi,
I am currently working on a “portsdown” 4&Pluto based DATV project so I collect the needed parts and try to put all the key points on the table … I already read a lot of things and run some basic testing …
I need some clarifications on the powering of the system: I analyzed a lot of discussions around this topic:
- Cheap power supplies which could lead to severe issues for the equipment’s
- Same kind of troubles with USB hubs
- And finally some discrepancies between the RPI4 and the Pluto: RPI requires 5,1 V and Pluto rather requires 4,9
I aim to have something usable in /p conditions so powered with 12V
So why not using some bulk converters (DC-to-DC converter with adjustable output voltage) ?
We could have one converter for the RPI and another for the Pluto:
- This allows to adjust the voltage in a distinct way
- This provides a comfortable margin regarding the current which can be delivered
Has someone investigated such thing ?
Is there any known issues with such bulk-converter ?
In advance thank you !!!
73
Christophe F5IWN

[Basil]

Be careful, some of these Chinese buck converters fail with the full input voltage on the output terminals. They can also have a noisy output and appaling multi turn pots that have minds of their own.

Finding quality adjustable ones is hard. You might consider building your own with conservatively rated parts, a decent heat sink and quality capacitors.

[g0mjw]

I have used switching regulators from 12V to 5V quite a bit. In the original minitiouner boards I added a 5.7V Zener diode to protect against that sort of thing. The concern there was failure of the adjustment pot but it is the same idea. I didn't have that in the PicoTuner, maybe I should have but there is no pot and the LM2576 is pretty reliable, as long as you use a genuine one.

On my PI4 GPIO board I included a 1N5339B, a 5.6V 5W Zener. That's one reason why it needs a fuse, there are other reasons, like reverse polarity or a short, but basically it needs one. The regulator can be set for 5.1V for the PI4. A similar regulator could be used for 4.9V for the Pluto, but that's another regulator. If you are using an Ethernet adapter, that's the way to do do it,

If you just plug the pluto into the PI4 USB socket, it is going to see the 5.1V powering the PI. In that case it is probably easiest to modify the USB lead to put a series diode there. A suitable diode should have a fairly low forward voltage drop, a Schottky diode like an SB130 will drop about 0.5V so the pluto will only see 4.6V. It will probably be OK but if that's a bit low, or there is a significant drop in that cheap USB cable you found in the junk bin, you might get away with a simple restive dropper, about half an ohm or less.

Mike

[Basil]

Good points Mike. What happens if you parallel two diodes, is the voltage drop halved?

[radiogareth]

I have salvaged suitable diodes from dead buck regulators (usually fake Hi V-in modules that expire within minutes of meeting 50V, they will have been selected for low Vf but any 1 amp Schottky diode (check the data sheets) works well. I run my Portsdown at 5.25V and feed the Pluto via a decent but hacked (diode in +ve line) USB lead. It ended all my 'pluto not found' messages.
Gareth

[g0mjw]

Good points Mike. What happens if you parallel two diodes, is the voltage drop halved?

No, well not quite no. Each diode has a curve and the voltage drop depends on the current, so for example the aforementioned SB130 diode:

Screenshot 2024-12-10 113717.png (35.91 KiB) Viewed 809 times

drops 0.45V at 1A but only 0.4V at 300mA so there is a little difference. In F1 that would matter, but here, not really. Also note the higher voltage diodes drop more for the same current. That matters with a switching regulator, don't over specify the reverse breakdown voltage.

Mike

[G3GJA]

I run my Portsdown at 5.25V and feed the Pluto via a decent but hacked (diode in +ve line) USB lead. It ended all my 'pluto not found' messages.
Gareth

I've found that the Pluto is quite happy on higher voltages provided the dedicated power socket is used to power it. Just disconnect the +5v red wire in the lead to the main USB connector and then power it from the other micro USB socket.

This has been used in GB3EY since January and has not misbehaved.

Clive G3GJA

[Basil]

Mike, thanks for the paralleled diodes explanation. Appreciated.

[f5iwn]

Many thanks for all these explanations. Appreciated. I will analyze them carefully !
73s
Christophe F5IWN