FlightAware Discussions

DC-DC converters (for running your Pi)

Triggered by the discussion of DC-DC converters I decided to trough together a test setup and see what really happens.

The regulator module shown is a cheap chinese made unit using a LM2596 (possibly real, probably a clone).
These are cheap, readily available and very convenient.

I’ve use 6x 3R3 resistors in a 2x3 config to give 2.2Ohm load
I’ve set the output voltage to 5.3V to allow for plenty of headroom through a Pi.


Parameters:
Input:
Bench PSU providing 15.0V, 1.15A (17.2W)

Output: 5.3V
Load: 2.2 Ohms
Current: 2.4A
Power: 12.8W

Result: Heatsink Temp: 91C

I then repeated with a more realistic load (2x2 3R3 resistors)

Bench PSU providing 15.0V, 0.75A (11.2W)

Output: 5.3V
Load: 3.3 Ohms
Current: 1.6A
Power: 8.5W

Result: Heatsink Temp: 65C

Something to remember is that this reg isn’t in a sealed box in the full sun.
De-rate accordingly.
The conclusion is: don’t put you finger on the reg.

1 Like

Industrial Power Supply 36V-75V > 8.2V 45A
Voltage converters, compact, powerful, galvanically isolated
for
$10

Output voltage is determined by a single resistor:

The model with an output of 8.2 volts can be adjusted in the range of 3.4-8.7 volts with 2kΩ and 10 kΩ resistors:

Impressive specs and much better build quality than those I’ve been using, but (for me at least), the input voltage is a killer.
I have 24V to play with. Changing that would be impractical.

Input: 8-35V
Output: 5V 8A

Price: $5


1 Like

I did buy a board very similar to those, but didn’t end up using it.
I bought a pair of these (one was DOA)

I used a linear regulator chip because I considered the amount of RF noise those switched power supply chips can push out.
I feed it only with a moderately high voltage 9V, so the power dissipation it is not an issue… For the 100mA my preamp is using.
This was one of the many reasons why I choose to send the RF signal away, on a cable, to be processed in a controlled environment, instead of trying to setup the Pi at the antenna and send the network cable to the router.

1 Like

Without a doubt, a final stage linear regulator is the easiest way to minimize ripple on the output.
However I’m not sure how much benefit it will be when feeding an SBC with a dozen clocks onboard all generating their own noise.
The RTL dongle has it’s own linear regulator (AMS1117-3.3 or similar) which leaves the preamp - that should have a clean supply (or failing that, well filtered).

For my site using the regs above, I only have 24VDC available, so can’t use a transformer or wall wort to get the supply close enough to use a liner reg. (24V to 5V would create too much heat/losses)

Like I said, I am not feeding the Pi from that lin reg. Just the filtered preamp, via coax.

Here is another I’d forgotten I had.
I ordered 5V USB PoE adapters, but 12V 2.1mm adapters were supplied instead. While I was waiting for the correct parts, I used this to provide 5V.
The display cycles between output Voltage and Current.

Right, this isn’t good and I need to do something about it.

I’m using a PoE injector along a 25m length of CAT6 to my feeder up the mast.

In the box is a Pi3B+ with a fan connected to the 5v rail and an Airspy Mini plugged directly into the Pi.

I have a one wire temperature sensor on the 3.3v rail (although this has stopped working) and a bias-tee injector connected to the 5v rail on the Pi which is powering an RTL-SDR LNA.

I haven’t slapped a meter on the output of the PoE injector to see what the actual voltage is because it’s up in the air and anyway, it’s not doing it continually. The actual injector itself feeds 48 volts at 1 amp up the cable which seems to be the standard figure. Either the regulator at the top end is breaking down or there’s simply not enough current available and the voltage is dropping.

The PoE box at the top has three possible voltages out, the first thing might be to try selecting nine volts and then somehow regulating it down to a good 5 volts but if that fails then the only solution will be to run a separate cable up with 13.8v on it from my 30A Daiwa shack PSU and regulate that down.

[Sat Jun  8 12:38:14 2019] Under-voltage detected! (0x00050005)
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[Sun Jun  9 13:02:48 2019] Under-voltage detected! (0x00050005)
[Sun Jun  9 14:07:07 2019] Under-voltage detected! (0x00050005)
[Sun Jun  9 14:49:06 2019] Under-voltage detected! (0x00050005)
[Sun Jun  9 14:51:04 2019] Under-voltage detected! (0x00050005)
1 Like

I have found that the 3B+ is more sensitive to POE issues than older models.
Ensure that you have a quality POE switch/injector and POE adapter.

If you case can handle it, try using the RPI POE hat. It has a built in fan but only works with a few cases.
I managed to put a heatsink under the POE hat fan.

That will reduce the amperage on the output side of the power supply and should help unless the device is just bad.

Then you can just use a buck converter that’s a little beefier for the actual 5V output.

Optimally one with an adjustable output, so you can fine tune it to 5.1V to compensate the power input circuitry on the RPi and the mini USB cable.

It’s not cased and has this HSF attached. The PoE injector was cheap and cheerful!

The PoE hat is rated at 15W (3A) which should be enough to power everything so it might be worth just grabbing one of those to try first.

I keep forgetting I have a bag of these already! Not quite a buck each, I think they were £1.95

Slap a heatsink on the device and that’ll be a lot quicker and cheaper than buying a PoE hat.

Even with adjustment.

Give it 9V from the bench, load it to 1A and tune it to 5.1V :slight_smile:
Should then be good to go into the box.

Pretty much, yes. Making sure I adjust the voltage again when it’s in the box though to be absolutely sure it’s right before I connect it to the Pi!

The plastic case doesn’t have good thermal bonding to the die and as poor thermal conductivity regardless.
Have a look at he underside of the PCB and see if you can fit the heatsink there without shorting anything.

1 Like

Good call. The bottom of the PCB is well lacquered and the heatsink is held in place by a thin ‘thermal pad’ (double sided sticky) as well so it’ll be slightly clear of the board. I’ll heatsink the underside of the board as well as the device itself.

If the weather holds off, I’ll do it this evening.

All done. It’s messy in the box but it’s all working. I set the voltage to 5.1 and it’s all back up and running. The temperature sensor is working as well which is a bonus. I’ll keep an eye on it over the next few days and see if I get any more under voltage or throttle messages.

Could voltage be added to the performance graphs? It might be a useful addition.
/edit - No, I don’t think it has any voltage measurement circuitry built in. Shame.

Replying to myself.

I started getting under voltage messages at around 12:00 today and you can see that the rate started bouncing up and down at about that time.


home-rate2

The rate went up after I sorted the power out.

So did my range.

home-range

These graphs make me happy.

2 Likes