Re: 12v->5v noise


I haven’t had THAT happen yet.  Our local social-media-over-ham-radio project has about 40 of this model. I can attest that the output voltage display isn’t accurate though, as bad as +/- 0.2.  There is supposed to be a calibration procedure but I’ve never played with it.  The good news is that the output voltage doesn’t change on its own.  Once you set it to 5.1 or 5.2, it stays at where you set it.  I have a few USB test devices, one of which can put up to 4 amps load on a USB source, and others that measure the voltage and pass-thru current.  I recommend getting a USB voltmeter.  
Here’s an example of one for $12.  The 2 units I have weren’t anything special.
This will give you a clue of how bad the voltage swings on a USB cable when real current is drawn.  If you are getting the lightning bolt icon on your Raspberry PI GUI, this will explain why.  USB cables, unless they specify the wire gauge (and even then) use really crappy wire.  What kind of voltage drop would you expect drawing 3 amps across a 36 inch  24 gauge wire? 

This is a useful Linux command if playing with Raspberry PI supplies:

grep -a Under-voltage /var/log/syslog | tail -10


Tadd / KA2DEW
Raleigh NC  FM05pv

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On Sep 24, 2020, at 10:50 AM, N5XMT <dacooley@...> wrote:

Don't trust the voltage display on those.  Several people on our reef-pi forums for aquarium controllers set the display to 5v, then powered up the pi with it and smoked the pi... One was putting 12v out when the display showed 5v

On Sep 24, 2020, at 07:14, "Tadd KA2DEW in NC via" < target=_blank>> wrote:
A Raspberry PI 4B can only accept 3 amps on it’s USB-C connector.  It does desire 5.1V though, not 5V.  
It’s tricky finding a 5.1V USB source that isn’t 110VAC powered.  

I have been using a switcher supply made by DROKing in the far east. 
This is a $15 or so device which has a multi-turn pot to set the output voltage and accepts a wide range of DC voltage on the input.  
A key feature is that when main power is turned on, the unit immediately outputs the desired voltage to the USB connector.  Some of the variable output USB converter items power on with the USB output disabled.  

The bug with this particular unit is that it appears that DROKing has discontinued it in favor of a menu driven device (at the same price) but which powers on with the USB output disabled.  However, the unit continues to be available in various places including Amazon.  However, for a while the ISBN number kept changing and so any link to the offering seemed temporary.  That sounds like somebody is cloning the device and it isn’t DROKing anymore.  Dunno.  Maybe DROKing changed their minds when the Internet blew up after they discontinued it?  

I haven’t had any particular noise on HF from this, though I live in a suburb and have plenty of noise already.  YMMV. 

Tadd / KA2DEW
Raleigh NC  FM05pv

North Carolina Off-The-Grid Social Network:
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On Sep 24, 2020, at 12:38 AM, Tom Flynn via < tflynn6693@...> wrote:

 I think a 7805 puts out 1.5 amps his device puts out 15.  Does he need 15 amps?

On Sep 23, 2020, at 10:45 PM, Siegfried Jackstien < siegfried.jackstien@...> wrote:

If you have enough power... Use an7805
That sure is quiet even if it wastes some power in heat...
Sure its possible to make a switcher quiet... Meanwell psu are really good... But for shortwave i would additionally add some filters and ferrites.. If a switcher used... 
Or use 7805... Grin
Greetz dg9bfc sigi

Am 24.09.2020 05:16 schrieb Randon Loeb < randonloeb18@...>:
Was all excited to try my remote shack (rpi3 controlling ft-891 over wifi) but unlike my in-house testing with pi powered off a wall wart, when I power it off battery through a 12v->5v converter I get noise that (at least on 20m) raises an s1 floor to s9. Basically the rig is in my car powered off a lifepo (not the car battery), I mention because space is tight.

I believe it is mostly coming from the converter, this one:

It seems to send noise through cables connected to it (well technically the usb power hub I made), but also through the pi and into the radio via the usb cat cable. But any cable plugged into seems to sort of act like an antenna.

If I power the pi off of a 5v cellphone battery pack, noise drops down (seems like maybe a little, but at least S6 floor drop). As I say, I tested this pi in close proximity to the radio before off a wallwart, so I don't think pi itself is creating the noise.
Only thing I want to mention is that I created my own little USB power hub to connect to the converter using some jacks I have, and I assumed it is correct or at least harmless to ground the usb jack cases, was that wrong?

For now I am assuming the converter is noisy.

Anybody have experience with that one? Is there a better go-to? Besides ferrites (will try in the morning) any other way to quiet this one down? I have a few buck converters lying around and will see if I can come up with something, but open to any suggestions.
Randy KN4YRM

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