5VDC from USB comes out at 12V plug

[DL9SAU]

Hello,

usual setup: Cable from tablet to mini-usb-plug. Powers the radio and communication over usb-serial data. -> Have fun with ft8cn.
"By accident" I measured 5V (coming from USB) at the 12V plug of tr-usdx.
(To be more exact, abt 0,25V less than thow voltage at USB). I was surprised.
Is this intended, or is it a hardware bug?

Long story:
I experimented with a USB-C--trigger-module for connecting my powerbank to the 12V plug. USB-trigger tells the device likes 12 V. -> The power bank delivers directly 12 V; no step/up converter needed (and thus no noise from those parts). My USB trigger has an LED.
After I disconnected the cable from the USB-trigger to the powerbank (while my tablet was still using the USB connection to tr-usdx's usb-plug), the led of the USB trigger module did not go off. -> There had to be a voltage coming out of the 12V port of the tr-usdx. I measured -> yes, 5V from the usb-port.
I don't feel well with that, because the usb-trigger-chip is re-powered over it's output line from the tr-usdx-12V port. I had luck the usb-trigger-module tolerated that.
It also explains the strange behavior of the usb-trigger module:
it worked ok if tr-usdx was not connected to (and thus powered from) the tablet.
But the usb-trigger-module refused to work if I powered the TRX via tablet and afterwards tried to plug in the usb-trigger-module to the 12V line. It refused to tell the power bank to power up the line.

Vice versa: Voltage via the12 V plug never lays on at the usb-connector -> this is good and expecgted, else usb-hosts may die.

vy 73,
- Thomas DL9SAU

[DL2MAN]

This is normal behavior, as USB is protected from 12V by a simple Diode.
In case 12V is present: 12V is supplied to the whole 12V line and 7805 supplies 5V line
In case only USB Voltage is present, USB Voltage is supplied to 5V and 12V Line.
Only L1 seperates USB_5V from 7805_5V
Look at D3/D4 in Schematic:
https://dl2man.de/wp-content/uploads/20 ... _1_2-1.pdf
Even though it is a "dirty" solution, it made Voltage priorization happen with very few parts.

73 Manuel; DL2MAN

[DL9SAU]

Thank you for your information.

I admit I'm not an electronic expert (but I understand the schematic you pointed at). I just have a bad feeling offering 5V to the output line of a power supply (which, in my case with the usb-pd-trigger-module, does not work properly).
-> I decided to add a diode and the usb-pd-trigger-module is happy.

vy 73,
- Thomas dl9sau

[pe1nnz]

Hi Thomas,
This is not something to worry about.
While you CAN measure 5V on there on the power jack, you cannot really use it (e.g to lit a lamp or create further damage in the ps) as the current is extreme less.

The low current is the diode leakage that is in reverse mode.

Hope this helps understanding where it comes from.