KVM Multiple Power Sources - USB + DC (USB Backfeeding power)

Level1 Store KVM Helpdesk
1

KVM Model:

2 Port Triple Video KVM

EMI Model No: DAE7B58B47

PAAG-E3132B

USB Interface (5-gigabit or 10-gigbit):

5gbps

Problem:

tl;dr: Is it safe to have power plugged into the KVM via the included power adapter to the 9-12v DC power adapter AND have power provided to the KVM via the USB-3.0 A “R” port?

I have the L1T KVM and a Coolgear CG-U3MINI7PH (7 Port USB 3.2 Gen 1 Mini Powered Hub w/ ESD Surge Protection). I’ve noticed that the USB hub is able to power the KVM entirely without the DC barrel jack plugged in[1]. I was quite surprised to find out that the USB hub could power the KVM entirely, and have been concerned that this is unexpected and that the two power supplies could “fight” each-other[2] and end up killing components.

Is there anything to be worried about here?

Other Notes:

I haven’t actually had any obvious problems with running this configuration[3], but I’m still concerned that I’m doing something “bad” :sweat_smile: I did look, but I haven’t been able to find any existing clear answers to this in the forum.

Here’s a diagram of the whole chain:

828×1848 57.6 KB

Diagram Source 
flowchart TB
    %%% # Relationships
    %% note: the link id's `pp` means "power path"

    %% ## Mains Power to adapters
    Mains pp1@-- mains power -->
    UPS pp2@-->
    PowerStrip
        
    %% ## Adapters to devices
    
    %% ### Mains Power to USB HUB
    PowerStrip pp3@-- 120vac -->
    DC5V("5v DC Adapter (CG)") pp4@-- 5VDC -->
    USB-HUB

    %% ### Mains Power to KVM
    PowerStrip x-. (currently disconnected) \n 120vac .-x
    DC12V("12v DC Adapter (L1T)") -. 12VDC .-x
    KVM

    %% ## How the USB Hub is connected to the KVM
    %% Note: Power seems to be traversing this connection, and powering the KVM
    USB-HUB pp5@<-. USB3 (Power) .-> KVM
    
    %% This is the "true" direction of the USB link.
    %% KVM -. USB3 .-> USB-HUB

    %%% # Styling
    classDef powerFlow stroke:red,stroke-dasharray: 9,5,stroke-dashoffset: 900,animation: dash 25s linear infinite;
    class pp1,pp2,pp3,pp4,pp5 powerFlow;

    %%% # Node Definitions
    %% Main Devices

    %% %% Subgraph with empty name just helps the graph look better
    subgraph KVM-Graph [" "]
        USB-HUB[USB Hub] 
        KVM
    end

    %% Power Devices/Nodes
    subgraph Power
        Mains@{ shape: bolt }
        UPS["UPS \n (APC SMT3000)"]
        PowerStrip([Power Strip])

        DC5V
        DC12V
    end

    %% %% Commented out for simplicity, included for detail
    %% subgraph Desk
    %%     subgraph PCs
    %%         PC1 -- USB3 --> KVM
    %%         PC1 -- DisplayPort x2 --> KVM
    %%         PC2 -- USB3 --> KVM
    %%         PC2 -- DisplayPort x2 --> KVM
    %%     end

    %%     KVM --> Monitor1 & Monitor2
    %% end

    %% USB-HUB -. USB .-> USB-Devices@{ shape: processes }

  1. I, being buzy/lazy/lack of energy, have been running the KVM on just the USB HUB’s power for many years now, because I’ve been concerned that two power supplies would conflict :sweat_smile: ↩︎

  2. I don’t really have any substantive grounding for this concern, which is why I’m asking about it. ↩︎

  3. The most common issue I run into is devices not reconnecting to my linux system after switching sources, but I have been assuming this is an issue of my devices/linux/the USB hub, and not the KVM. ↩︎

2

Main power (red LED) and USB power (green LEDs) are different, and both provide power. When you say the USBs power the KVM, it should not actually power on the whole unit for video capability.

What your listing in your diagram sounds normal enough

3

I now realize I said “USB 3.0 B”, but I didn’t realize it was an A port. But regardless, I am driving the entire KVM unit, including the video capability, off the USB A port on the KVM.

image
image1000×483 123 KB

mermaid-diagram-2026-11-02-100341
mermaid-diagram-2026-11-02-100341470×1662 41.2 KB

4

Can you send a video of this, or attach a link to a video of this? Lmk if I need to elevate your permissions.

5

@Level1_Amber Sorry for the vertical video, that was what I could do :sweat_smile: (Video has no audio)

Video:

  1. Showing the cables that are powering it
  2. Unplugging the connection to the powered USB hub
    1. Monitor goes off, because KVM loses power
  3. Replug
    1. Monitor comes back, KVM recieves power
  4. Showing that the DC barrel jack to the KVM is not connected
  5. One last unplug, this time, removing the power to the USB Hub.
    1. Monitor & KVM goes dark, because power to USB hub was removed
  6. Replug power, KVM had some issues and started beeping because I was struggling to plug it back in fast/directly enough with one hand :upside_down_face: (the sound it made was . . . . -)
6

So I got with @wendell about this,

The USB-A port is backfeeding power into the KVM, which is always bad. Even if it continues to work like it is for you, you’re bound to cook the KVM sometime and brick it.

I’ll give another example. DisplayPort cables are literally physically 20-pin. However, the 20th pin is power, and it should never be connected. Good quality DisplayPort cables have only 19 pins connected.

Therefore, to use this USB hub safely into the KVM, you are going to want to purchase a cheap adapter that blocks power:

Edit: I have never used this before, if the power adapter works for you, can you update me on this to confirm?

Thank you!

7

Got it, this is exactly what I was worried about :sweat_smile:

I’ll see about this, I believe I’ll have to find one that supports the extra pins from USB 3[.2 Gen 1] though I’m not sure it’ll work :confused:

For now, I have unplugged the USB Hub power and switched the KVM to power.

1 Like
8

Questions:

  1. Since it’s back-feeding, any thoughts on having both power supplies connected? KVM and USB HUB?
    I was thinking they’d “fight” each other inside the KVM if the KVM wasn’t isolated from the USB power (which since back-feeding is able to power the KVM, I assume not)

  2. Any DIN mountable, mountable in general, or otherwise “industrial” USB hub’s (or brands) y’all would recommend?

    I’ve seen the Anker recommendations, but I’m hoping for other options

    I’m a bit wary of Coolgear now :confused: