Two 5v power sources and one to USB

[praveen_khm]

Hi,

I am designing a board which is powered through USB. The 5V, 500mA from the USB is more than enough to power the board. However, I have an additional board connected to it. The additional board uses an external power supply, flowing though a 5V regulator (xx1117) and powers it. Now, what happens if 5V from the second board is connected to 5V USB board? There are no diodes across either boards to avoid reverse voltage.

The connection is a simple pin-to-pin connection where 5V from external supply is directly connected to 5V USB supply which is in-turn connected to a computer. Now, what happens? Does the higher current from secondary supply flow into the computer's USB and kill the port? Or it makes no difference apart from increase in current? If there may happen any damage, how to avoid it?

Kindly help.

Thanks,
Praveen

[praveen_khm]

Bump~!!
Just to get to the top again for someone to check and help.

[rgawron]

Is positive rail of one device connected to positive rail of the second device?

If your two devices share only mass and signal lines, then they share only signals in the signal lines, not theirs power supply voltages. It should be fine.

If at least one device has a mass connected to ground (your computer have it if it's not a laptop running on a battery), both devices will be grounded - this .

[praveen_khm]

Both the boards share ground and power lines. One board sits on top of another board and 5V from first board is connected to 5v of second board while each has its own power supply (one from USB and other from 5V regulator)

[jeroen79]

Most likely:
If you connect two powersources together then the one with the higher voltage will provide current to the consumers.
The one with the lower voltage would not provide current as it's output is too high.

That way you could endup with the USB powering both devices.
If you do not want that then you could add a diode to stop current flowing from A to B.
Or disconnect the 5V lines of A and B and only have them share the ground.

[Ian.M]

That configuration is strictly forbidden by the USB standard.  The *ONLY* time it is permissible for a device to supply power to the host is when an OTG host and device have negotiated host charging in compliance with the USB Power Delivery (PD) v2.0 specification.   The easiest fix, if the external supply can power both boards, is to disconnect Vbus, although, depending on the device chipset and USB stack used, you may need to be able to sense if it has voltage on it for proper initialisation.  Otherwise you need to split the power to the two sections and handle the cases when one or the other is not powered by preventing either outputting logic '1' if the other is not powered.

[Lovely_Santa]

If you are driving it from an USB connection, so I suppose it's in a PC? (explains also the 500mA)

BUT... If you know a computer, it's all driven by an ATX power supply like an example below:


It has standard connections, Just buy a splitter, so you can split the power out of it (most likely one for DVD driver or something like that) so you can get a power line with 5V power (red color) without a problem...
So then you can easy use 5V without blowing your USB port by accident...

Just a quick... it's the same power as the one comming out of the USB port...

On topic: If you want to power something, you need to know if it's a power supply that can source only or can sink current on it's output... The power supply in a computer can't do that (only source). If one of the 2 power supplies can sink current, then you can have a problem, becose when there is a different in potential, it will sink the current to maintain it's voltage level...

Example: the usb give 5.02V and your other supply give 4.98V... If your other supply can sink current, current will flow out of the USB driver, into your other supply... Not a huge problem, your devices will still be powered, but you are only able to drive 500mA out of your UBS port, so that may cause for problems...

2 Solution:

(...)
If you do not want that then you could add a diode to stop current flowing from A to B.
Or disconnect the 5V lines of A and B and only have them share the ground.

[Zbig]

That configuration is strictly forbidden by the USB standard.  The *ONLY* time it is permissible for a device to supply power to the host is when an OTG host and device have negotiated host charging in compliance with the USB Power Delivery (PD) v2.0 specification.

I totally agree, just have to add that this is not always followed by the gear manufacturers, unfortunately. I was shocked when I realized my self-powered Unitek USB hub was back-feeding my PC motherboard. The RAM refresh LED was on as long as the hub was plugged in even when PC itself was off. The damn things (USB hubs) often have their DC barrel jacks wired straight between USB ground and +5V

[praveen_khm]

Thank you all for the responses. I am planning to use FET's to isolate both the power supplies. Let me check how it is done...