Electronic Modification - Advice Sought

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PC Pilot
Posts: 1
Joined: Fri Jun 19, 2020 2:17 pm

Electronic Modification - Advice Sought

Post by PC Pilot » Fri Jun 19, 2020 4:26 pm

Hi everyone,

I am seeking your advice in respect of a USB device modification project and whilst my technical abilities are sound, my electronic technical knowledge is best considered basic given that it is many years since I explored my way through electronic circuitry or circuit diagrams :oops: !

On the face of it the project is relatively straightforward. The device in question is a USB 3.0 header extender (i.e. it sits inside the PC case connected to the 19 Pin USB 3.0 motherboard header (it's input) providing further 19 Pin headers & USB sockets (it's outputs) thus expanding the available internal on-board headers/USB Ports by a factor of 4). Unfortunately the device has an inbuilt flaw, when powered by the (optional) SATA Power connector the ports remain active and produce return to the PSU (through the 5V pins to SATA Power) which disrupts any shutdown and may potentially damage other components. Similarly, if the device is left unpowered it readily becomes overwhelmed and so the USB devices begin to fail as the total number of connected devices demand power resources which exceed the 1.8A (900mA on each of the two ports provided by the 19 Pin motherboard feed) unpowered threshold, so rather a double whammy!

Helpfully one reviewer of the device (clearly with electronic knowhow) proposes a simple modification to provide unconstrained protected power provision using some simple components, though he/she does not elaborate on the component values required and only rather generally upon the work involved. The following is an edited transcript of the relevant passage from their review.

"The problem is caused because some, if not most motherboards support charging devices when the computer is off. This means that the 5V on both ports is still active when the power is turned off. This feeds back about 4.7V (Schottky diode) to the power supply through the SATA power connector, causing the power LED light to remain ON even if the power is off, and keep some devices, like the RAM powered up, which you do not want when the PC is turned OFF. This can also prevent powering up the PC again, or even damage some parts in your PC.

A good voltage isolation would have been to add a Schottky diode to the SATA plug power also, but this would have dropped the voltage to 4.7V instead of 5.0V, which means some USB 3.0 devices would not work and/or the length of USB header cable would end up greatly shortened. The optimal solution therefore would be to use the 4.7V, put it into a DC-DC converter and boost it back to 5.0V.

So basically, if you want to use the hub for USB stick, and external USB hubs, and external hard drive, just don't plug into the SATA port. If you want to loose the connected device constraints cut the circuit trace between the USB 3.0 19 pins plug, add a Schottky diode, and then cut the trace after the two voltages are combined and add your own little buck boost regulator to raise it up to 5.0V, and then you're good to go with, or without the SATA power port!"

Applying my own rudimentary knowledge here I believe that for modification 1) what they are suggesting is to add a Schottky diode to EACH of the 5V pins (Pin 1 & Pin 19) on the 19 Pin (Input) Header so as to prevent the voltage leak returning to the motherboard (through the on-board 19 Pin Header at the other end of the header to header link cable connecting to the device's 19 Pin input). Thus, by breaking both paths such that the anodes would now be connected to the end leading to Pin 1 (or Pin 19 for the other port) and the cathodes to the track leading into the device, the circuit is protected from the voltage return leak.

In the case of modification 2) I believe that what they are suggesting is that the two 5V inputs (from Pin 1 & Pin 19) are internally combined (to supply 5V to each of the 'output' 19 Pin header connectors Pin 1 & Pin 19?) and at this point to cut the track and insert a 5V Step-Up/Step-Down Voltage Regulator with the input terminal connected to the track leading from the input header, the output terminal to the track leading to the selection of output headers and the ground to a suitable ground termination (such as those in use for 19 Pin Header pins 4, 7, 13 or 16?). This would ensure that each of the 5V pins (1 & 19) for each of the output headers would always receive 5V.

So firstly, does the summary of my understanding of their transcript make sense, or have I misunderstood the concept and/or procedure required?

Secondly, I require some assistance in identifying the correct components required, especially the Schottky Diode values with which I am by no means conversant. For the 5V Step-Up/Step-Down Voltage Regulator I have identified the following device as potentially suitable but would of course welcome all comments from the forum experts.

Pololu S7V7F5 - 5V Step-Up/Step-Down Voltage Regulator See <https://www.pololu.com/product/2119>

available here for £6.96 inc P & P <https://www.hobbytronics.co.uk/s7v7f5-5v-regulator>

I look forward to hearing your comments and thank you in advance for taking the time to respond.

PC Pilot

Posts: 217
Joined: Sun Oct 04, 2015 6:18 pm

Re: Electronic Modification - Advice Sought

Post by David » Wed Jun 24, 2020 8:56 am

Have you considered making a separate PSU, with a mains transformer/bridge/cap/three-terminal regulator (such as LM 7805 - there are very many about)? You common the input to the regulators and take the output to feed the USB. A small cap, say .01 uF should decouple each USB +ve line. Don't forget to remove the existing +ve line from the computer motherboard.

I have not tried this, and so you are really on your own. There are very many computers and motherboards about so I don't want to make any specific remarks! Let me know if this helps.

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