bi-directional I/O pin - 5v out, 24v in ?? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: April 06, 2016, 06:15:59 pm
content: I've run out of pins on a connector that, for various complicated reasons i can't go into here, can not be now changed.Currently, the device with this connector takes USB power (5v) from a host device (pc/laptop etc) abd sends out that 5v to a pin that powers the target microprocessor.However, i've been asked if there is a way of making the device "standalone" and be powered by the system that the target microprocessor is in. That means accepting anywhere from 12 to 28Vdc back in that same pin and using that to power the device.So, can anyone think of a nice way of doing this?normal diodes are out, because i've only got a 5v source, so the voltage drop would mean significantly less than 5v out, so i'm wondering about using FETS or something as a low drop diode?The 12 to 28V coming in would need to go via a voltage regulator to be dropped to 5v to power the device of course as well

author: mariush
author_rank: Super Contributor
timestamp: April 06, 2016, 07:49:46 pm
content: The simple and relatively cheap way would be to just use a switching regulator in SEPIC mode (buck and boost) and a diode to block the higher voltage from flowing into the 5v usb voltage. Take 4.5v - 24v in, output 5v .There are also voltage regulators that have one input and one battery "backup" input, so that they'll do "fail-over" to the battery backup if the input goes below a certain voltage.Here's for example some chips from Linear (good but quite expensive) :http://www.linear.com/products/PowerPath_Controllers_*_Ideal_Diodes

author: station240
author_rank: Unknown
timestamp: April 06, 2016, 11:22:17 pm
content: Yes a SEPIC DC-DC converter would do this, and also cope with low 5V inputs.I tested one that would take 5V-30V in and output a stable voltage (adjustable), sadly it was too cheaply made and didn't have proper over current/over temp protection.I'm in the process of designing a better version for my own use.ps: Make sure you label the special cable your using to inject 28V into the USB port, to prevent explosions.Or just build the DC-DC converter into the cable.

author: ovnr
author_rank: Frequent Contributor
timestamp: April 07, 2016, 12:05:03 am
content: I think people misunderstood your problem. Well, or I misunderstand it.So:You have a pin that will either receive 12-28V, or if the device is bus-powered, output 5V? And the 5V outputmustbe within a few percent of 5V?You also follow the USB standard and will not back-power the USB connector when the device is connected to the 12-28V source?You will probably want a so-called ideal diode. Just using a MOSFET transistor isn't going to work terribly well without some control, and the easiest solution really is just to buy a finished controller.Here's a. It'll also tolerate being connected to the 12-28V source and a USB port at the same time without any fireworks. Note that part numbers are best described as "picked haphazardly"; you need to find whatever is best for your application. The ideal diode controllers I picked are expensive at $4.5/pop (1-off); you can probably find cheaper options. The MOSFETs and voltage regulator/diode are random picks; find your own.

author: max_torque
author_rank: Super Contributor
timestamp: April 07, 2016, 10:37:40 am
content: thanks ONVR thats exactly what i'm needing!? >>>>>>goes off to google ideal diode controllers 😃