Bidirectional Power Pin? - Page 1
Forum section: electronics / projects

author: sonyqrio
author_rank: Newbie
timestamp: June 26, 2014, 04:09:31 pm
content: I'm designing a circuit and I need to have a pin that can act as both a power input and power output.When I put 12v into the pin, I want it to power my circuit. When no power is applied to the pin (power applied to the circuit elsewhere), I want my circuit to output 5v on that same pin.TheP82B96is an IC that accomplishes exactly what I want, but it comes no where close to being able to handle the amount of current (~1A) that I require.I'm guessing that I'm going to have to end up implementing something that involves two buffers with MOSFETs that can handle the voltage and current that I'm after.Any ideas/suggestions/schematics/resources would be greatly appreciated.- George

author: attie
author_rank: Contributor
timestamp: June 28, 2014, 09:09:12 am
content: That sounds like quite a crazy / scary idea to me... If you were looking for12v inor12v outthen it would be much less scary.The trouble is... what happens if the input drops to ~9v, do you start driving the 5v out? and then risk either bringing your internal rail higher than 5v, or needlessly loading your 9v input by trying to make it 5v.The same goes for a "12v" input that has dropped or otherwise gets to ~4v - do you output your 5v, and bring the external 4v up? What happens if the external stuff tries to regulate the 4v supply to 4v, thus drawing lots of current from your internal 5v supply.12v in / 12v out can be very simply achieved by a diode if you're willing to accept the losses.

author: jeremy
author_rank: Super Contributor
timestamp: June 28, 2014, 11:41:01 am
content: Quote from: attie on June 28, 2014, 09:09:12 amagreed.Can you solve this mechanically? Why not just have a plug with 3 pins (or 5 so you can put it in backwards)?

author: amyk
author_rank: Super Contributor
timestamp: June 28, 2014, 12:17:10 pm
content: Quote from: attie on June 28, 2014, 09:09:12 amI read the OP's requirements as more of "you can draw up to 1A of 5V from this pin, but put more than 5V into it (like 12V) and it starts sinking current and powering the circuit (I'm assuming your circuit can work just as well from 12V as it does 5V. This can be done easily with some diodes/MOSFETs.It's similar to how laptops are powered; the main power rail is at the voltage of the batteries when the AC adapter isn't plugged in, and rises to the voltage of the adapter when it is. There's usually a diode/MOSFET to stop the input jack sourcing current, but some designs don't have this and youcandraw current from there.

author: dannyf
author_rank: Super Contributor
timestamp: June 28, 2014, 02:49:09 pm
content: QuoteSo whatever on that pin will see either 12v or 5v?What if a 11v power source is applied? What about 3v?Think through those cases and you will arrive at the right conclusion about this arrangement.

author: sonyqrio
author_rank: Newbie
timestamp: June 28, 2014, 05:11:21 pm
content: Quote from: amyk on June 28, 2014, 12:17:10 pmThis is the correct interpretation of my requirements.If 12V is put into the circuit, it is regulated down to 5V regardless. The circuit can be powered by voltages as low as 3.3V, so any voltage from 3.3V to ~26V (the maximum input voltage of my regulator) can be applied to that pin.If and only if power isn't being applied to that pin will 5V be fed out of the pin. The proper protection circuitry will need to be implemented.I've been pondering the idea and I've arrived at this circuit.