How to power a circuit from two sources?

[zapta]

I am building a one-of circuit that will be connected to two power sources and wonder what is a reasonable way to do the power arbitration.

Here is a simplified schematic diagram


The circuit include a 5V Arduino Min Pro https://www.sparkfun.com/products/11113 , a 5V FTDI Basic @115.2K https://www.sparkfun.com/products/9716, a ATA6663 Lin transceiver chip, and a 12V/5V Linear regulator (for higher max Vin than the Arduino's LDO). The USB side of the FTDI Basic will be used with a laptop computer running on batteries (floating) so no ground issues.

The board should be able to operate in these three configurations

1. VBAT power only (USB is disconnected or laptop is off).  Arduino should function (TX, RX, leds, etc). FTDI/USB not used though the Arduino sends 0/5V signals.

2. USB power only (USB connected, laptop powered, VBAT off) - FTDI and Arduino should function (e.g. to upload a new program). Lin chip not used though the Arduino sends 0/5V signals.

3. Both VBAT and USB power. Everything should work.

Of course it is also important that nothing, including the computer, will get damaged in the above scenarios, so possibly, some protection on the signal lines may be required (hopefully not).

Thanks,
zapta

[Jon86]

You could either use some kind of Diode/MOSFET circuit just to stop the current flowing into a source, or I think what would be more suitable is to use a real power management IC that switches between sources when needed, there's plenty of those devices out there so I'm sure you'll find one for your needs. USB/Battery backup is going to be the most common arrangement anyway.

[Macbeth]

eh? just use a 7805 on the 12v, surely?

[smashedProton]

Look at the circuitry on the top left of daves power supply schematic.  that may be what you need

[Jon86]

Look at the circuitry on the top left of daves power supply schematic.  that may be what you need

If you're talking about the MCP73213 and that part of the circuit, that's a battery charger, zapta didn't mention charging the battery, just working off two supplies.

[calexanian]

How about just some shotkey steering diodes. Higher voltage wins.

[Jon86]

How about just some shotkey steering diodes. Higher voltage wins.

But if the battery wins, you're wasting battery aren't you?

[calexanian]

Be sure your other power source is always higher silly!  O0

[zapta]

How about just some shotkey steering diodes. Higher voltage wins.

This is for the Arduino power, right? 

The FTDI is hard wired to the USB power so it will be off if the USB is disconnected or the laptop is off (we don't want to inject power into the USB cable). In this case, the FTDI will have no +5V but will get +5V signals from the arduino. Will this damage the FTDI? Will resistors in series to the signals from the Arduino, e.g. 10k,  will be sufficient protection?

[zapta]

[But if the battery wins, you're wasting battery aren't you?

This is the car battery and the power comes through the ignition switch battery consumption is not an issue in this case and so is the laptop battery power that is fed via the USB.

[calexanian]

Then how about a relay switched set up. When shore power is applied it automatically switches in.

[Jeroen3]

Maybe you are looking for something like an LTC4411 powerpath switcher or other hot-swap controllers.

[sleemanj]

See

https://www.eevblog.com/forum/projects/primary-(eg-usb)-and-auxillary-(eg-battery)-power-steering-ic/msg299942/

Short version is that it's a harder problem than it sounds, and the couple of IC's mentioned in that thread are probably both the easiest and actually cheapest way to do it.

I still haven't gotten around to trying out my NOR based solution.

[Mechatrommer]

if you listen to a "mechanically inclined" guy, he will suggest you...

R1 = current limit to relay coil (save energy)
C1 = storage tank (avoid blackout during switching)
S1 = press closed to supply from both sources
D1, D2 = backflow protection
KISS, suit yourself.

[dannyf]

Everything should work.

What does that mean? Who is going to power whom in this case?

[zapta]

Everything should work.

What does that mean? Who is going to power whom in this case?

It's mean the three sections should work, from left to right: LIN tx/rx, Arduino, FTDI/USB.  The LIN must be powered from VBAT, the FTDI must be powered from the USB power (hard wired within the FTDI board), the Arduino can be powered from either.

[zapta]

Thanks Mechatrommer.

1. Why the relay, can't I just OR the power using the diodes?

2. If USB power does not exist, the Arduino will be power from the battery (good) but will send 0/5V signals to the unpowered FTDI chip. Can this damage the FTDI chip? If so, what is a reasonable protection? Will a series 5K resistor be sufficient?

[Mechatrommer]

1. Why the relay, can't I just OR the power using the diodes?

thats the problem with "mechanically inclined" guy he cant think clear enough. yes you can... according to him but a relay will provide extra isolation to your circuit and avoid prolonged reverse biased to the diode, which he not sure how long the yield/life time will be.

2. If USB power does not exist, the Arduino will be power from the battery (good) but will send 0/5V signals to the unpowered FTDI chip. Can this damage the FTDI chip?

put a sense line in PSU from laptop, if its goes low dont send any signal. plus the 5K current limit resistors as you suggested.

[zapta]

put a sense line in PSU from laptop, if its goes low dont send any signal.

That's a good idea.  I can do the same also for the other side, the LIN tx/rx.

[dannyf]

It's mean the three sections should work ...

Two diodes will work then.