2 5V power source for a circuit

[galapogos]

Hi,

I'm wondering if it's possible for a circuit to have 2 different power sources for 5V? I've seen many USB bus powered drives that normally use the USB 5V voltage to power the drive, but they often also have a DC jack. All of the ones that I've seen have both the Vusb and the Vdcjack connected to both the drive's 5V and the circuit's 5V input into the LDO regulator, i.e. both 5V sources are shorted. When an AC adapter isn't being used, this isn't a problem since there's only 1 5V source(Vusb), but when an AC adapter is used, isn't this potentially dangerous due to differences in voltage levels between the 2 5V sources, as well as 1 source overpowering the other? What are the chances of things blowing up?

Thanks.

 

[Ldanielrosa1]

The voltage requirements for USB is that at worst case (longest cables, highest drain, poorest connections) there must be a minimum of 4V3 _at the device_.  If the device in question can stand to run on as little as 3V6, then a diode in series with the supply will cause problems.  A diode in series with each supply is probably how the device in question prevents supply contentions.

 

[audioguru2]

The AC adapter jack probably has a switch in it to disconnect the USB when the adapter is plugged into it.

Battery powered devices also have an adapter jack with a switch to disconnect the battery when the AC adapter is plugged into it.

 

[galapogos]

As far as I can tell, there's no switch. The circuitry for these cheap enclosures are pretty simple, and the PCB is small with very few components.

What would happen when they are shorted? Worst case, would something blow up?

Also, how's this for a solution? I'm thinking of either 2 FETs or BJTs with low Vce(sat), one for each power source, but opposite polarity. The base/gate would be controlled by the 5V from the DC jack, so that whenever there's power from the DC jack, the DC jack FET/BJT is switched on and the USB one is switched off.

Would this work or am I crazy?

 

[audioguru2]

An adapter jack with a switch in it looks the same as one without the switch except it has a 3rd terminal.

Here is a pic of a chassis-mount switched power jack. They are also made for pcb mounting.

 

[galapogos]

OK, that schematic looks like most I can find on DC jacks. How exactly does the switch work? I thought pins 3 and 2 are just shorted together.

 

[audioguru2]

The barrel of the plug pushes the spring contact #2 away from contact with terminal #3.
Terminal #3 is the internal battery which is disconnected when the adapter is plugged into the jack.

 

[galapogos]

Makes sense. I assume pin 1 is Vcc and pins 2 and 3 are GND? Hence the USB Vcc is actually shorted with the DC jack Vcc but the USB GND is disconnected from DC/system GND when a DC plug is inserted, hence disallowing power delivery from the USB connector?

 

[audioguru2]

Usually the positive is switched, not the ground. It could be either way since the jack doesn't have polarity.

 

[galapogos]

OK, in that case pin 1 has to be Vcc then. I'll pull out my multimeter tomorrow, test it out, and report back! Thanks!

 

[galapogos]

OK, so I tested 2 of the enclosures I have here that have DC jacks. They all have 3 pins, so I assume they're switched jack. I found 2 of the pins that are shorted with a multimeter, and then I plugged in the DC jack(unconnected to the mains of course) and tested for continuity on those 2 pins again. Doh, they were still shorted.

So either the 3 pin jacks aren't actually switched jack, or the enclosure maker simply shorted the 2 on the PCB side both to GND, which means they're still shorting both power supplies rather than switching them.

Anyway, the center pin(pin 1) is positive, so the 2 pins that are shorted are GND. This contradicts what you say about "usually the positive is switched, not the ground."? Does it matter which one is switched? Would it still break the circuit if the USB's GND is disconnected from the circuit GND?

 

[audioguru2]

You can buy adapters with the center pin positive or the center pin negative. Yours is positive.
Maybe your circuit uses diodes to separate the supplies.

 

[galapogos]

Hmm, I thought the adapters are pretty much standard. Is one more prevalent than the other?

Also, I don't think switching the GND would work right? Since the USB ground is from the PC, which is from the wall socket, which is where the AC adapter GND is from too, so they sorta share a common GND.

 

[audioguru2]

I have never seen an AC adapter with its DC grounded from the AC wall socket. Usually the DC is isolated. In Canada an AC adapter has two AC pins, hot and neutral, but no ground pin.

 

[galapogos]

What about AC adapters with 3 pin plugs? Is the DC ground still isolated from the AC wall socket? Anyway, does it even matter? Will breaking the GND on the USB side cause it not to supply power to the circuit? Coz I did some googling and it seems that positive polarity AC adapters are a lot more common than negative ones, so it'd be good if I can just break the USB power by disconnecting it's GND.

 

[audioguru2]

There is no positive or negative adapter. They all have two terminals.

it is their plug polarity we are talking about. Is the center pin positive or negative?

 

[galapogos]

Sorry, that was what I meant by positive or negative polarity. Positive meaning the center pin is positive and negative meaning otherwise. It seems that most AC adapters out there have positive center pins.

 

[galapogos]

bump?

 

[galapogos]

I took the easy way out and simply placed 2 schottky diodes on the 2 voltage sources to prevent any back flow of current. It seems to work, except my voltages are down to ~4.75-4.85V. My device has a 5V tolerance of +/-5%, which puts me on a borderline case. I'm wondering if there are any low drop out schottky diodes that have a lower forward voltage? I'm currently using a Zowie MSCD202 that has a Vf of 0.45V@1A