- Tue Jan 22, 2013 7:13 pm
#154496
Hey Guys,
I have a project that I'm working on that has me a little stumped, and need advise.
Its a small USB 5v powered device/widget, the 5v supply is limited to 500mA as per USB spec. But there will be situations that I may require more than 500mA. So I'm looking for the best way to isolate the USB 5v from a second 5v source, wall wort for example, while allowing both power sources to share the load of the device, The additional 5v is for supplemental power to ease the burden of the primary USB power.
I had thought of adding a diode, but the trouble with that is the voltage drop across the diode is such that the supplemental voltage is lower than the USB voltage and therefore the device does not draw any current from it (Both power sources equalling 5v starting points.
The other idea is to use a P-chan fet, Source is connected to the external power source, the Drain connected to the device, gate connected to gnd. Now this works as I had hoped, when the external supply is connected I see a drop in current from the USB connection, so the current from the two power sources are both powering the device.
But I was not sure if the fet is going to give me the isolation that I was looking for, am I heading down the wrong path here?, or is the fet option workable?
thanks,
Croc4
I have a project that I'm working on that has me a little stumped, and need advise.
Its a small USB 5v powered device/widget, the 5v supply is limited to 500mA as per USB spec. But there will be situations that I may require more than 500mA. So I'm looking for the best way to isolate the USB 5v from a second 5v source, wall wort for example, while allowing both power sources to share the load of the device, The additional 5v is for supplemental power to ease the burden of the primary USB power.
I had thought of adding a diode, but the trouble with that is the voltage drop across the diode is such that the supplemental voltage is lower than the USB voltage and therefore the device does not draw any current from it (Both power sources equalling 5v starting points.
The other idea is to use a P-chan fet, Source is connected to the external power source, the Drain connected to the device, gate connected to gnd. Now this works as I had hoped, when the external supply is connected I see a drop in current from the USB connection, so the current from the two power sources are both powering the device.
But I was not sure if the fet is going to give me the isolation that I was looking for, am I heading down the wrong path here?, or is the fet option workable?
thanks,
Croc4