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Does this seem like a reasonable inline USB "external power" circuit?

Beginning to think I should have put it across ground on the target device?

@gudenau Oh, yeah. For an NPN Darlington, that's how you'd want it.

@gudenau It looks alright, assuming I understand your purpose correctly. Why the Darlington rather than a MOSFET or something? How much current is the connected device going to draw?

@danbsmitbo Yeah, was doing some research and that was what it said. It's probably .5a-1a, not exactly sure.

@gudenau Other than that, the only major thing is that you're right in your second post that the Darlington should go between the load return and ground.

@gudenau Bipolar transistors are like a diode drop each. The spec sheet for the TIP120 shows a 2 volt drop at 3 amps. Logic fets would be a better choice as they have only resistive characteristics but I would use a P fet and inverter.

@gudenau Fet's are voltage to current amplifiers. An increasing voltage on the gate causes the drain to pass increased current. Logic fets are designed to be fully turned on (saturated) at 3.3 to 5 volts.

@gudenau There are others besides this one:onsemi.com/pub/Collateral/MCH3

Note that the voltage drop is a function of the on resistance of 0.069 ohms. The gate merely needs to be driven >2 volts negative from the source which would be connected to the 5 volts supply. The drain can supply 3.5 amps (I always recommend a little overkill and fuses for resilience).

Here is a simpler description of high side switching with P-Fet's.
vishay.com/docs/70611/70611.pd

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