Nick Oliver

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9 years ago.

Output 5V on battery vs USB

I am having a slight issue, My setup: TFT 4wire serial connected to ground and VU (5v out), using an LPC1768.

When I connect the MBED to usb the screen turns on and 5v is measured on the VU pin. But whe i switch to a 9V battery input on the ground and VIN pins the tft screen wont power on and the measures voltage starts at 2V and then slowly ramps up to 5V.

What is going on with the device? Could it be broken?

2 Answers

Andy A
9 years ago.

VU is the USB voltage output. If USB isn't connected then no power is supplied on that pin, any voltage you do see is leakage backwards through a diode.

The LPC1768 board will always supply 3.3V out but the VU output is only usable when powered via the USB input, not when powered by V IN.

If you need a regulated 5V output you'll need to add an external voltage regulator.

Robert Spilleboudt
9 years ago.

This is normal look at http://developer.mbed.org/platforms/mbed-LPC1768/ the 5V is labelled 5V USB Out: only when there is USB power!
If your TFT needs 5V you have to provide a regulator battery -> 5V
If your TFT accepts 3.3V, the Mbed provides 3.3V when VIN is powered.