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11 years, 8 months ago.
Doubt about my configuration on VU pin
Is it safe to connect the regulated 5V external DC source in parallel with both the 5V device and microcontroller's VU pin?
(I'm not trying to power the mbed through VU pin, picture shown below is just a small part of my project)
The 5V external source will power up the device when no USB is plugged into computer. The 5V Vu pin will also power up the device when the USB is plugged into the computer while the 5V external source turned off.
4 Answers
11 years, 8 months ago.
I actually do power my mbed through the VU pin, probably not what they intended but it means that all my external devices (gps, imu ect) can be powered by usb when available or usually the 5v regulated battery power that my system runs on.
I've also plugged both usb and 5v into vu, didn't cause any problem.
I've been doing this a long time with no problems, but I guess I really should check the schematic to make sure its ok.
(better add I take no responsibility for blowing things up, this is just my experience, and the fact VU is just connected to usb 5v)
11 years ago.
I think part of the reason for the slightly inconvenient power configuration is to minimise the risk of current flowing back into the USB port. Also you have some freedom to select a different external supply voltage instead of 5v. Assuming you are using 5v one option would be to use a mini-USB "charger" to power your project, that way the power would appear on VU even when running standalone.
You might be able to run everything from 3.3v instead of 5v. Since Vout is auto-switched this simplifies things a lot.
Otherwise if you can stand the voltage drop then a pair of diodes could be used to pick up whichever 5v supply is present. If you can't deal with the drop then you need some form of active switch, for example the ICL7673 which uses P-type FETS. Unfortunately the 7673 suffers from a relatively high on-resistance unless boosted.
Also I didn't realise this was from 7 months ago, oops. Maybe questions should be auto-closed after a month?
11 years, 8 months ago.
Hi WB, first some things:
- VU is 5V USB Out not In, and VU is only present therefore if you have connected your mbed via USB cable
- If you want to power up the mbed with a 5v external source, it have to be connected through the VIN pin not VU
I hope this will help you
Greetings
11 years, 8 months ago.
I'm not trying to power up the mbed through VU pin. The picture shown above is just a small part of my project. I have the 5V external source connected into my VIN pin. There will be instance whereby i want the device to be powered by VU pin via USB cable while the external source is switch off and vice versa.