Do I need a diode in my circuit when using a USB-powered DC power supply to power a laptop camera?

Question

I have made a USB webcam from an old laptop camera, a 5 V/3.3 V DC power supply powered by USB, and an old USB cord. When I looked at similar setups, however, I found they tended to have one or two diodes in them, and I cannot figure out why. I have a rather basic knowledge of diodes, as far as I know, they are used to prevent current from running in the wrong direction (for example. if a battery were put in the wrong way), but that does not explain why some of these tutorials use 2 diodes.

On to the actual question: I am using a small DC power supply intended for breadboards to convert from the 5 volts of the USB to the 3.3 V my camera requires. Is there any reason I would need to include a diode in my circuit?

Answer 1

2 silicon diodes in series will have a nominal voltage drop of 1.4V. 5V source from the USB - 1.4V drop from the diodes = 3.6V. Most 3.3V devices are probably maybe fine with 3.6V.

This isn't ideal as the voltage drop will vary based on the current drawn, so it could lead to frying the USB cameras parts. Maybe. A non-zero chance of it happening.

Using a regulator is better than using dropping diodes which is better than trying to use a resistor voltage divider.

Alternatively, diodes also can provide reverse voltage protection. But it's unlikely someone is using it for that. With USB, the power connections are fixed, so reverse voltage is an unlikely scenario.

You could use one, if you think you may accidentally connect the power backwards

Answer 2

If you are using an SMPS, such as a desk top power supply, they have a 3.3 volt rail which can be used to power a camera, for example. With a switched-mode power supply, such as a desk top supply, the voltage and current are regulated, so running a laptop camera would be just right amount to power it. The pictures on the web show two diodes cause they are running it from the usb 5 volt power rail, which does give the cam some protection from being hooked up backwards and also brings it close to it's operating voltage. The 3.3 volt regulator is the way to go if you are using a usb port to power it, which is regulated also.

I use a desktop power supply to run a few things in my lab, hands down the best way to run your electronic gadgets and test circuits. Most modern electronics use 3.3, 5, and 12 volts to power them, which the desktop power supply has. Even if not the exact voltage you need, a buck converter will bring it down to about 1.5 volts, as another example.