Diodes in place of 3.3V LDR?

General discussions about V-USB, our firmware-only implementation of a low speed USB device on Atmel's AVR microcontrollers
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morrog
Posts: 2
Joined: Thu Nov 06, 2008 8:21 am

Diodes in place of 3.3V LDR?

Post by morrog » Thu Nov 06, 2008 8:24 am

On the homepage for AVR USB, the schematic shows two diodes in place of a 5V to 3.3V regulator. Would that really work? I can't image how two diodes are supposed to drop the voltage down from 5V to 3.3V.

Thank you.
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henni
Posts: 16
Joined: Mon Sep 08, 2008 4:17 pm

Re: Diodes in place of 3.3V LDR?

Post by henni » Thu Nov 06, 2008 11:54 am

morrog wrote:...two diodes in place of a 5V to 3.3V regulator. I can't image how two diodes are supposed to drop the voltage down from 5V to 3.3V.


A silicon diode has a forward voltage (Flussspannung) of about 0.7 V. Two diodes will drop the 5V down to 5V - (2* 0.7V) = 3.6V, which is sufficient low voltage for the USB lines. An extra advantage is that diodes have no quiescent current, therefore, standard conform USB sleep mode (less than 500 µA current) is easiest with these two diodes.
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morrog
Posts: 2
Joined: Thu Nov 06, 2008 8:21 am

Post by morrog » Thu Nov 06, 2008 7:41 pm

I'm still pretty new to electronics, so I had not known about the forward voltage on diodes. Thank you for the explanation. Very helpful :)
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