- What is the purpose of D3?
- Which is the correct orientation?
- Does it depend upon its intended purpose (whether to protect the USB or the 7805)?
I have two ATtiny85 development boards, one is a DigiSpark and one is a generic mini development board.
As the schematics below show, they are essentially the same:
Mini development board:
However there is one small but significant difference - the orientation of D3:
- The DigiSpark has the anode to 5V and cathode to USB 5V;
- the Mini Dev board has the anode to USB 5V and cathode to 5V.
I have tested and checked the orientation of D3, on both boards, with a diode tester and both of the schematics are indeed correct1, and both boards work from USB power, as required. However, the orientation of D3 on either board is opposite to that of the other.
For the Mini development board, I assume that the diode (with the anode connected to USB 5V) would protect the USB from externally connected VIN feeding back, via 5V. Would that be a correct assumption to make?
However, for the DigiSpark board:
- How can the board be powered from USB, if the diode is blocking the USB 5V (anode to the 5V output of the 7805)?
- Is D3 protecting the 7805 from USB power?
- If so, then the USB is left unprotected (when an external supply is applied)?
For completion, here are annotated photos of the two boards, showing the component identifiers:
Annotated mini development board
For full disclosure, these images come from my blog Digistumped?, which has some additional reasoning explained, which I don't need to go into here.
I have checked and metered out both boards and checked the components and their connections. All components are accounted for in the schematics... There are no hidden components (which aren't shown on the schematics) and no hidden traces/connections/contacts.
1 I have searched and found links discussing the orientation, but none have a suitable explanation:
Diode D3 is not in reverse. as suggested in post#2 of digispark ATTiny85 not running on independent power:
Is D3 really connected as per the schematic? I think it should have the anode connected to the USB connector.
as this Bypass Digispark 5v 500ma current limit? post states
we can see that there is a diode that drops the voltage to the VIN pin on the board...as i understand it that pin is there to prevent external power from feeding back into the usb port...
I do think the schematic for the original digispark is wrong... the diode is pointing towards, the 5v, not away from it, so it is allowing the USB to provide power to the 5v pin, but preventing any power present on 5v from feeding into the 5v line. Just to be clear, this is the schematic I am looking at for the Digispark...