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Topic: Understanding the Uno's USB Isolation Circuit (Read 2 times) previous topic - next topic

UNTEngineer

Dec 01, 2012, 07:52 am Last Edit: Dec 01, 2012, 09:12 pm by UNTEngineer Reason: 1
Can someone explain to me how this circuit is working? I dont get what its doing. I see that its turning on and off the USB Vin based on some input, but other than that I dont understand how it is operating or what the application for such a circuit is? (Its part of the Uno circuitry btw)

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retrolefty

#1
Dec 01, 2012, 08:17 am Last Edit: Dec 01, 2012, 08:22 am by retrolefty Reason: 1
It's purpose is to isolate The USB +5vdc from the on-board +5vdc voltage regulator for the case when there is both an external DC power source connected to the board and also plugged into a PC USB port at the same time. It's called the 'auto-voltage selector' circuit. It's never a good engineering practice to have two voltage regulators 'hardwired' together so this circuit is designed to prevent that from happening. The T1 mosfet is the active switch element and the op-amp is wired as a simple comparator to drive the mosfet on or off. The presence of a Vin voltage above a minimum valid value cause the current path from the USB power to be cut off.

Lefty

UNTEngineer

So if I have a 32u4 connected to USB and a voltage regulator at the same time, that would be a no-no, correct? I'd have to implement this circuit to separate the USB voltage from the circuit voltage to prevent frying my 32u4? Cant I just use a diode on the USB voltage line, or is that still not enough?
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retrolefty


So if I have a 32u4 connected to USB and a voltage regulator at the same time, that would be a no-no, correct? I'd have to implement this circuit to separate the USB voltage from the circuit voltage to prevent frying my 32u4? Cant I just use a diode on the USB voltage line, or is that still not enough?


Well it's not an automatic 'it will fry my board and put my eye out' sort of thing. Lots of people power their arduino boards with an external regulated +5vdc power supply via the 5V and ground shield pins and that does effectively wire the USB +5vdc and the external +5vdc together if you plug in USB at the same time, and most have reported no problems. However the two regulators can fight each other and current flow in the wrong directions if the two voltages are too far apart. The USB voltage standard is something like 4.75 to 5.25 and who knows what the actual voltage of a specific regulated +5vdc supply might be as it too has a voltage tolerance value. Simple diode isolating is certainly a simple and effective means to isolate the two voltage sources, with only the slight disadvantage of a slight voltage drop that might or might not be an issue using the analogRead() function and assuming the reference is 5.0 when it might by 4.4vdc due to the diode drops.

Lefty

dhenry

Quote
what the application for such a circuit is?


It detects the presence of Vin and switches power supplies: U1A is a comparator. It takes Vin, divides it by 2 and compares it to 3.3v. If Vin / 2 is greater than 3.3v (aka Vin > 6.6v), U1A outputs 1 and turns off T1, which isolates USBVcc from U2. So U2 is powered by +5v (which hopefully is powered by Vin at this point).

If Vin / 2 is less than 3.3v (aka Vin < 6.6v), U1A outputs 0 and turns on T1, which switches in USBVcc.

So 6.6v is the cut off point for Vin to power the device.

They could have done a better job around U1A.

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