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Topic: "Phantom Power" to Arduino via input pin??? (Read 3 times) previous topic - next topic

SirNickity

If the two sub systems can be independently powered then a seriese resistor and catching diodes to each rail are a must.


Is that considered ESD protection?  I thought the inputs already had this anyway?  (Honest question -- I'm not that advanced of an engineer, and would love to know what would make for good design practices.)

How can a USB device be plugged in and not powered?


Well...  How many desktop-sized drive enclosures have you found that are bus powered?  :D  (In my case, I'm talking about -- for e.g. -- a Plextor half-height optical drive in a USB-to-IDE enclosure.)

Grumpy_Mike

Quote
Is that considered ESD protection

Yes it is a start, you sometimes add inductors and ferrite beads as well.
The ESD protection built into this device is minimul and needs supplementing if it is going to do a proper job.
You need to withstand between 2 to 5 KV discharges depending on what standard you are trying to meet.

Zeph

Grumpy, could you explain further what circuit you are suggesting?  Suppose you have two ATmega's connected to each other, with separate power supplies, such that it's not possible to power and unpower them at exactly the same time.  What series resister would you use, and what kind of diodes connected to what,  to allow them to communicate at maximum speed when connected, but to keep any input from ever being above 0.5 volts when a microcontroller was unpowered?

I am imagining something like 1K resistor in between, with schottky diodes on each input connected with the  anode on the digital input and the cathode on the VCC input of the microcontroller.  Is that what you mean?

The real voltage difference involved compared to the "naked" inputs (ie: besides the current limiting) would be due to the Schottky diode voltage drop being less than that of the internal protection diodes which it would be paralleling.  With a regular diode, the voltage difference between the input and VCC on the unpowered microcontroller would be about the same as the naked situation, right? 

So in that scenario, it would be essential to use a low forward drop diode paralleling the protection diodes if you intend to protect the inputs of an unpowered processor from ever rising above 0.5 volts over the processor's VCC.

If you have something like that in mind, the interesting thing is that the unpowered processor might still operate on phantom power and the LEDs on the unpowered processor board might still light up - depending the resistor size and the loads being driven - this is due to the wide VCC range.

Given how easily this "input still above 0.5v while VCC unpowered" situation can occur (eg: a processor monitoring a battery or solar panel, or connected to a peripheral device running from a separate regulator  with different capacitors), I would think this required resistor and diode protection must be documented in many application notes.  Can anybody reference one to help us all?  Thanks.

Grumpy_Mike

Look at the circuit here with the clamping diodes, make it a 220R resistor between them and have the diodes on the input side.
http://www.thebox.myzen.co.uk/Tutorial/Protection.html

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