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Topic: "Phantom Power" to Arduino via input pin??? (Read 3471 times) previous topic - next topic

Nick Gammon


No it is not common at all. Driving a signal into a device that is not powered is hardly ever encountered.


Well as a few examples:


  • A DVD player with digital output plugged into an amplifier (or TV) which is turned off at the power point.

  • An Ethernet cable plugged into a Ethernet switch which hasn't been powered up.

  • You plug your iPhone (via USB) into a PC, and then turn the PC off when you go to bed

Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

Grumpy_Mike

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A DVD player with digital output plugged into an amplifier

Digital output plugged into an analogue audio amplifier!

These examples are not at all the same as sending an input into a powered chip. In consumer electronics there is all sorts of protection on inputs and outputs, you never connect them directly into a chip. Most have at least catching diodes in like I said. Audio amplifiers are invariably AC coupled.

Nick Gammon

I'm not trying to argue with you Mike. And no doubt consumer electronics is designed to be idiot proof.

However modern amplifiers have digital inputs (co-ax sockets in some cases). I have no idea what's inside the box, but theoretically you can be putting a digital signal into the amp, where - possibly after navigating some protection circuits - it hits a digital processor chip.

I suppose with things like I2C you might expect that both ends would be powered, and on a single board that would certainly be the case. But once you start talking about "cable runs" for I2C/SPI and how it is a 2-Wire protocol (SDA+SCL+GND) then one presumes that each end is individually powered. And therefore one end powers up before the other one.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

Zeph

As people seem to be agreeing, the plug and play cables of consumer electronics are going to tend to be somewhat protected from common scenarios, in various ways.  For the most part (due to static and EMI consideration as well), they are not going to connect a microcontroller pin directly to some jack on the exterior of the device; if they do, it will be specially designed for that.

This situation arose when connecting one naked microcontroller output to to the input of another microcontroller (no special protection except what is built into the chip), with separate power supplies such they won't both be turned on or off at the same millisecond.  Since I was connecting both ways (master out to slave in and slave out to master in), there was not way I could power down both at the exactly the same time.

Basically, such situations can and often will occur any time you are jumpering microcontroller based electronics together in some circuit with more than one power supply.  So if your Arduino is powered by USB but connected to a breadboard circuit with its own power supply (say a sensor or actuator of some sort), can you turn the two supplies on and off at exactly the same time?  What if one side is battery powered?

One concern is damaging a microcontroller.  My suspicion is that while it's not a good idea, it probably isn't super sensitive to this because most people who have ever used two power supplies while prototyping have probably run into it without noticing.  Using a resistor between the microcontroller might help some. 

If one accepts Grumpy_Mike's model that an unconnected VCC in the slave is effectively at 0V, then no input to an unpowered microcontroller should ever be above 0.5v, period.  In addition to two processor situatins, think of something like in analog input connected via voltage divider to a battery or solar panel, used for voltage monitoring when operating normally.  I suspect we have a bit more slack than that model (and that there is a difference between leaving the VCC input unpowered and actually tying it to ground), but you can decide for yourself and caution is wise.  I would certainly not advise deliberately using phantom power, but I'm not going to say "never, ever use two power supplies that cannot be powered or unpowered within milliseconds of each other".

Total current is another aspect.  In my case, the slave ATmega328P was powering itself, two small on-board LEDs, and the output that returned to the master.  If it was in a circuit to put out 40ma on several pins, obviously it would have attempted to draw more current from the 40ma output of the Master (from which it was drawing phantom power).  Maybe that would have been more problem for the protection diodes, or even the Master's output.  So the more power your controller might draw via an input, the more you might want to be concerned.

(Since I'm using DIP based Arduinos, I may order another ATMega328P from Mouser for $2.50 next time I have an order, and keep a spare around).

The new caution I learned was that we should be aware that the "unpowered" microcontroller might actually begin or continue operating.  It could even send out well formed commands over SPI or I2C or RF; in a few situations that could be problematic - you think a system is off but it continues to function.  Probably not often a concern, but if we are ever working with systems where having the microcontroller operate when we think it's unpowered could be a problem, it's good to be aware of the possibility of it operating under "phantom power" supplied via an input pin.

Grumpy_Mike

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And no doubt consumer electronics is designed to be idiot proof.

No it is not that, it is the ESD specification that any consumer product has to meet. The components for that will protect the circuits from this situation.

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

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

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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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