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Safe voltage range for 5V input


#1

I’ve got a power supply that produces 5V and 12V, but the 5V is a bit high… 5.18V as measured by my DMM. Is it safe to use this to power my FEZ Panda II?


#2

Yes, panda only needs 3.3v


#3

If I power the board with 3.3v, the digital IO lines will only have 3.3v as “true”, right? Does it work the same way if I power the board with 5v? As it happens, this power supply also has a 3.3v output (it was an ATX power supply in a previous life…)


#4

You need to power it from 5v but then internally it is 3.3v


#5

Right, it’s the overvoltage on the 5V pin I’m concerned about, because as I understand it, I’d be bypassing the 5V regulator on the board by powering it that way?


#6

[quote](it was an ATX power supply in a previous life…)
[/quote]

Most/all multiple output switch type power supplies require some minimum load to regulate properly. If you add a dummy load to the 5V section you might see it start regulating better.


#7

I’ve got a 10 ohm resistor on the 5V rail (25W) to provide a static load. Just want to make sure that the 5.18V isn’t going to kill it when I plug it into the 5V pin…


#8

If you read the data sheet for the processor, the pins are 5V tolerant, but the Absolute Maximum Rating is 6.0V, so you won’t harm the processor if you put 5.18V on it, but ONLY IF you put that voltage into one of the digital pins, if you put it in one of the analog pins, even if it’s configured for a digital input, you will harm the processor. The analog pins are not 5V tolerant.


#9

I think we are confusing 3 things.

The maximum power to 5V pin on FEZ board
The maximum power on the processor
The maximum voltage on an input pin

What exactly are you tying to do? Why not power up FEZ from USB cable for example? Or why not use a regular power pack on the power connector?


#10

This is the one I’m concerned about. I’m running a stepper motor (using an EasyDriver) from the 12V power supply, and I have to have a common ground for the setup to work. I figured the 5V output from the power supply was perfect, as it shares a common ground with the 12V used for the steppers, but when I measured it, it was .18V higher than it should have been. I just need to know whether it’s safe to power the FEZ through the 5V pin using 5.18V from that supply.


#11

Yes connect that to 5V pin on FEZ Panda and you will be fine. This voltage goes through 3.3V before it ends at the processor so it can be 7V or more!


#12

Ah, perfect, that’s what I was hoping to hear :slight_smile:


#13

One more question…

Is it safe to power the FEZ through the 5v pin while it’s connected to the computer via USB?


#14

Topic is continued here http://www.tinyclr.com/forum/1/2782/