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[FEZ Panda II] User manual voltage confusion


#1

I’m trying to figure out the absolute ratings for externally powering my FEZ. I’ve found the following values:
[ulist]Manual says the external connector accepts 6-9 volts.
Board writing says the external connector accepts 6-9 volts.
Manual says the [italic]Vin[/italic] pin accepts 6-12 volts.[/ulist]

Now I measured the voltage on [italic]Vin[/italic] with an 8.4 volt battery connected to the external connector, and I was able to measure 8.4 volts between [italic]Vin[/italic] and [italic]GND[/italic]. This seems to indicate that [italic]Vin[/italic] and the external connector are connected somehow, which confuses me a bit as their ratings are different.

What I’m looking to use as external power is an 11.1 volt LiPo battery, and I want to know why I have to connect it to [italic]Vin[/italic] and can’t just use the external connector?

Here’s the manual:
http://www.ghielectronics.com/downloads/FEZ/Panda_II/FEZ_Panda_II_UserManual.pdf.


#2

12V will work, even 15V or higher will more. It is all about heat dissipation.

I suggest you stick to what the board says, 9V max.


#3

So what you’re saying is, if I connect the 11.1 volt LiPo battery the voltage regulator will have to dissipate more heat? Is this a constant dissipation, or is it relative to the amount of current being drawn by the FEZ outputs?

In any case do you have any recommendations for what would be an easy and efficient way to regulate the 11.1 down to something more suitable for the FEZ? I assume a simple resistor isn’t an option as that would depend on the current the FEZ draws. I’ve heard about switch-mode voltage regulators being very efficient and easy to use, but I have no clue which ones are popular.


#4

Hi there,

the more current the device uses (not just Fez but all the attached peripherals) then the more heat the onboard VREG has to dissipate. The onboard vregs are good, but are not large and do not have great heat dissipation.

Tell us what peripherals you might be running from the 11.1v battery, including things running from the regulated power on the Fez (so shields and touchscreen etc).

As Gus says, much over 9v and a “usual” load with just the Fez, you start to get hot. If you have a display connected for instance, or ethernet shield, then the current draw is too much if you’re running over 9v. Ideally, if you are drawing a lot of current, then you want to be down near 7.5v ideally.

Something like an LM317 or LM7808, while not a switchmode vreg are reasonably efficient if you just want to drop the current down from your pack’s 11v1 to ~7.5 and then feed the onboard VREGs with it’s output.


#5

Currently, the only things I’ll need to drive from the FEZ is
[ulist]L293 H-Bridge (http://www.toddholoubek.com/classes/pcomp/wp-content/uploads/2009/02/l293.pdf)
RX433 module (http://www.apogeekits.com/PDF_Files/RF_Receiver_Module_Datasheet.pdf)[/ulist]

The H-Bridge will control two motors using one PWM pin and two IO pins each, so a total of two PWM pins and four IO pins.
The RX433 module is connected to the FEZ 5v supply and uses COM1 RXD, and will be receiving continuously at 2400 baud.

I don’t know how to estimate the current drawn from those components in total, so help would be much appreciated.

I don’t mention the motors as the H-Bridge allows me to source the motors directly from the battery pack, circumventing the FEZ circuit, and the batteries are 30CC while the motors are only using ~120 mA on normal load, so they should never cause any drops in the battery supply.

I don’t know if this is the “usual” load of a FEZ, but I will be adding more later components later, so I’ll be on the lookout for a good voltage regulator anyways. However, with the current details, am I safe to do some preliminary testing without frying my FEZ?


#6

for small amounts of testing, I suspect you will be ok, but this is a problem you will need to address sooner or later in the configuration you’re proposing so I’d get on that sooner rather than later. Given you are driving motors, perhaps you should consider a 2 cell Lipo pack just to run the Fez as well as the 3-cell for the motors? Are you space/weight restricted in your application? Also, what is the draw from the motors at/near stall ? That’s a more reliable estimate of howmuch battery sag you’ll experience.


#7
Voltage             =  12 V
No-Load Current     = 3.5 mA
Normal Load Current = 120 mA
Stall Current       = 360 mA

Those are the exact motor specs. I think I’ll give the 2+3 cell pack solution a go, as I have plenty of space to work with, and it seems the easiest.