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Fez Panda supply voltage


#1

Hey guys, quick question, whats the absolute minimum voltage that I could supply to the VIN pin of FEZ Panda?

I’ve got a spare 7v2 battery laying about, and was wondering whether I can plug that straight in, or would I need to regulate it myself to 5V and supply it via the 5V pin.

Thanks :slight_smile:


#2

I use a lion 7.2 volt battery. The on board 5v reg only really needs 6+ volts to operate.

The absolute minimum would be around 5.7v but I wouldn’t run it at this voltage…

Cheers Ian


#3

http://www.datasheetcatalog.org/datasheet2/8/0uzr32fcc5ych0637cho90kh7f3y.pdf
The datasheet of the LM1117-5 says operating max is 15V (abs max of 20v)


#4

Thanks guys - while I’m on the topic of voltage levels - What are your preferred methods of interfacing with 5V level devices? I’m thinking about a PWM signal I have going to a VNH2SP30 motor driver, that requires 3v25 for a high signal. I’m a bit worried that with lower duty cycles the signal won’t be picked up.


#5

[quote]The datasheet of the LM1117-5 says operating max is 15V (abs max of 20v)
[/quote]

yes, but…the larger the voltage drop the across the regulator the more power it must dissipate.

P = I * E

If you have a large E as I increased the power dissipation rapidly increases. Your regulator will get hot and you will be wasting lots of power as heat.


#6

William! The MINIMUM voltage…

Xarren! The nature of PWM motors means that if the motor sees the signal it will work… I used a 48v PWM motor and my PWM output signal was only 5v and it was fine. If it doesn’t see the signal you’ll have to use a transistor to level change.

Jeff! I agree. As the current in this case will be in excess of 100mA. However you can still put a heat sink on the regulator.

I wouldn’t personally put more than 9V on the regulator with this current!!

Cheers Ian


#7

Ian - This is the motor controller I’m going to use:

What do you recon, 3v3 PWM signal - Would that be fine?

The reason I’m asking in advance instead of testing is that it is going to get delivered something like a week before all my coursework is due in, so I kind of need to make sure everything works, at least theoretically, and get the PCBs for it etched before it comes, then just hope it works.

Also - transistor, or a low voltage FET? Wouldn’t a transistor be too slow? (20Khz)


#8

That chip is really designed to interface with a 5 volt micro. With 3.25 volts VLh it’s close. This is a guaranteed operation spec. It may work fine and it may not. There are level translator chips that could solve this problem. As an alternative Infineon has the BTN7960 half bridge chip that is 3.3 compatible. Robot Power has a shield and board using these chips.
http://www.robotpower.com/products/MegaMoto_info.html
If this is a design project, they do have a schematic.


#9

Xarren! Yes, I have looked at the datasheet and the PWM input IS CMOS compatible, if it were TTL there may be cause for concern. The worst case in this instance would be a 5v pullup resistor on the PWM input.

Gary! Your knowledge of motor drivers will exceed mine as I don’t use motor controllers as such, my motors aren’t directional so the controller is built in to the motor.

Cheers Ian


#10

Thanks for the input guys - I’ve already ordered a board using the chipset I’ve specified earlier.

Wouldn’t a pull up resistor from the FEZ Panda output damage the output pin? I was thinking to put a low voltage FET on it to 5V.


#11

No!! The panda’s pins are 5v tolerent…

Cheers Ian


#12

Thanks, what value resistor would you suggest? I have never used pull up resistors to change the voltage of an output, and am still trying to work out in my head how that’s going to work. What stops the voltage being 5V while the signal itself is low, if it connected to 5V via resistor?


#13

Worry about that later… I’m sure it will work okay without the resistor. If you are designing a board then about 2.2k will do… but I would check operation first.

When high there will be 3.3v on the panda’s output pin there is a residual resistance in the output so there will be a voltage divider between 3.3v and 5v thus yielding more than 3.3v, when low its at ground potential. So you will have a 0 -> @ 4v signal.

Like I said logic high on MOST 5v systems is 3v +, so all should be okay

Cheers Ian


#14

Thanks :slight_smile: