Power and Processing problem on Fez Panda 2

I power my FEZPanda2 from a 5V output coming from my motor driver (Sabertooth 2x25) and noticed that when I drive the motors at a higher speed, the serial port or the tcp/ip channel on the FEZ stops communicating and/or the program on the Fez stops running (I think it’s this one). I noticed that some other people in the forum had similar issues with some boards, so I measured the voltage and I have a steady 4.98 volts going into the 5V IN/OUT socket, and the one of the GND sockets going to 0V. And, weirdly enough, if I plug in the USB cable to my PC, the program starts running correctly again…and I think that USB provides about 4.5-5.0V, too.

Has anyone come across this issue before? I have a 12v source in my robot - would it be better to power this board by providing 12v to the Vin socket (under the GND sockets), instead? (I wasn’t sure what the 12v socket was for exactly)

After further testing, I noticed something else: my motor driver drives two motors. If I drive one motor at high speed, the program on Fez continues to run okay. If I drive both motors at high speeds, the program stops running at some random point. So, I’m starting to suspect a current draw problem, via the motor driver, because my voltmeter indicates that the power is still about 4.98 volts at both low or high speeds. My next test will be to measure the change in current, I think. Thoughts?

Hi Wayne,

This kind of issue is related to current source capability. The source that you’re using can’t keep up with both the motor demands and still provide enouth power to the Fez to continue operationg it. What happens is that the voltage sags sufficiently to cause you an issue. I suspect your 4.98 measurement isn’t showing the sag when the motors drive - either start or change in speed - when the load is higher.

the voltage may be 4.98 but that does not equate to power. ohms law is your friend here.

remeber that V/.IR substitute out for the unknown.

note power can be found by
P = VxI
P = V^2/R
P = I^2xR

invaribly in your case it does sound like you are tryng to pull too much current from th power supply you have available. i would alway recommend powering motor that can have large power requiremens seperately from the supply that is used to power the processor and accompanying logic.

hope this helps

Hi, Brett & Nubiarn:

Thanks for the advice, you’ve confirmed what I was thinking…I’ll power the card separately. I know there is the 6-9 volt plug, but can I power it via the 12V socket (the one right next to the two GND sockets) with 12 volts? I was under the impression that the Fez would only accept either 5 volts via USB, 5 volts in the 5V socket, or 6-9 volts in the plug. But, since I happen to have a 12 volt source in my machine, can I use the 12V socket without burning out the Fez?

Th FEZ panda can be powered by the USB or 6-9dc power supply socket.

•Through USB port or an external DC 6-9V power supply (connecting both is safe).
•3.3V regulated DC output is available.
•5.0V regulated DC output is available

i see that the Sabertooth has a built in 5V 1A Switch-mode BEC that can provide power to a microcontroller or R/C receiver, i assume this is the supply that doesnt have enuogh power output to keep the panda alive when running at higher motor loading

In this case i woud power the fez panda via the usb preferably via a powered usb hub, or through the power jack from a seperate psu. I would then use your original 12vdc source to only power the sabretooth motor controller. This way your panda will not drop out under high loadings.

@ Nubiarn:

Yes, the 5V 1A BEC is the supply that doesn’t have enough power to keep the Panda running at high motor load. Do you know what the 12V Input socket is useful for on the Panda? (the one below the GND sockets)?

@ Wayne, on my Panda2 I don’t have a “socket” that has 12v written anywhere near it - can you tell us where exactly that is on yours? I have one input that is marked as “6 to 9V” which is the input into the onboard regulator (the barrel jack). As @ Nubiarn says, the only options are unregulated power that can be processed by the onboard regulator (which could go to 12v if you needed to but it’d generate a lot of heat which is why you should avoid it if you can), or you can provide regulated 5v power (what the USB connection gives)

Sorry, it’s the pin labeled Vin, which the documentation says is “Vin for external power 6 to 12 Volts”. (But, I got the card powered and working by supplying 5 volts to the 5V pin, which the documentation says “5 Volts In/Out”.)

OK, VIN is the same as the barrel jack input. Yes, the brochure does say it’s capable of up to 12v, but again I’d be careful doing so, the silkscreen value only says 9v. It would depend if you have any high power devices on the Fez, like a screen or something. But if you’re feeding in 5v that should be ok; the 5v is still filtered and regulated by the 3v3 circuit onboard, so even if the BEC isn’t outputting the cleanest power you should still be ok.

Ah, now I see…that makes sense, then, that the VIN is the same as the barrel jack input. I’m feeding in 5v to the 5V pin, but I took the earlier advice and am feeding the power directly from my 12v source (instead of from the Sabertooth motor driver) and using a switching regulator to step it down to 5v. This seems to work nicely so far - thanks.