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Panda II : Vin is 12V or 9V max?


#1

I have a FEZ Panda and I plan to migrate to Panda II.
In my project, my Panda is powered by a 12V battery. I wonder if I can still use the same powering with my Panda II.
The Panda II specs are a bit confusing :
In the schematics (http://www.ghielectronics.com/downloads/FEZ/Panda_II/FEZ_Panda_II_sch.pdf) => It’s 12V
On the board itself, and in the manual => It’s 9V

9V? 12V?


#2

The regulator chip, LM1117mp, is the same in both versions of the Panda, so I don’t see why you can’t continue using 12 volts.

I’m currently testing a Panda II running with 14.6 volt input to test for any overheating or other problems. So far so good although I’ve been advised that this is flirting with overheating.

The real experts may have a better reason for the down-rating to 9 volts on the PII, but I’ve not seen this addressed.


#3

12v is fine but when connecting fez connect shield then even 9v may heat up the regulator


#4

the larger the difference between the input voltage and the output voltage of a regulator, the larger the amount of power that must be dissipated by the regulator. Also, the higher the current being consumed by items downstream of the regulator, the higher the power dissipation requirement. All that dissipation is in the form of heat - so the more components you drive and the higher the input supply, you really need to be aware of the impact on heat from the VREG. And a surface mount VREG on a PCB is not a highly effective heat dissipator on it’s own.

If you have to have a higher input voltage, and you have to have a large current draw, then considering adding another regulator prior to the Fez, where you can add an appropriate amount of heatsink - you can bring the voltage back down to ~6-7v and feed it to the Fez, or possibly bring it down to 5v and feed it direct onto the panda.


#5

Brett’s answer gives the complete picture. If you aren’t drawing a lot of current in your app and don’t mind dissipating power in the regulator you have some headroom for using a higher Vin.


#6

OK, 6V should be the most efficient in term of energy dissipation.

One question is remaining : I have notice that when powering the Panda with 6V (4 AA batteries, so maybe 5.2V in reality), everything becomes “weaker”. Piezo is weak, and most important, LCD display is weak (weak contrast). With the 12V battery there is no comparison.

I don’t know if it is due to small voltage, or to small power (A) of my 4 AA batteries.


#7

You should measure your 5V supply. 6V is maybe too low. The regulator drops a certain amount - usually you should plan about 1.2 - 1.5 volts above the regulator rating. That’s why everyone is advising you to use 7-9 volts. You probably need 5 batteries for safe operation, otherwise the 5v output may drop below 5v.


#8

The drop out voltage for the LM1117 is about 1.2 volts. The drop out voltage is the input/output differential at which the circuit ceases to regulate against further reduction in input voltage. It is measured when the output voltage has dropped 100mV from the nominal value obtained at VIN = VOUT +1.5V.

With 6 volts in - 1.5 volts =4.5 volts methinks the Panda doesn’t have enough juice to get a solid 5 volts out on the first regulator (the second one is for the 3.3 volt bus).

I’m dealing with the same problem and that is why I’m using a 12 volt battery with back up charging. Please critique, if you have better information.


#9

When I’m feeling lazy and need to drop a few volts I just put a few rectifier diodes in series. They will drop about .7 volt each so 5 gives you enough voltage drop to run from 12 volts where 9 is desired.

I did this a couple times with my DVD player which has a 12 volt lighter socket car adapter power cable and I have a very large 16V external battery pack for computers. Could not find a spec for maximum input to the car adapter but figured it must be safe at 14.5 volts as that can be expected on a vehicle so used four 3 amp rectifiers. Worked fine and I watched movies all the way from the US to Thailand but the rectifiers got so hot they melted the plastic body of the car adapter I had taped them against.

Next trip I used a 3-Terminal adjustable regulator (don’t recall which one, one of the LM3XX series) on a large aluminum heat sink and again the heat sink got very hot but everything worked.

Would like to find a small switching regulator module so as to not drop the extra voltage as heat. But then again not sure they would let me bring such a large battery onto a plane these days. It is about 5 in by 9 in by an inch thick.

A related and funny story was the time I forgot my laptop power brick when travelling to Thailand (my wife is Thai – we go often) but had my airplane power adapter which takes in 12 volts. So was walking around Bangkok and spotted a electronics store but only 12 volt power supply that had was huge and weighed probably 15 pounds. So had them wire up a lighter socket to it and used it the entire trip. Made a few flights with it as carry on that trip and while it raised a few eyebrows it was allowed on the plane. But that was in 2000.


#10

There is a variable voltage regulator LM350 that can deliver 3.5 amps anywhere from 1.2 to 35 volts( from a suitable DC wall wart, 12V battery or similar). You can put a circuit together with a bare minimum of parts and feed Panda II at 7.V which will not break a sweat on the internal regulator and provide lots of power for external ancillaries.

I guess I should add that you can’t get 35V unless your supply is greater than that. I use them lots and have not had a failure. Very inexpensive.