This weekend I finished assembling my FEZ Tinkerer Kit, and plugged in a Panda II and a FEZ Touch.
This kit is great
Everything is well built and fits perfectly. The case is very nice too. It is the perfect combination for three projects I’m currently working on for my home: a Security Controller, an Automation Controller, and a Sprinkler Controller.
My only very minor complaint is that the location of the XBee mounting holes precludes mounting the FEZ Touch without a cable. I wish the XBee had been moved down towards the edge about 1/2 inch.
I’m kicking myself for not ordering a couple more of these when they were on sale last month
I just bought two more of these kits, and am looking forward to using them
I am using a Panda II with the Tinkerer board, and would like to solder in a super capacitor for the real-time clock.
[ulist]
I noticed that GHI uses a 0.22 F capacitor on the Connect shield. Is a 1 F capacitor okay?
Can I solder it in anywhere that Vbat is exposed?
Does it need to be near the Panda II board?
Do I need to worry about other shields “interfering” with this voltage?[/ulist]
if you take a look at the skiz for the panda II board.
You will find D1 that goes from VCC to the Vbat pin which also bridges off to VBAT, where you would connect your backup cap to.
Now here is the deal, when power is supplied it will charge your 1F cap through that diode. 1F is rather large. Depending on the current of the diode it may fry it or the foil going to it or to the cap.
what would be better is to limit the inrush current to the cap by placing a 50ohm resistor in series from VBAT to your cap. this way if you cap is dead and you apply power it will slowly charge it and not put to heavy of a strain on the power supply, foils and diode.
Can I solder it in anywhere that Vbat is exposed?
you can solder it anywhere you see vbat.
Does it need to be near the Panda II board?
no
Do I need to worry about other shields “interfering” with this voltage?
i have not looked at all that can be used, so dont know.
the Connect shield schematic shows the cap connected directly to the VBAT pin. so if you do want to hang the larger 1F cap i would recommend a series resistor.
I want to use this LiPo charger [url]http://www.sparkfun.com/products/10300[/url] by connecting the VCC and GND output to the 5V and Gnd rails on the Tinkerer. These rails connect to the 5V and Gnd pins of all boards plugged into the Tinkerer. The LiPo will be powered from an external wall wart.
So far I think everything is okay, but I would also like to be able to plug a PC into the USB port of the Panda II for programming. I believe that this is okay for the Panda II because the Vin of the USB connector is connected to the +5V rail in the Panda II via a diode. I’m not sure that it is okay for the LiPo charger. Here is a schematic of that device [url]http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/PowerCell-v12.pdf[/url]
The only concern i would have is if you don’t connect the battery, and then plug in the USB cable into the Panda to power the system. The 5V power from the USB cable would back feed into pin2 of the TPS61200 IC. Though the block diagram shows it as a fet output, i have found that many times IC manuf, use generic block diagrams to help explain things when there is actually more in there.
Its best to play it safe.
To make it 100% safe, i would remove the series diode off the panda from the USB jack, so the USB cable cannot power the board. To power the board you would always use that external power supply.
-OR-
you could go at it the way you explained, and put a series diode from the external power supply +5v to the panda +5v rail this way it would block any back feed.
Another big thing to check into is this, what is the min discharge voltage of the li-Po that you will be using. This is a pet peve of mine with Sparkfun concerning there li-po stuff. They do not tell people that many li-po’s cannot go below 3.2v. Draining a li-po below 3.2v is a VERY bad thing.
You MUST make sure that your battery can tolerate a discharge to that voltage.
it would be a good idea to solder the tabs of your cell to this for full protection both ways. I.E. charging and discharging.
Li-Po are great battery’s, but they are also the most sensitive to over charge and drain. If you have ever seen one catch fire you would understand my concern. They are violent when they catch on fire!!!
i had a 2200ma cell puff up and blow a 3’ intense flame out of it for ~8-12 seconds. Good thing i was in the garage when it started to puff up to throw it on the floor. I hate to think if i was not home and that happened.
According to Sparkfun the [quote]Battery includes built-in protection against over voltage, over current, and minimum voltage[/quote].
The datasheet says
[quote]3 Specification
Item Specifications Remark
Nominal Capacity 2000 mAh 0.2C 5A discharge
Nominal Voltage 3.7V Average Voltage at 0.2C 5A discharge
Charge cut-off Voltage 4.20±0.03V
Standard Discharge Current 0.2C 5A
Max Discharge Current 2.0C 5A
Discharge cut-off Voltage 2.75 V[/quote]
The also mention in the datasheet that they stress tested it and it didn’t catch fire
Your welcome.
I know i probably came off a bit paranoid, but i have designed many battery charging systems over the past and have used the same boost IC you want to use. I have personally seen these devices fail when we took them to the lab to have them stress tested in Hot/Cold & humidity. So by default i always now add the battery protection board on all my batteries as an added protection level just in case.
Nothing worst to be sleeping at night while this guy is charging and the IC fails allowing the battery to overcharge.
