XBee Pro adapter current

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grapeghot
 
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XBee Pro adapter current

Post by grapeghot »

The current rating of the regulator on the XBee adapter board is 250mA, while the current rating for a XBee Pro Series 2 is 295mA when transmitting. Is it recommended to use a larger regulator?

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Re: XBee Pro adapter current

Post by adafruit »

its not the -best- engineering practice... :D but the xbee doesnt use 300 mA all the time, only in bursts, so it should be OK. just make sure you have about 4V into the regulator in case the drop out is high

grapeghot
 
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Re: XBee Pro adapter current

Post by grapeghot »

Ok, that sounds kinda reassuring.

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ericwertz
 
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Re: XBee Pro adapter current

Post by ericwertz »

I saw this, too. But it's not just the Series 2 Pros, but the XBee-Pro "Classic"s also that can peak at 250mA (340mA if using the antenna'd version).
I was checking on this before I got too far along (perhaps) designing my own (first) board. The MCP part does specify a *minimum* maximum current (try to get your head around that one :-)) of 250mA.

I'm still not loving the 250mA part here, given that the LEDs and buffers could be good for 13-30mA altogether. Plus, no telling what somebody's gonna want to hang off of the broken-out I/O pins.

I'm not going to push it and will swap-in another regulator. I'd rather not risk getting caught with my buffers down around my ankles.

Margin is your friend, unless you're a lousy stock-picker.

-e

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Re: XBee Pro adapter current

Post by adafruit »

the LEDs are less than a mA total (1K resistors)
the buffer also uses less than 1 mA, its CMOS logic
considering how 'marginal' most DIY projects are, this is probably on the low end of things i'd be concerned about. its not like all of a sudden at 255mA the regulator dies. itll just droop down a little. and the xbee can run on as low as 3V which gives it a lot of droop to drop

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ericwertz
 
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Re: XBee Pro adapter current

Post by ericwertz »

I don't see 1K resistors on the schematic, I see 200 on one, and 200-1000 on the other. Looks more like 5mA to me total if one's 200 and the other 1K, ~7mA total if both 200ohm, or ~1.5mA total if both 1K. If nothing else, it looks like the BOM and schematic don't exactly match.

I did see that 4mA in the datasheet for the buffer (typical source/sink), but I'll readily admit that it wasn't clear to me if this was per gate, or for the whole package. If it's per-gate then it'd be 16mA (typical/recommended) or simply 4mA if not. I(max) for the whole quadpack is 50mA though, if for some reason it's famished.

Digi specifies 215mA typical Pro xmit, 250mA max, and 340mA max for their RPSMA-and-chive-flavored chips.

My point is that it's getting close even if you assume typical numbers, but without anything useful connected to it. The concern is that if someone starts loading up the post-regulated output by sourcing/sinking through the I/O pins, or switching with an external transistor, they could get busted "texting (xmiting) while driving".

As long as people know that they shouldn't use the 3V3 output for driving their XBee Pro and toys (like unlatched relays or LED arrays), then.... sweet. As a practical matter, I'd be more worried about relays unlatching everytime the XBee transmits (although latch-hold currents are probably a lot less than latch-pull currents). Hopefully they'll figure this out if/when their relay-switched loads toggle on and off when transmitting.

I worry a little that not all of the non-EEs (like moi) aren't necessarily going to appreciate that some V-regs aren't 1A capable like the 7805s that they may be used to, and that some wireless designs can take even more current than many of the eletromechanical (or even 20mA LED) goodies that they might already be familiar with. I actually really like that it's possible to do a lot with the XBee without having to bring your own uC with the I/O peripherals that it's got, but if they can't be driven (good luck finding sink/source current specs because they're not there) with the regulator, then it's for naught.

I also worry that the homepage says "Level shifting circuitry means that its trivial to connect it to 5V circuitry such as an Arduino without risk of damage", when this is only really correct for the comms+RST pins. Any connection to the I/O pins on the XBee from the Arduino are still at risk. This wasn't obvious to me until I looked at the schematic. The compounded problem is that Digi doesn't (currently) document that their I/O's are not 5V tolerant.

Contrary to appearances, I actually don't enjoy hearing myself talk, and so certainly wouldn't expect you to either. Just trying to help. Sorry if it doesn't appear that way.

regards,
-e

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Re: XBee Pro adapter current

Post by adafruit »

well the power supply rating is clearly noted on the project page and in the store. of thousands of customers, there hasnt been a single report of a power supply failing. (and people always report any design flaws, believe me. :lol: )

the LEDs use 1K resistors in the kit (check your kit) and they're not both on during TX so thats 1mA
the Icc of the '125 is 0.05mA (check the datasheet again, you're not reading it right)

if you find me a better power supply chip that's to-92, ill consider it for the next version
& if you want to use the biggest xbee out there with this design but the supply isnt good enough: the adapter is open source hardware, if you have improvements you can always make modifications for your own use and re-publish them!

anyways, the pages have been updated to note that the 300mA Xbee pros may not work and the wiring page makes it clear how the shifter functions

grapeghot
 
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Re: XBee Pro adapter current

Post by grapeghot »

I found two TO-92 300ma regulators at mouser:

Sharp PQ033ES3MXPQ
AME AME8811AEATZ

Maybe for better margin, you might think about going to a surface mount package. The Microchip MCP1825S-3302E/DB looks a good candidate since it can handle 500ma, but unfortunately double the cost. Also, this would make the kit more difficult for beginners to solder.

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ericwertz
 
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Re: XBee Pro adapter current

Post by ericwertz »

Well, I hear the Keyboard Cat, so I guess I'm done here. I appreciate the help. Have a good time at Maker Faire.

Sensei, arigato.

adafruit
 
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Re: XBee Pro adapter current

Post by adafruit »

Hmm, I vaguely remember seeing those parts when i specced the psupply and the thing that annoyed me about the sharp part is a .7V dropout (compared to MCP1700's .17V) and the AME part was often out of stock (whereas the MCP1700 is available at every distro)
unfortunately, they're not pin compatible so its not a drop in replacement and would require redesigning the PCB so we'll consider it as an upgrade. and still they're under the 340mA max... annoying because the LD1117 series is 800mA in a SOT23 package and currently im trying to keep the kits THM only :(

grapeghot
 
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Re: XBee Pro adapter current

Post by grapeghot »

Having just one SMD part wouldn't hurt or will it? A SOT-23 is easy to solder, although it might scare off some customers. But, wouldn't that be great a way to get people introduced to soldering SMDs? IMHO, I wouldn't hold back trying to improve a design, just because I had to use through hole parts.

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karlgg
 
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Re: XBee Pro adapter current

Post by karlgg »

If you look at some of the soldering problems new people have, there is a LOT to worry about by giving them a single doodad that has nothing to hold it in place before soldering but your hopes and dreams. In the grand scheme o' things, it's not that big of a deal... if you know what you're doing, or are careful and/or lucky. But you're increasing the danger zone for those who don't, and aren't.

And I think LadyAda has enough fires to put out with the kits as-is!

adafruit
 
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Re: XBee Pro adapter current

Post by adafruit »

im looking into using a THM 300mA LDO for the next rev, but SOT-23 is fantastically hard to solder

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len17
 
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Re: XBee Pro adapter current

Post by len17 »

I for one am grateful for the through-hole designs.

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richms
 
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Re: XBee Pro adapter current

Post by richms »

Everything I DIY is surfacemount - yeah its harder to solder, but without needing to drill holes in boards its faster - when making a kit where you are getting stuff made by machine where the holes are not hard then I can see the attraction with thruhole however.

To me a SMD is not a discouragement and really, its a skill that you should learn IMO.

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