3.3v regulator and external power

Status
Not open for further replies.
T

turtle9er

Well-known member
Hi,

I have a project where I need a subject to wear a teensy while they walk. There are already a few threads that mention this, by applying 5v or 3.3v., however I am still confused exactly how to connect to the 3.3V. I am also using the ADC, therefore would like the cleanest signal possible. If I get a 3.3 LDO regulator (any recommendations?), can I just cut the pad and apply the 3.3v to the 3.3v on the teensy, or do I only cut the pad when bypassing the usb power? Can I also keep the usb function and get power from that if needed during programing etc? I am still learning all of this, and I learn best by seeing pictures or schematics, since I get so confused by all the jargon some times and therefore can't follow some of the threads that already exist on this subject. I am wondering if anyone has done something similar, and if so if they might have a picture of this set up or a schematic. My ideal setup would be when I need to collect data from a subject I would hook up the battery pack and flip a switch and start collecting data. If I ever need to change code, I can then just unplug the batteries and plug in the usb.
 
I tend to think the simplest approach is just to power it via the USB port. You get cell phone chargers that put out 5v, and which have a battery that you recharge in turn from a computer or wall charger plate. I bought the EzoPower 5000maH battery from Amazon for under $25 (http://www.amazon.com/EZOPower-Universal-Rechargeable-CellPhone-More-Silver/dp/B004I0J4F0/ref=sr_1_10?ie=UTF8&qid=1377401098&sr=8-10&keywords=ezopower+charger+5000maH) 1-2 years ago. Now, I haven't done a timing test on the Teensy, but on my Arduino Uno, running the blink program with no special attention to power saving, it ran for 4-5 days before running out of power. If you need it, you can get batteries that have a higher maH rating for a longer runtime, or batteries that are smaller, which might be more convenient for the Teensy (for the Uno, the battery is about the size of the Uno).

In one sense, you are wasting power somewhat, because you have to go from whatever the batteries internally put out to the regulated 5v of the USB power, and from there the Teensy has to drop the power to 3.3v, while it might be more efficient to go directly to the 3.3v.

For the stuff I anticipate running Teensy (or other embedded microprocessors) on, having it run for 18 hours or so, is all I need, and I can recharge it at night, and I haven't worried as much about getting the most charge out of the batteries. If I did several day treks through the wilderness without a power station, than it would be a different answer.

What you might want to do, it get 2 sets of batteries (no matter what type of batteries you get), and have one back at the base station charging, while you use the other one in the field, and then just swap batteries.

Now, if size is more important, you can go the lipo route to get 3.3v batteries and bypass the USB voltage regulator. I don't like that as much, because it would be yet another battery that I have to get a charger for, etc. With USB phone chargers, ti uses the standard mini/micro cables, I already have, and I can charge it anywhere.

In the electronics store near me, they also have a charger that takes 4 AA batteries and puts out a regulated 5v of power. You can use rechargeable AA batteries for this. Since I have a number of AA batteries for my flashes, I've thought about picking one up for an additional backup, but right now, I have several batteries, and I don't really need more. Here is one from a US seller (http://www.ebay.com/itm/Portable-Emergency-Cell-Phone-Battery-Charger-4AA-Fits-All-Phones-w-Micro-USB-/360573278429?pt=US_Cell_Phone_PDA_Chargers&hash=item53f3d798dd), though the one in my store is cheaper (and you can get it cheaper from China if you want to wait). However, if you don't have a stockpile of rechargeable AA batteries, I would just go with the self contained unit.
 
Last edited:
The Teensy has a 3.3V LDO regulator integrated (fed via diodes from the 5V USB). According to this thread:
http://forum.pjrc.com/threads/13030-Teensy-3-0-Internal-voltage-regulator-18650-batteries
the voltage drop is 0.4V. The Teensy 3 LDO regulator has a dropout voltage of 0.3V. So the input voltage range (assuming you want to get 3.3V VDD) is something like 4V - 5.9V. 4x NiMh AA or AAA batteries would work perfectly.

If you are not using VDD/VDDA as voltage reference for the ADC, a LiIon battery directly connected to the 5V input should work perfectly fine.

I doubt an external voltage regulator will buy you much for ADC accuracy, if you don't separate VDD/VDDA.
 
Thanks for all the info.....never thought about using an external battery backup system. At most I will be collecting data for 2hrs, so not to worried about getting every single maH out of the battery. However, if I use a normal AA battery holder and connect directly to Vin, do I have to cut the pad to isolate it from USB, or can I just make sure there is no external power if I need to use the USB to program. I just don't want to cut the pad, and then have to keep soldering or un-soldering that connection if I need to use it for another program. Sorry, just trying to go from the external power info page http://www.pjrc.com/teensy/external_power.html however this seems specific to the teensy 2++. Thanks again for every ones help.
 
Last edited:
turtle9er said:
do I have to cut the pad to isolate it from USB, or can I just make sure there is no external power if I need to use the USB to program.
Click to expand...

My understanding is yes, you have to cut the pad if you want to apply 3.3v externally. Otherwise you'd be backfeeding the regulator, which is apparently a bad idea. Some regulators may feature the snubber diode referenced in the link, for all I know, but my presumption is that Paul added the jumper trace between the pads for a reason, i.e. he recognized in advance that some people prefer providing regulated power externally from the Teensy board.

I prefer providing power externally for the simple reason that it keeps the heat out of the processor and allows me to tailor the power supply to a specific need. The boards I have mounted the Teensy 3 on feature either pin headers to select the source of 5V power (V-USB, on-board power, etc.) or use schottky diode arrays to select among the voltage sources in an automatic manner. The pin header is easily the most reliable / foolproof way to do things, the Schottky array makes things super simple for users. In the end, it all depends on the manner in which the Teensy is used that determines the best configuration.
 
If you connect the battery to the 5V/Vin pin/pads, you are fine, no need to cut anything. Just make sure not to connect USB at the same time. You could also cut up a USB cable and just connect the power wires to the battery.
 
Status
Not open for further replies.
Back
Top