View Full Version : Externally Powered Teensy 2.0++ with 4 AA-Bateries.

01-04-2013, 05:28 PM
I want to power a Teensy 2.0++ externally with 4 AA batteries and still use USB power when connected to the USB port. I've read the documentation using option 3 which snips the 5.v pad lead and adds 2 1N5817 diodes, one between the pads, and one inline with the battery's positive lead.

I want to use either 4 alkaline(1.5v) or 4 Enloopes NiMh(1.2v) batteries. I want the chip to run at 16Mhz.

Will both these battery options work???

The akalines will provide 6 volts minus the .3 volts drop over the diode = 5.7 volts...is this too much?
The Enloops will only provide 4.8volts minus .3 volts = 4.5 volts...is this too little?

What happens when I plug in the USB 5-volt cable when the 5.7 volt alkalines are there too?
I guess USB 5v will be powering the USB components and the alkalines will be powering the rest of the chip??????
On the other hand, if Enloops are installed, I guess the 5-volt USB would be powering everything?

Any info would be greatly appreciated.

01-04-2013, 06:09 PM
I chose to use option 1 which cuts the trace. This works fine during development of my project where I had 5V power and usb connected. Then I soldered the trace back together when I put the teensy on a circuit board when the project was working put into service.
Using the diodes (option 3) will work fine with both power sources but if you go over 5V you will blow the teensy. So the alkalines are out. And the rechargable nimh seem to look OK except for when they just come off charge. Each battery may be as high as 1.4V depending on the charger.
So for the battery power I'd use 5 batteries and then use a low dropout(LDO) 5V regulator.
Hope this helps.

01-04-2013, 10:50 PM
This discharge curve seems to show 4 eneloops as being suitable:
Source: here (http://www.stefanv.com/electronics/sanyo_eneloop.html)

although I agree the data sheet (http://www.eneloop.info/fileadmin/EDITORS/ENELOOP/DATA_SHEETS/HR-3UTGA_data_sheet.pdf) shows a peak discharge voltage of 1.4V.

Edit: I see the graph above is at 1.2 amps, which explains the overall lower voltages. Discharge at lower current gives higher voltages.

01-05-2013, 03:48 AM
Hey Guys,
Thanks for the quick response.

I looked on the Teensy website concerning external power, and saw something I hadn't noticed before.
Based on the information on that page it says 6 volts is the max...not 5 volts.
It specifically states that:
Caution: Reverse polarity power or greater than 6 volts can damage Teensy.
There is also a table stating that 4.5V to 5.5V is the range for running at 16 Mhz.

My dilemma is, I really want to stick with 4 AA batteries ...two by two lengthwise.
That's the size of the form factor I can work with.

If these enloops can put out 1.4 volts at low amps, it sounds like they don't have much less peak voltage than the Alkalines at 1.5 volts.
That's 5.6volts vs. 6 volts.
The inline diode will knock those maximum voltages down by .3 volts keeping us within the stated limits.
Also these batteries will loose voltage as they run, so although they will be runnining near maximum initially, they will taper off to more normal ranges.

Another question: can inline diodes be used in series? Two diodes in series may be just the ticket knocking things down another .3 volts for freshly charged batteries.

As far as LDO's, I may be wrong, but doesn't an LDO requre more voltage to work in this case?
Can an LDO drop as small amout as .6 volts?
Also, does an LDO have internal reverse battery protection...or do you still need to include an inline diode between the LDO and a Battery.

Let me know if the two diodes in series will work.
Or...if there is a standalone LDO solution that will work with 4 AA batteries?

01-05-2013, 03:27 PM
As far as LDO's, I may be wrong, but doesn't an LDO requre more voltage to work in this case?
Can an LDO drop as small amout as .6 volts?
Also, does an LDO have internal reverse battery protection...or do you still need to include an inline diode between the LDO and a Battery.

A linear voltage regulator typically requires substantially more voltage than they put out (the one on Arduino wants 6.5 to 12V in for 5V out, for example). Special low drop-out (LDO) regulators work with only slightly higher input voltage than output voltage (300mV for example).

They do tend to be surface mount devices however. LT3060-5 example (http://www.farnell.com/datasheets/1359603.pdf).

However, while a 5.5-in, 5.0 out LDO would be fine with 4*1.4=5.6V, it would stop working with 4*1.35=5.4V - and eneloops spend most of their discharge cycle at 1.2V

Perhaps running a Teensy at 3.3V and 8MHz is an option? For example, LT3060-3.3 needs 3.85V < Vin < 45V so would be fine with 4 eneloops anywhere from their maximum discharge voltage (1.45V?) through their normal of 1.2 and right down to 0.97V each.

01-05-2013, 04:52 PM
FWIW, I've run Teensy2 from 4xNiMh, with one series diode in the power lead, and it hasn't blown up yet.

01-05-2013, 06:32 PM
Atmel 32U4 (Teensy 2.0) safe operating voltage. At 8MHz it covers a fair range of voltages.
Source: atmega32U4 datasheet.

It looks like 4x AA eneloops with a peak discharge voltage of 1.48V and a low voltage of 1.1V before running right down, plus a 1N5817G Schottky diode with a voltage drop at 0.1A of 0.32V will provide a range of 5.60V to 4.08V, reverse voltage protection up to 20V, and be totally within spec at 8Mhz.

Would work at 16MHz too, down to 1.2V per eneloop; just change them earlier before they really drop off. Though for battery operation, lower current draw is better; @5V and USB off, Teensy 2.0 is 18.9 mA at 16MHz and 11.0mA at 8MHz (source (http://www.pjrc.com/teensy/low_power.html)).

Hmm. To disable USB:

Disable USB
The USB port consumes power. If you're placing Teensy inside a project where no USB access is need, disabling the USB saves power.

TODO: code samples

01-05-2013, 09:33 PM
My thought:

Use a zener to hold the voltage at 5V coming out of the battery pack, and diodes to prevent the circuits from colliding.

01-06-2013, 04:18 AM
Thanks guys for all the input.

I think I'll stick with the option #3.
It's a single diode in series with the battery. Plus a second diode that bridges the 5v pads and lets you use a combination of external power or 5v. USB power.