Running Zx-24 from 4x AA cells

sturgessb · Post by **sturgessb** » 26 March 2009, 11:53 AM

Hi Guys

I'm replacing the innards of a RC transmitter with a zx and xbee and just wondering what to do regards power.

It carries 4x AA 1.5v cells and currently has a 3.3v regulator inside. Am I best to run the ZX on the 3.3v or straight from the cells (so possibly over 6v with fresh batteries) into pin 21?

Thanks

Ben

stevech · Post by **stevech** » 26 March 2009, 12:41 PM

3.3V regulated from two cells? Not much headroom as the cell voltage declines.

Battery powered: for long life, you need to get the XBee to sleep per the 802.15.4 protocol for an end-device. Or just change the batteries often! They take about 30mA on receive as I recall. Transmit mode power is usually unimportant as transmissions are normally infrequent/brief.

sturgessb · Post by **sturgessb** » 26 March 2009, 12:44 PM

4 cells not 2.

changing the batteries often is fine. Im also only using a 1mW xbee as it is a secondary controller that talks to the master controller only a few feet away.

sturgessb · Post by **sturgessb** » 26 March 2009, 17:56 PM

I guess my options are..

1) 3.3v into pin 21
2) unregulated from batteries into pin 21 (what is the max voltage this can take, 6v?)
3) unregulated from batteries into pin 24 (what is the lowest voltage the 5v regulator will operate at?)

Which would be best?

GTBecker · Post by **GTBecker** » 26 March 2009, 18:02 PM

> 1) 3.3v into pin 21
> 2) unregulated from batteries into pin 21

Consider your ADC calibration.

Tom

sturgessb · Post by **sturgessb** » 26 March 2009, 18:13 PM

good point, although surely that rules out batteries into 21 not 24?

Ben

Post by **dkinzer** » 26 March 2009, 18:17 PM

sturgessb wrote:Which would be best?

I don't think options 2 and 3 are viable. Option 2 would apply an out-of-spec voltage when the batteries are fully charged. The maximum operating voltage is 5.5v but four fully charged AA cells will probably be about 6V or more. Option 3 would work except that the on-board regulator has a dropout voltage specification of nearly 2 volts at 100mA. That means that the input voltage needs to be at least 7V which you won't see even with fully charged batteries.

You could add an external regulator circuit using an LDO (low dropout) regulator. You can probably find one with a dropout voltage around 0.2V. The reason that we don't use such a regulator is that they typically have fairly low maximum input voltage specifications which would lead to a high failure rate when users apply an out-of-spec supply voltage.

Note, too, that although option 1 may work it is operating the device outside of the manufacturer's specifications. Running at 14.7MHz, the minimum operating voltage is specified to be about 3.7V if I recall correctly.

GTBecker · Post by **GTBecker** » 26 March 2009, 18:41 PM

More expensive, but a solution perhaps, is to use lithium AAs, ~8v from
4x2.0v, into pin 24.

Or, if you have the room, you might add another pair of alkaline AAs, to
get ~9v from 6x1.5v.

The discharge curve of alkalines might not be helpful, but lithiums stay
up longer and fall more steeply, I think.

Tom

sturgessb · Post by **sturgessb** » 26 March 2009, 18:55 PM

ok great thanks ill look into those two options.

LDO regulator
2v cells

Ben

weatherman · Post by **weatherman** » 26 March 2009, 20:04 PM

sturgessb wrote:ok great thanks ill look into those two options.

LDO regulator
2v cells

Ben

You may want to look at a LP2954A. I typically use a 6V battery power supply in combination with this regulator for many of my projects. If you can get by with 200ma output or less, this is an excellent choice. In my experience it works well with an input as low as 5.5V.

kurakaira · Post by **kurakaira** » 27 March 2009, 2:53 AM

Hello ,

Why not use a cheap DC / DC converter ?
Here's a part i use when connecting ZX-24's to a 12 V car battery .
Traco Power TMR1211 , it small and cheap , cost's around 10-20 € .
It gives nice and clean 5 V power at a max 400 mA , it does not even get warm at 300 mA .
Alltough specified at 9-18 V input , it's turn on is somewhere above 3.5 V and it works below 2 V !

spamiam · Post by **spamiam** » 27 March 2009, 11:27 AM

I think that you NEED a regulated power supply, not just straight battery power.

Also, you probably should use an efficient converter. A switching regulator is going to be noticeably more efficient than a linear regulator. Some switching regulators cost not a lot more than linear, and some even have the same pin-out! Check out Recom. (http://www.recom-power.com/pdf/Innoline ... xx-1.0.pdf). They have lower power units as well.

-Tony

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