Single cell AAA to 24V @ ~20mA

Any thoughts?

Why not use 2 cheap 23A12 12V alkaline batteries in series. Maybe even takes less space than an AAA plus converter,

I need the 24V to be somewhat regulated with varying current loads and over the life of the battery. Doesnt need to be Super stiff but +/-1V or so until the battery discharges below 1V. then it can shut down.

Looks like these will fall off 12V fairly quickly with use. The Ah rating is down to 6V

Looking at the mAh charts and figures for discharg to 6V (1/2

0.5mA = 115hours 2mA = 28 hours 5mA = 9 hours 10mA = 3.5 hours 15mA = 2 hours

I'll extrapolate here

20mA = 1 hour to reach 6V

looking at the shape of the chart and assuming a similar S shape I move the 6V y intercept from 115 to 1.

I'll assume 10.5V is my minimum which crossed about 1/3 of the way to the 6V cutoff time.

so 1/3 of 1 hour at 20mA should be 10.5V (neglecting internal resistance, etc)

which would be 20m minutes.

At 10 seconds/use that would be 120 uses. at 5 uses / day that would be ~ 1 month/ battery.

hmmm.... might be reasonable of the internal source resistance is OK.

same analysis for the AA battery. (duracell has a better datasheet)

24V at 20mA is 480mW.

assume boost supply efficiency of 80%. = 600mW draw from the battery

at 1.5V = 400mA at 1V = 600mA

assume 500mA to simplify calcs.

Run time to 1V at 500mA is ~ 2.5 hours

assuming 10s/use and 5 times a day. That should give 900 uses or

180days of use. 1 AA every 6 months would be more than acceptable.

Sorry for the long response, Just thinking out loud to make sure a simple solution, that i honestly hadn't consered, would not work for this applicaiton.

Thanks for thaking time to suggest something though! If you have any other thoughts please follow up.

Personally I gave up on AA/AAA, and only use lipo in new designs For 1$88

plenty choise, use a small lipo 1Ah, 80% converter efficiency, why not. And then add a MCP73831T charge chip and recharge from USB.

Use lipos with internal protection.

scroll a bit for deals, 4 x 800mAh for 13$, etc

Depending on what you're doing LiFePO4 is likely a better chemistry than LiPo, though it does require different chargers.

At 1V gets hard to find fets that switch well, you may find one of Diodes/Zetex low saturation voltage BJTs is a better fit, as in:

piglet

Cool circuit. I'll have to play with it in spice.

So you pull down on\ the PNP flows current and turns in the Npn.

Then the NPN pulls current from the inductor and from teh RC-feedback to the PNP drive which further turns it on.

What causes the switch to turn off and throw current into the capacitor? Inductor saturation? VCE sat build up on the NPN coupling back to the PNP?

How do yo tune the switchin frequency?.... besides carefully. :)

Yup fets at 1V are nearly a no go expecially at the currents requred from the 1V.

I was hoping to find suggestions for something like this but with a higher voltage output.

It is basically a constant on time converter so the switching rate is highly load dependent. You correctly describe the switching on process - then once the timing capacitor has charged the PNP begins to switch off which in turn begins to turn off the NPN and is made snappy by positive feedback. It is a useful circuit to know for uncritical conversion at levels of tens of milliwatts to about one watt where the load current does not vary wildly.

piglet

For such a small amount of power intermittently I think the closest thing to a "canned solution" would be the two-stage boost, a chip with a low Vmin first to go from as low as 0.5 to 5 and then 5 to 24.

If you need it yesterday then I think that will work fine and is the way to go, you can buy little breakouts for a couple bucks with all the SMT stuff including inductors wired up appropriately and drop the two of them onto a set of protoboard or PCB pin headers.

Looks similar to the circuit used in some of those inexpensive solar garden lamps that use a small panel and a single NiCad cell for storage

Is the ON node tied to low/gnd to turn it on?

George H.

-

the chip in those is usually the QX5252. Max pin voltage = 5.5v, i_out 3-

300mA.

NT

Yes, and high or float to shutdown.

piglet

Does it have to run off a single AA cell? Life would be much easier if you could accommodate a 9v battery and a more modest multiplier.

--
Regards, 
Martin Brown

Rechargeable 9V Lithiums have a pair of cells and all the protection stuff. Wonder if the extra size isn't less than all the stuff you have to use to make an AA work?