stepping up battery voltage

Hi. I want to step up voltage of my AA 1,5 battery to 2.5V but i need to limit the current to 500mA.

Ampmeter shows that the current of the battery is around 6A and dropping to 3A.

How to make this 2.5 / 0.5A output ?

Reply to
Igor
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Usually, amperage is drawn by the circuit, not "produced" by the battery.

Remember ohms law, I = V/R. The ampmeter is design to have a low R, so as not to effect the whole of the circuit. If you connected the meter directly to the battery, you've basically shorted the battery. Which AFAIK is dangerous.

If you want 2.5v with .5A, that means the rest of your circuit needs to have a resistance of at least R = 2.5v/.5a = 5ohm. You'll be hard pressed to find useful circuit with < 5 ohms.

In any case, are you *sure* you need to limit the amperage, and that the circuit isn't going to just use what it needs?

Reply to
Daniel Pitts

Yes I'm sure. The output should be 2,5V/0.5A. I want to make this as simple as possible and as first step I want to see how will this work with AA battery. However, if you know how to get this output from some other source, let me know.

I do have 3v/5A AC/DC converter (12/9/6/3V) but again current is much to big for such application. I can switch this ac/dc converter to 12V (1A) and put some load , like some motor, so that the voltage and current will drop to needed value..

Reply to
Igor

--
If your load requires 0.5A with 2.5V across it, then it doesn't matter 
how much current the supply can source as long as it's not less than 
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Reply to
John Fields

Eaby 290590670347 step up converter nominal input voltage 3 volt, but is works lower.

see also 290714206877

Run two of them in parallel: Ebay 280711749967

w.

Reply to
Helmut Wabnig

A popular method of stepping up low voltage is the blocking oscillator (plenty on the internet) but you're basically at the mercy of the flyback voltage developed by the collector winding, there's nothing to stop you winding a secondary winding with fewer turns to make a lower voltage flyback converter - might even be possible to have a secondary with more turns than the collector winding (only works out if you're using a rectifier on the output) to make a step up forward converter.

You can wind the blocking oscillator inductor on one of the bigger ferrite toroids from a scrap motherboard, collector winding for 1.5V single-cell =

15T, same for the base winding. You can wind as 30T with a tap at 15T (for B+) to give you a sense of how the winding is arranged - the base winding is connected to the base via a 100 Ohm pot in series with a 15 Ohm limiting resistor (which will probably give a fair bit of control over output current) - the fixed resistor limits maximum base current with the pot at minimum.

Experimenting with secondary winding ratio & configuration is left as an excercise for the student.

Reply to
Ian Field

You are sitting on the border of not to many switch IC's that can start up reliably at 1.5 volts. I do remember one that could start at .8 volts but don't have the number at the top of my head..

you could make a blocking oscillator on a small hand wound transformer, but regulation maybe a bit tricky.

Also, if I remember, the Cmos version of the 555 timer will operate on a single cell. With that, you can make a buck boost supply.

I guess it all depends on what you're trying to do..

Jamie

Reply to
Jamie

"Ignorant"

** Makes no sense.

You cannot have a fixed voltage AND a fixed current limit too.

** That is the current under near short circuit conditions.

Try measuring the VOLTAGE at the *same time* and I bet it is SFA.

.... Phil

Reply to
Phil Allison

I think you're misunderstanding something. Voltage is typically an "exact" rating for a circuit and power supply. Amperage is a "maximum" rating, meaning it can't safely supply more than that (and still maintain the voltage level).

So, if I have a device which came with a 12v 500mA power supply, I can plug in a 12v 2A power supply with no ill effect (assuming the same polarity is maintained)

On the other hand, if I plug in a 16v 500ma power supply, I could cause problems to my device. Or, if I plug a 12v 250ma power supply, I could damage the power supply, or the device would simply not work.

So. I ask again. Are you sure you need "exactly" 500mA, not "at least"

500mA? Perhaps you could tell us what you're using this for, so we, as a community, could better help you.
Reply to
Daniel Pitts

"Daniel Pitts"

** Not if done only briefly.

he short circuit current of a AA cell depends on what type it is:

Heavy duty Zinc = about 3 amps.

Alkaline = about 9 amp

NiMH = 30 to 40 amps.

That last one is dangerous cos it can set the meter probe cables on fire.

.... Phil

Reply to
Phil Allison

It pretty much has to work down to 0.9V or so if you're going to get everything there is to get out of the battery -- a dry cell only puts out

1.5V when it is very lightly loaded for it's charge level.

With a 100% efficient converter, 2.5V at 500mA demands around 833mA out of a 1.5V cell, and the current required goes up as the cell voltage goes down.

--
Tim Wescott 
Control system and signal processing consulting 
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Reply to
Tim Wescott

Then you don't understand electronics very much at all, or you're stating your needs very poorly. Because what you're asking for is physically impossible.

The load on a power source determines the relationship between current and voltage. Usually this means that there is a 1:1 relationship between the voltage applied to the load and the current it pulls; never does it mean that the source can dictate both the voltage at and the current through the load. You can make a 2.5V source and apply it to a load that will pull 500mA at 2.5V, or you can make a 500mA source and apply it to a load whose voltage will ride at 2.5V at 500mA, but you absolutely positively cannot make a source that will deliver 2.5V at 500mA to anything at all.

An immediate counter-example to what you're asking for is a resistor: a 5- ohm resistor will pull 500mA from 2.5V, but if you exchange that for a 1- ohm resistor, it will pull 2.5A from 2.5V. There's no way that your source can make a 1-ohm resistor act like anything other than a 1-ohm resistor other than magically changing the nature of the resistor.

There is no simple about what you are asking for, because the first step in achieving your goal of a source that will always put out exactly 2.5V and 500mA, regardless of what load is attached, is to move to a different universe with different physical laws -- then figure out how to stay alive there. Until you manage that, you need to obey the laws of physics here.

No you can't. What you are asking for is not physically possible.

Tell us what you're really trying to accomplish. Not the 2.5V/500mA bit

-- tell us what you're building, and what you expect to do. Something along the lines of "I want to build a pocket electric cigarette lighter..."

--
Tim Wescott 
Control system and signal processing consulting 
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Reply to
Tim Wescott

Yes, very well understood..

Yes ago a made a concoction where I used a diode to isolate the Vcc of a basic timer and had that drive a mosfet to buck switch a coil which got its (+) before the diode. Then I back fed the final output of the switch circuit which was 12 volts after regulation to the Vcc of the timer.

The source was a 6 volt arrogation battery system, a very large one at that, however, there were some electronics that needed 12vs from that and it needed to stay operating when the 6 volt battery drained down to the point where it could no longer supply enough output in the buck circuit to maintain 12 volts and back feed the timer. I had a comparator switch in there to shut things down if the output reached 10 volts or less.

During the day solar panels were used to recharge this system if possible and a emergency generator if needed. THe gen would start if the buck circuit switched into shutdown.

The only problem with all of this of course, once the battery gets down like that, you are not starting that circuit until it is up and happy again :)

Oh the days of what I could do with timers, I also love the cmos versions. I know there is more integrated IC's these days for that and i've used a few, but those aren't as much fun!

Jamie

Reply to
Jamie

The PR4401 LED driver can start that low but probably won't shift as much current as the OP wants - not sure whether you can buffer those with an external transistor or not, but I think they've since released an upgraded version anyway.

There is a variety of single cell inverter chips; probably LT, maybe maxim.

Reply to
Ian Field

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