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Problems with potentiometer and battery

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stefano85

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Hi,
i'm Italian, so excuse me if i write incomprehensible phrases in English. :p

The problem: i have a 22 ohm / 5 w linear potentiometer and i would like to regulate with it the exit of a 12V battery, from 0 to 12V. The battery is a package of 2/3 AAA, ni-mh, 300 mAh. I tried to connect the exit of the pack to the 2 extremities of the pot. and then to check the voltage outgoing from it, but the voltage is always very low, 2..3 V.
Using a series of 2 batteries (1.2V AAA) with the same pot. i can regulate the voltage from 0 to 2.4V. Also adding other batteries to that series i have the same success.

So I thought that the 12V battery was gone.

But making a third test with an alcaline 9V battery and the potentiometer, i have the first result: a lower voltage than the possibility of the battery, 2..3 volt at most.

Why? Could it be, because in the 2nd test i have used a 800 mAh battery, instead in the other two test, a lower capacity batteries (300 mAh and 190 mAh)? Buying a potentiometer with more ohms than 22, could i have a good results?

Thanks :)
 
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At 22 ohms you'd have over half an amp flowing through your pot, for most batteries this is a lot of load. I would definitely suggest a larger pot.
 
The resistance is too low, so the current exceeds the amount the batteries can supply before the voltage falls really low.

The output voltage won't be regulated, adding a resistor will reduce the output voltage.

Why are you doing this?

How much current do you need?
 
Ok, let's say i have a 27 volt battery (3 x 9V in series, 200 mAh), my load isn't higher than 50 milliampere and my most used voltage is about 19-20 V even if in some cases i need other voltages for other applications (this is why i need a potentiometer). What can it be a balanced resistance for the potentiometer, in that conditions? Also considering the waste of power.

Thanks for the interest.
 
Hi Stefano,

hope you're not located at Napoli. :) A German saying: "See Neaples and die" I experienced some 30 years ago. Three shoe polishers waxed and polished my board engineer's sandals waxing his white socks black in the middle of an intersection.

Here is a primitive "regulator".

Adjust the pot for the desired output voltage. The maximum will be approximately 0.7V below supply voltage.

(The "electronic work horse 2N3055" can stand a lot of pain before it dies).

Boncuk
 

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Hi Stefano,

hope you're not located at Napoli. :) A German saying: "See Neaples and die"
No, i come from Rome.
Also Italians says that proverb; but we also say: "What is the first thing that impress you of Naples?" "A bullet" ('impress' for 'hit') :D
But we must keep it.
However...
Here is a primitive "regulator".

Adjust the pot for the desired output voltage. The maximum will be approximately 0.7V below supply voltage.

(The "electronic work horse 2N3055" can stand a lot of pain before it dies).

Boncuk
Sorry but i'm a beginner and i think i'm not able to understand those circuits. Should i use a transistor with the potentiometer?

edit: my circuit is **broken link removed**
 
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Hi Stefano,

(The "electronic work horse 2N3055" can stand a lot of pain before it dies).

Boncuk

That is so true. I reckon I have seen 3055's used in more application's than any other power NPN transistor.

They were well designed from the start in the 1960's. And still widely available today.

I wonder what other Power Transistors or IC's are still legend to this day?? Oh ...the 741 Linear IC is still available too. In spite of it being the basis of original Op Amp design...

It provided the rock solid foundation upon which most of today's Op Amps are now built.

Good design is the secret that so few engineers can unlock.
 
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The transistor acts like an amplifier which boosts the current.

Two transistors are better than one.

I suggest you try building a few circuits and see for yourself.
 
Even better, replace the single transistor with an Sziklia pair.

Sziklai pair - Wikipedia, the free encyclopedia

Hi Hero999,

why so cruel with a newcomer? Is it because he is Italian?

We all know we could knock out an (ancient) roman division with just one machine gun. :D

Nice detour to describe a darlington transistor though. :)

Here are the revised schematic and PCB layout. But I don't see any advantage over using a single NPN power transistor - except for lower maximum output voltage.

Regards

Boncuk

To Stefano,

using a pot to regulate power the maximum allowable current depends on the capabilities of your pot. If it is wire wound for high current you might use that. Better use a normal (carbon film pot = cheaper) and a power transistor.

You don't have to know much. Just build the circuit as shown in the PCB layout. Nothing can go wrong if you don't mess it up with a short.

I have been using that kind of "regulator" to charge the batteries of my electric razor for more than six years without problems.

Regards

Boncuk
 

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