Getting 5V from a 7805 connected to AA batteries.

[AceOfHearts]

I am using a 7805 to get 5V from four AA batteries in series (6V).

However, I have noticed that when I connect it, it does not give me 5V...

Right now I have 5.47V from my battery, when I connect it to the 7805 I get 4.41V but not 5V.

Any idea what I am doing wrong? Or if it is possible to get this 5V this way?

Thanks for reading.

 

[Optikon]

I am using a 7805 to get 5V from four AA batteries in series (6V).

However, I have noticed that when I connect it, it does not give me 5V...

Right now I have 5.47V from my battery, when I connect it to the 7805 I get 4.41V but not 5V.

Any idea what I am doing wrong? Or if it is possible to get this 5V this way?

Thanks for reading.

the 7805 is not a low drop out regulator. In otherwords, you need ~ 1.5 or 2V higher input to maintain 5V output regulation. Input 7V and see what you get on output.

 

[AceOfHearts]

the 7805 is not a low drop out regulator. In otherwords, you need ~ 1.5 or 2V higher input to maintain 5V output regulation. Input 7V and see what you get on output.

Thanks very much.

I just tried it. You're right.

I can see a dropout voltage of 2V on the datasheet. I suppose this means it needs minimum of 5+2=7V to output the typical 5V.

Thanks again!

 

[Hero999]

How much current do you need?

If it's fairly low (<50mA) then I'd go for the LM2936-5 which has a droput voltage of just 200mV so it'll give 5V until the battery drops to just 5.2V and most 5V stuff will still work at around 4V meaning that it'll still work when the battery drops to 4.7V.

 

[picbits]

If its low current then consider running it off a rechargable 9v battery. It will give you a steady voltage until near the end of its charge.

 

[audioguru]

You don't want the 7805 regulator to operate in dropout. Then it is already not regulating and its output voltage has already begun to drop.

A rechargable 9V battery can supply hardly any current.
Its voltage drops like a rock if the current is more than about only 10mA.

 

[Hero999]

You don't want the 7805 regulator to operate in dropout. Then it is already not regulating and its output voltage has already begun to drop.

Sometimes it doesn't matter, suppose a circuit will work from 4 to 5V so providing it doesn't drop below 4V it'll be alright.

A rechargable 9V battery can supply hardly any current.
Its voltage drops like a rock if the current is more than about only 10mA.

I agree but 7.2V batteries are crap, you want 8.4V.

**broken link removed**
**broken link removed**

 

[audioguru]

The 6 cells in a "9V" battery rated for 7.2V are bigger and have a higher capacity than the tiny 7 cells in a "9V" battery rated for 8.4V.

The voltage drops quicker when the battery's capacity is less.

 

[Hero999]

That's true, I suppose it depends on the current draw and the minimum voltage requirements of the circuit.. I have as multimeter that won't even work properly at 7.2V, it displays the low battery symbol which isn't any good when run of a 7.2V battery.

 

[audioguru]

A 9V alkaline battery works for years in my Fluke multimeter.

My cheap Chinese multimeter also works for a long time on a 9V alkaline battery but it doesn't automatically turn itself off. I must remember to turn it off.

 

[justDIY]

use a TPS61040 from texas instruments, 1.8 to 3v in, 5v out, 400mA max. There are chips that'll go lower in voltage, or provide more current, but the tps is easy to use and at 1mhz, needs only small passive components to get the job done.

 

[bananasiong]

There is another boost regulator from TI, TPS61200 which has the input voltage range of 0.3 V - 5.5 V with the output voltage adjustable up to 5 V. The max output current is 600 mA.
But the SON package makes me cry while doing soldering T_T