I have a 9V battery and i want to turn on a microcontroller. I am using LM7805 as a voltage regulator. what caught my eye is why i need to use capaciter ??

I tried without capaciter and it work!

The thing that amaze me is the suggestion of manual LM7805 to use capaciter.

See diagram below

Attached Images

lm7805.JPG (22.8 KB, 17 views)

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Hesham Ismail Mohammed Sharif
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hi uaefame,
Some of the older 780X Vregs can go unstable under certain conditions, the low value caps are there to prevent the instability, I would recommend you fit them.

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Eric
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Use the capacitors. I was using a 7815 voltage regulator without them one time. Sure enough I got 15V out of it when I was using a multimeter, but when I hooked up an oscilloscope it showed there was a large AC voltage superimposed on the 15V at a frequency of just over 1MHz. A multimeter will never catch anything like that. The data sheet for this specific regulator said it didn't need any capacitors, but when I put an output cap in there the high frequency oscillation went away.

7805's rely on the low output impedance of the emitter of their pass transistor to ensure stability. That impedance is not something that should be relied upon to be 'low enough'. Use a capacitor. 0.1uF ceramics are cheap.

 

A 7805 voltage regulator needs a minimum input voltage of 7V.
But a little 9V battery's voltage quickly drops to 7V so its life will be very short.
You should use a "low-dropout" 5V regulator that still works fine when the 9V battery's voltage drops to 5.5V.

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Uncle $crooge

 

I have a doubt on this. When a 9V battery drops to 5.5, where is the capacity in the battery to deliver power, even if the LDO regulator accepts the input and tries to give 5V DC as output?

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Regards,
Sarma.

 

Hi Sarma,
The internal resistance of an alkaline battery increases which causes its loaded voltage to drop. Then a low dropout regulator simply turns on more to keep the regulated output voltage at 5.0V.
If the load current increases then the voltage from the battery drops. If the load current decreases then the voltage from the battery increases.

Here is a graph of the loaded voltage dropping from Energizer's 9V alkaline battery:

Attached Images

9V alkaline, 53mA load.PNG (7.8 KB, 15 views)

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Uncle $crooge

 

Yes, the datasheet also explains *why* you need the capacitor. Just because one test shows that it *can* work without does not mean that it's a good idea to skip it.

Trust the datasheet. The people who wrote it know more about the 7805 than you do.


Torben

 

You should be reminded that the microcontroller also needs its own capacitor from 5V to GND. This may be in the µC data sheet, but even if it isn't shown it is needed.

 

Don't forget that there is the low-power version in a TO-92 package, the 78L05, which takes up a lot less space and may consume less overhead power. It's rated for 100ma maximum.

Dean

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Dean Huster, Electronics Curmudgeon
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R.I.P.

 

And once he gets tired of buying alkaline batteries and uses a rechargeable "9V" with a nominal voltage of 7.2V, the LDO becomes even more important.
Not to mention that a 9V battery will be hard pressed to supply more than 100ma for very long anyway.

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With 60% of your batt power being lost as heat in a linear reg. This little guy looks pretty cool.
http://www.perel.fi/pdf/dms-78xxsr.pdf

 

I recommend the LM2936Z-5, it only drops 200mV, has a low quiescent current 15µA, it's cheap, robust and reverse polarity protected.

The only downside is it's only rated to 50mA but if you want more than that, then use the unregulated supply and you shouldn't really be using a 9V battery if you want more than 50mA anyway.

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I personally have been using the LM2940, as it can drop to 5.6VDC (off the top of my head).

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