Which capacitor to use with 7805 voltage regulator?

[the_merovingian]

I'm using a 7805 voltage regulator to convert the 9V from a PP3 battery down to 5V for supplying a PIC microcontroller.

I've taken precautions and am using a diode to prevent the battery being connected incorrectly.

What size capacitor should I use between the +VE and GND rails? Would a 10v 220uF electrolytic be suitable?

If anyone could supply the theory behind this too (i.e. equation / datasheet / application note) that would help my learning.

Many thanks in advance.

 

[srikanthsamaga]

Since you are using battery as input source for the regulator, capacitor is actually not required, else you may use a 10uf to 100uf electrolytic.

The actual purpose of this capacitor is to filter out the noise and voltage variations from the input source and is usually necessary when transformer and rectifier is used for supplying input to the capacitor.

 

[betwixt]

I disagree with srikanthsamaga. the capacitor IS required.

Inside the regulator is a high gain amplifier, always monitoring the difference in output voltage with an internal reference supply. Like most amplifiers, it can become unstable if variations in supply voltage cause a change in output voltage. Given the battery has quite a high internal resistance, it is very likely it's voltage will vary if the load changes.

You should always connect a capacitor of more than 100nF and preferably in the region of 1 to 10uF across the input terminal and ground with the wiring as short as possible. A capacitor on the output is less important but it is preferable to have one close to the output and ground pins as well. If you make the output capacitor too big, it has a detrimental effect on the regulators transient response. Between 1 and 10uF is usually a good choice.

Brian.

 

[the_merovingian]

Many thanks betwixt.

Any particular reason why 1 to 10uF?

 

[betwixt]

I suggest 1 - 10uF because that's what the regulator manufacturers suggest. The value isn't critical but if you make it too small you risk it being inadequate to damp oscillation. If you make it too big, at least with a single capacitor, you run the risk of the high frequency impedance of the capacitor being too high. Most cheap aluminum electrolytic capacitors have a relatively high internal inductance so while fine as reservoirs in power supplies, they become less effective at damping high frequency oscillation. You will find that in 'better' designs, there are two parallel capacitors used, one will be electrolytic to give a high value in a small space and the other will be ceramic to improve HF performance.

Brian.

 

[srikanthsamaga]

Thanks for the information betwixt.

 

[Audioguru]

A PP3 9V battery quickly drops to 7.2V. The diode in series reduces the voltage to only 6.5V.
But the minimum input voltage for an old 7805 is 7V so it will not regulate the output at 5V.
Use a more modern low-dropout regulator and the capacitors that are recommended in its datasheet.

 

[Joe.mok]

Just choose the recommended value suggested in the manufacter's datasheet...

 

[ledscree]

I disagree with srikanthsamaga. the capacitor IS required.

Inside the regulator is a high gain amplifier, always monitoring the difference in output voltage with an internal reference supply. Like most amplifiers, it can become unstable if variations in supply voltage cause a change in output voltage. Given the battery has quite a high internal resistance, it is very likely it's voltage will vary if the load changes.

 

[Rf engineer]

u have chosen the correct capacitor....however as a further reservation u can easily use 25 V 1000uF capacitor..............

these number are easilt available....and secure for your purpose