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New Member
Ok i'm going to buy the LM7805 voltage regulator in a couple hours time after checking out its datasheet. I saw at the input there is a 0.33uF capacitor and a 0.1uF capacitor at the output but how do i know what voltage on the capacitor i should ask for?...20V...50V?

I read on this forum that the voltage rating should be the voltage used in the circuit multiply by 2 times root 2(1.4142). So the 0.33uF capacitor at the input should be, if i'm using a 9V battery at the inout voltage: 9V X 2 X 1.4142 = 25V? & the 0.1uF capacitor at the output should be 5V X 2 X 1.4142 = 15V?


the input capacitor should be rated at least double the voltage you are supplying so if you are supplying 9V then 16V or 24V capacitor rating is ok. The output capacitor need to be 12V or more.

note that 0.1uF at the output is fairly small, I'd go with minimum 47uF

Also, if you are going to use battery, the 7805 is very old and not so power friendly regulator, it disipates a lot of heat and has voltage drop of 2V so I'd suggest some low dropout voltage regulator like BA05T or BA05CC0T (get ones in TO220FP3 case - same as 7805)


New Member
Ok thanks ;) ... i got it to work with the 0.33uF and 0.1 uF both rated 50V and i see your point about the heat dissipation of the LM7805... my 9V battery is already 8V with one day of testing :( but at least i got my dtmf decoder and off hook detector working finally :) . Thanks a lot ;) Working on a ring detector now and then would feed those signals to a PIC18F452 for monitoring of call times and caller ID together with activation of an electrical device like a lamp. I couldn't do any of these things without all the great help from this site :D Thanks thanks thanks to all so far and i am almost sure to have more questions as i go along. :p


Well-Known Member
Most Helpful Member
A 7805 stops regulating and its output voltage drops when its input voltage is 7.5V or less.
A 9V battery voltage quickly falls to 7.2V then a 7805 will not work.

I use a low-dropout regulator for all my 9V battery powered circuits. It still works perfectly when the battery voltage drops to 5.5V.
All semiconductor manufacturers make low dropout regulators.
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