DC Power Supply PCB

Nigel Goodwin said:

You can't get 3V from a 7805, you would have to add an extra regulator after it - so may as well not use the 7805 in the first place.

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jemch72 said:

The diodes will not ruin any regulation, 7805 will all the time have 5 volts in the output pin, the diodes are a easy way to solve the issue,the only thing can affect diodes is temperature, it cause to them to drop more or less than 0.7 volts... i will use 7805 as variable regulator but seems to nobody undertand voltage divisor in the ref pin of the 7805QUOTE]

The voltage drop across the diodes will vary with the current through them, and for a small current will hardly drop any voltage at all - regulation will be terrible!.

As for using the 7805 as a variable regulator, I'm sure almost everyone here knows perfectly well how to do it, and that it's not a very good option - for a start the current limiting no longer works!.

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jemch72 said:

Nigel. diodes are not resistors, they are two substrates of p and n silicon, they drop over them self 0.7 volts if they are silicon, not germanium(0.4volt ), independently of the current that flow thru them, they can have a variable voltage over them if they are polarity reversed and in alternate current, if you place them in pure dc voltage (with low or very lo ripple) no variable voltage is over them, other wise they will disipate power/

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jemch72 said:

Nigel. diodes are not resistors, they are two substrates of p and n silicon, they drop over them self 0.7 volts if they are silicon, not germanium(0.4volt ), independently of the current that flow thru them, they can have a variable voltage over them if they are polarity reversed and in alternate current, if you place them in pure dc voltage (with low or very lo ripple) no variable voltage is over them, other wise they will disipate power/

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jemch72 said:

Did you realise that are milivolts? any way, again, i am giving a "get out of the problem" solution , I am not posting to try to help somebody that dont know much, of course is not the high tech solution..i suject the 2n3054 with a zener, but this guy burned a zener before, so bad idea, I sugested the lm317 and he only have a 7805, I sugested the voltage divisor in the ref pin of the 7805 to make the output varible, I even posted the formula to calculate it..but only I get post reply to say...diodes dont work...7805 cant get variable output, etc..I dont care, please , help the guy asking for help...and please, be wise...if you know thah you dont know everything you will progress in life, if you thing you seen all, know all, then you are gone, and not posthing , because the millions you made will have you in the bahamas with 10 girls arround...I am wrong?

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Hero999 said:

The voltage drop of a zener diode also vary depending on the current.

The only real solution is to use a 3V regulator.

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Nigel Goodwin said:

There's no point posting suggestions that are completely rubbish and won't work, your diode suggestion, although crude, would work under certain specific circumstances - but would be badly regulated, and would output near 5V under very low load. The variable 7805 idea is absolutely useless - you can only INCREASE the voltage with that, your example allowing adjustment between 5V and 10V (assuming you could get 10V with the input voltage you put on the diagram).

Your best idea was a discrete zener/transistor regulator - the reason the OP 'burned' a zener is probably because he simply stuck it directly across the output!.

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