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# how many ohm resistor do i need to get 5v from a 9v battery? Answered

hello i am writing up an instructable on setting up a wireless receiver , however i requires both 5v and 9v output, and the whole point of it is that it is simple and cheap, so i need to know, how many ohm resistor do i need to step down 9v down to 5v?

i do not wish to use a 5v regulator unless one can be bought for less than \$2, or else the instructable will cost too much.

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## 10 Replies

V=IR. You need to know the current draw. If the current draw is variable, then what you ask is not possible without a proper regulator, such as the 7805.

oldmanbeefjerky (author)2011-07-10

hmm, i have considerd that that regulator is cheap enough to use. may i ask, how is it wired up?
i cannot seem to find a simplified set of instructions for it!

rickharris (author)2011-07-11

the data sheet should show how to wire a 7805 - NOTE it needs some voltage overhead to do its work.

oldmanbeefjerky (author)2011-07-11

ah i get it now, the input and output negative can be either the ground middle pin or the bolt hole thing/heatsink (i dont know the name)

oldmanbeefjerky (author)2011-07-11

what do you mean? like it needs to have over 5v to work?

oldmanbeefjerky (author)2011-07-10

i think the current draw would be about 20ma, which means that
5V = 0.02 x ?
5 / 0.02 = 250!

thanks, ill have to try this out to see if it works before giving you a best answer

steveastrouk (author)2011-07-11

A standard 7805 needs a couple of volts of headroom to work, so 9 to 5 is fine. For reference, if you need to, there are low-dropout regulators, which can work with less than 300mV of headroom.

DON'T forget that there is no "magic" here and all the heat a resistor would dissipate will be dissipated by the regulator.

Steve

oldmanbeefjerky (author)2011-07-11

of course, though heat isnt really a problem here since it is only needed for temporary power, and at idle, only 3-5ma will be drawn so there wont really be that much strain on the regulator.

darn, why must you give such a good answer i cannot make "best answer"........

steveastrouk (author)2011-07-11

In that case you can use the tiny "LM78L05" instead of the chunky 7805.

Steve

oldmanbeefjerky (author)2011-07-11

i already bought 10x 7805's, though i will look into these smaller ones you mentioned as i will be selling them individually on my site for less than individual ones on ebay, still i must give kelseymh best answer