Help turning a 7805 into a 7808

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CronoTriggerfan
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Help turning a 7805 into a 7808

Post by CronoTriggerfan » Fri Sep 22, 2006 9:16 am

Okay, my 7808 regulator has not yet arrived, and I need one, so I want to use this guide to make one out of a 7805. I get most of it, but could someone explain it as if you were talking to a retard? :lol: I wanna make sure I fully understand it. Also, could I get the resistors needed for this project at, say, RadioShack?

Thanks,
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gannon
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Post by gannon » Fri Sep 22, 2006 9:26 am

I'm surprised that page doesn't even mention wattage for the resistors. You could try standard 1/4 W resistors, but don't be surprised if they die.
So, wire a 470 ohm resistor between the output and the ground pin (pin 3 & pin 2) and a 220 ohm resistor between the ground pin and ground (pin 2 & gnd). Just make sure you input ~ (7.2 - 5 + 8) = 10.2V to make sure it works (not sure if it can go lower, just computing the drop off compared to the std. 7805)

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Post by vskid » Fri Sep 22, 2006 9:32 am

gannon wrote:I'm surprised that page doesn't even mention wattage for the resistors. You could try standard 1/4 W resistors, but don't be surprised if they die.
It doesn't matter, its like a lm317, very little current passes through the resistors. So 1/4 watt will work. And radioshack should have them.
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Post by bicostp » Fri Sep 22, 2006 10:23 am

EDIT: Go <a href="http://bicostp.portablesofdoom.org/Articles/8v7805/" target="_blank">here</a>, I wrote up a page describing this better than the post does (I hope).

Make it look like this:

<img src="http://img96.imageshack.us/img96/9816/8 ... tlepv3.gif">
(the round grey things are solder)

You should get 8 volts out of the contraption if you attach the lead for whatever needs 8 volts AFTER the end of the 470 ohm resistor on the regulator's regulated power output. Connect the regulator ground to the resistor, then connect the resistor's other lead to ground.

Of course, always check it with your multimeter before you use it on anything important! (Once you biuld a couple of these it will be pretty easy, and you can make them pretty small. My latest one is barely bigger than 2 7805s)

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Post by MM007 » Fri Sep 22, 2006 1:36 pm

About how many mA can this thing put out @ 8V? 600? 650? I don't know its efficiency.
Warranty-Voiding fun!

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Post by CronoTriggerfan » Sat Sep 23, 2006 11:06 am

Thanks Bic, Sky, and Gannon! :D I'm going to RadioShack right now to grab the parts. GCp Ahoy!

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Post by vskid » Sat Sep 23, 2006 12:01 pm

bicostp wrote:Of course, always check it with your multimeter before you use it on anything important!
Make sure you have a load of some kind (not something important) on the regulator, I tested my lm317 and though I'd messed up until I hooked an led up to it and tested.
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Post by CronoTriggerfan » Sat Sep 23, 2006 8:32 pm

Thanks vskid, I'll keep that in mind.

Another question; I have all the parts, and I'm getting ready to modify my 7805. I've got a heatsink for the regulator body, but will the resistors put off any substantial amount of heat I need to dissipate? Thanks!

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Post by vskid » Sat Sep 23, 2006 9:04 pm

CronoTriggerfan wrote:Another question; I have all the parts, and I'm getting ready to modify my 7805. I've got a heatsink for the regulator body, but will the resistors put off any substantial amount of heat I need to dissipate? Thanks!

CTFan
On my lm317 they are very cool. On the lm317 voltage calcutator, it says that very lttle power goes through them. I think that the higher the resistance of the resistor, the less power it uses.
So to answer your question, most likely no.
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