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Lm1117-fixed vs adjustable
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Please be positive and constructive with your questions and comments.

Lm1117-fixed vs adjustable

by charliex on Fri Mar 19, 2010 9:43 pm

So i'm building a couple of protoype pcb circuits, and i'm teaching the GF how to solder and assemble, though really i'm just employing free labour to help my prototyping ;), and i make one board partially, she makes the other but only about %10, i check the voltages and its working fine after i fix one dry joint. So i finish mine put in all the chips and , check the voltages. uhoh 9V instead of 5V, so my At90CAN128 and MPC2515 are probably not too happy right now, so i start pulling stuff apart and wonder why mine is giving 9V instead of 5V, and her's is giving 5V. . Eventually i find that she has accidently used two 330 Ohm Inductors instead of the 330 ohm resistors and i've used the right resistors..

But ! it turns out I ordered the wrong component, a LM1117-5.0 instead of an ADJ ! so mine being at 330*2 reference made it about 9V, and her's being inductors was 5V. . so now she believes shes some sort of psychic electronics genius :)

Anyway, I'm needing a 3.3V and a 5V, is there any reason to go for the adjustable versus the fixed ? It'll cut my component count down by a few resistors with the fixed, and i'm using 1%'s so a little bit cheaper.

cheers.
http://032.la - Null Space Labs.
charliex
 
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Re: Lm1117-fixed vs adjustable

by zener on Sat Mar 20, 2010 12:58 am

Well, it's tomayto, tomahto. With the adjustable you have less part numbers to buy, but maybe more resistors, and definitely more more parts on the board. With the fixed you are limited to the fixed voltages. Mostly we use fixed, and use adjustabe where we actually want an adjustable voltage with a trim pot. But there is no harm from using adjustable for everything, you just have to load the resistors, and the more parts you load, the more opportunities for mistakes, shortages etc.

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Re: Lm1117-fixed vs adjustable

by charliex on Sat Mar 20, 2010 1:46 am

The main reason i ask is that i had a poke around other schematics from various vendors and they typically use adjustables, with fixed value resistors. Perhaps its just that its easier to carry one part , adjustable, than the various 5/3.3v/2v etc since the resistors are jellybeans, or habit.

I just wanted to make sure there wasn't something i was unaware of, thanks for the reply ! I think i'll switch to fixed vregs, and remove the extra resistors.
http://032.la - Null Space Labs.
charliex
 
Posts: 179
Joined: Sat Nov 28, 2009 5:54 pm
Location: Las Vegas

Re: Lm1117-fixed vs adjustable

by adafruit on Sat Mar 20, 2010 3:42 pm

we like the 1117's fixed - its hard to break it :)
see our parts list
http://www.ladyada.net/wiki/partselector/vreg#xx_series
for some of em

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Re: Lm1117-fixed vs adjustable

by charliex on Sun Mar 21, 2010 12:28 am

yep makes sense, and of course i popped my At90CAN128-au and now digikey. mouser and everyone else seem to be out of stock...

ahh well, i culled the extra resistors today, and the mp120 , so lots less to stuff by hand. just have to make a few more boards, i'm glad its a cnc.

cheers
http://032.la - Null Space Labs.
charliex
 
Posts: 179
Joined: Sat Nov 28, 2009 5:54 pm
Location: Las Vegas

Re: Lm1117-fixed vs adjustable

by zener on Sun Mar 21, 2010 12:30 am

If the CPU is socketed, definitely don't put it in until the power supplies are verified.

zener
 
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Re: Lm1117-fixed vs adjustable

by charliex on Sun Mar 21, 2010 10:46 am

It wasn't since its a TQFP64 and i skillet soldered the board. I did verify the circuit voltages with a test board, unfortunately the one i verified the gf had used two 0805 330 Inductors instead of resistors, which I missed, so on her's it worked.
http://032.la - Null Space Labs.
charliex
 
Posts: 179
Joined: Sat Nov 28, 2009 5:54 pm
Location: Las Vegas

Please be positive and constructive with your questions and comments.