adjustable regulator question

[new york cheech]

could someone help me, i bought an adjustable regulator at radioshack and i need to get 3.3v out of it, what do i need to get and how do i do it?

[bicostp]

You need to figure out how much power it puts out by itself without adjustment, in your case it should be 1.25 volts (you said you bought it at Radio Shack, and the LM317T is the only one they sell besides the 7805).

Concider this equation:

Vout = Vfixed + R2(Vfixed/R1 + Istndby)

Vout is the final voltage you need (3.3)
Vfixed is the output of the regulator

R2 and R1 are the resistor values.

Istndby is the standby current of the regulator (use 2.5MA for 7805 or zero for LM317T)



You need to hook up 2 resistors like this to get a variable output from the regulator. (I know it says "7805" on it, but they're the same layout; humor me for a sec here. )

Here's how the website I'm reading explained:

As an example, lets consider using the popular 7805 (5 volt) voltage regulator as a 12 volt regulator. In figure 1, lets assume 470 for R1 which means that a constant current of 10.6 MA will be seen between terminals 2 and 3. This constant current plus a regulator standby current of about 2.5MA will flow through R2 to ground regardless of its value. Because of this constant 13.1 MA, R2 can now be set to a value which will give us a constant 7 volts across this resistor. A resistor value of 533 ohms or 510 (standard value) will give us the necessary 7 volts. With 5 volts across R1 and 7 volts across R2, a total regulated value of about 12 volts will appear across terminal 2 and ground. If a variable resistor is used for R2, then the output voltage can be easily fine tuned to any value greater than 5 volts. The regulator standby current will vary slightly in the 7805 but 2.5MA will yield good results in the calculations. If an exact voltage (within .3 volts) is needed then R2 must be a variable resistor.

Of course if you want to you can use 2 1k potentiometers instead of resistors to get an absolute 3.3 volts. With these types of circuit there's about a +- .1v tolerance.

This is how I got the equivelant to a 7808 regulator for my PSone.

[*o*]

shouldnt you put this in n64?

[new york cheech]

yeah but no one answers me there

[Foxx373]

Knock yourself out:

http://spazioinwind.libero.it/andreabin ... index.html

[*o*]

THANKYOU!!!!! you rock foxx that is so useful!!!

[Foxx373]

any electronics questions you have, throw em' my way man. Remember, ive got over 10 years in electronics under my belt. Any they all thought it was a roll of quaters...

[Xorealix]

Hey Foxx, thanks for the link! Should I use 1/2 watt or 1/4 watt resistors to hook this up? Also, does it matter if I use a 470/770, or does it have to be 240/390? (R1/R2). Thanks for the help

[SpongeBuell]

The main difference between resistor wattages is the heat it gives off. If you can, use 1/2 watt for maximum cooling.

[Foxx373]

i use 1/4W resistors, but Spongie is right, 1/2W gives off less heat. You can use either combo of resistors. It might be hard to find a 770Ohm resistor. You can use any combo of resistors that will give you your output voltage. Also remember that resistors have a 5% tolerance on their values. I made a 7.5V power regulator for my screen, and it actually outputs at 8.2V. Measure the output voltage if you can to make sure u are not pumping out too much voltage for your device. If you want the best results, get a potentiometer (variable resistor) that is in the range you need, and tweak it to output the exact Voltage u need.

You can use a 1.3K and 2K resistor to get 3.27V. You might find it easier to find those resistors. You might be better off being under the desired voltage to compensate for resistor tolerances.

[Foxx373]

and for those of us that dont know how to read resistors, or hate remembering the color code table....

http://www.dannyg.com/examples/res2/resistor.htm

The way i remember the color code table is by remembering this phrase:

Bad Boys Rape Our Young Girls But Violet Goes Willingly

each 1st letter is a color

Black Brown Red Orange Yellow Green Blue Violet Grey White.

This has to be the ONLY thing i learned from high school electronics. That and most electroncis teachers, even the ones that worked for the governemnt, are not too bright in their "profession"

[Xorealix]

I tested my voltage regulator this morning, and found that it was still outputting 7.5V, instead of the expected 3.3V. I had my dad take a look at it, and his final conclusion (after testing it with a multimeter) was that "it has to be hooked up to the N64 to get the 3.3V out). I didn't say anything about it, because *he's always right*.

Hooked it up, N64 light lit up, no output to the PSOne screen. It's probably fried.

[bicostp]

parental athourity... sometimes...

You did have the regulator ground hooked up to the battery - terminal, right? (I made the mistake of not hooking up the ground ands the regulator passed the whole 12v I was putting into it.)

Yeah it's probably fried. Good thing GameStop N64s are only about $15!

[Xorealix]

Yeah I tied everything back to the ground, but still no luck. I think I'm going to resolder the power connector back on and see if I can get it to boot up.

Unfortunately N64's are $20 at my local Gamestop. Oh well. Can you get anything back for trading it a power cable / AV cable?

[new york cheech]

a few bucks u can i think