LM317T Adjustable Voltage Regulator HELP ?

[pete1986]

input voltage is 12v from a pc power supply

for some reason the schematic above is not working.i get an output of 1.29v when i should be getting 7.50v

however if i replace resister (R1) 240K WITH A 150K resister it works and i get a voltage of 10.35v (this was to test the lm317t to see if its in working condition)

i used a resister calculator and it said
1200k resister and 240k resister i should get a voltage of 7.50v
however i get a voltage of 1.29v...if i remove just the 240k resister i still get a output voltage of 1.29v

so i dont understand why this is not working and im not getting an output voltage of 7.50

can anyone help ? i used the resister calculator from this web page
http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

 

[BobK]

The resistors are way too large. Take the K off the end. I.e. use 240 and 1200.

Did you see the note in red that R1 must be between 100 and 1000 Ohms?

Bob

 

[pete1986]

The resistors are way too large. Take the K off the end. I.e. use 240 and 1200.

Did you see the note in red that R1 must be between 100 and 1000 Ohms?

Bob

that be why. cant believe i didn't see it lol thank you for pointing it out. just wondering can i replace R1 resister with a 0.15w Variable Resistor 1M ?

 

[duke37]

Use 240 for R1 and 5k or 10k variable for R2.
There should also be capacitors across input and output to ensure stability.

 

[pete1986]

Use 240 for R1 and 5k or 10k variable for R2.
There should also be capacitors across input and output to ensure stability.

thank you i have some Tantalum SMD Capacitor 10uf 16v 3216 A
would these be ok to use ? the input wont be anymore than 12v

and ill see if i can get some 10k pots online

 

[BobK]

If you want to adjust it around 7.5V I would use a 1K fixed resistor and a 500 Ohm pot. This would allow adjustment from about 6 to 9 Volts. With a 10K pot, most of the range would be wasted.

Bob

 

[pete1986]

Use 240 for R1 and 5k or 10k variable for R2.
There should also be capacitors across input and output to ensure stability.

just wondering. can i do this the other way round >

Use 240 for R1 and 10k variable for R2.

change it to

240 for R2 and 10k variable for R1 ?

 

[davenn]

no,
its R2 that must be the variable .... R1 sets the reference

Dave

 

[pete1986]

no,
its R2 that must be the variable .... R1 sets the reference

Dave

thankyou i purchased a 4290 variable resistor 10k and a 240 ohms resistor.

after wiring it all up it works great. i can alter voltage low as 1.23v and a max 10.43v
im powering it with 12v

just wondering how i could get it to be a max of 11v

also on the variable resister there was a leg not connected to anything i believe this is the other side of the resistor. because you only use the wiper leg and one side of the resisteor. anyway i connected the leg 3 to ground and it didn't change anything.so i leave it disconnected of is it best to connect to ground ?

this is how i have it connected>

 

[BobK]

The extra terminal should be connected to the wiper if anything.

Bob

 

[duke37]

The 317T needs about 2V across it (the droput voltage) in order to work so you will not get above 10V with 12V going in.

If there is ripple on the input voltage, then it is the voltage trough which is critical.

To get more output voltage you need more input voltage.

 

[pete1986]

The 317T needs about 2V across it (the droput voltage) in order to work so you will not get above 10V with 12V going in.

If there is ripple on the input voltage, then it is the voltage trough which is critical.

To get more output voltage you need more input voltage.

perfect thank you to everyone thats helped me that makes complete scene now and after looking at the ic info you are indeed correct. im running a pc fan at 9v to make is silent because at 12v it was loud. iv run the fan at 3v and there is no heat from the lm317 which i think is amazing

 

[davenn]

just wondering how i could get it to be a max of 11v

You will need more than 12V going into the regulator

Dave

 

[pete1986]

just wondering if anyone can tell me why some people recommend putting a 1N4002 Diode
is it a reverse polarity protection ?

iv only got 1N4007 diodes so just wondering if its worth buying the 1N4002

 

[duke37]

The 1N4007 will do all that the low voltage 1N4002 can do and then some !

The diode is there since when the power supply is turned off, the input voltage may fall faster than the output voltage. The reverse voltage on the 317 is not welcome.

 

[pete1986]

to protect it from reverse voltage could i just connect a 1N4002 positive side od the diode to the positive side of the input voltage.

 

[davenn]

to protect it from reverse voltage could i just connect a 1N4002 positive side od the diode to the positive side of the input voltage.

connect it as shown in the circuit you posted

Dave