lm317t power supply question - Page 1
Forum section: electronics / beginners

author: little_carlos
author_rank: Regular Contributor
timestamp: April 14, 2014, 11:06:38 pm
content: hi, im trying to make a power supply with this circuit, ive already made it but sometimes the lm317t melts, i think is faultybtw here is the circuitim making this to supply my experiments, it says it can handle 3 amps, but im actually going to use no more than 1 amp, so no big problemthis circuit is fine for what im up to, or should i see other ones?

author: mariush
author_rank: Super Contributor
timestamp: April 14, 2014, 11:50:59 pm
content: That circuit only uses LM317 as a sort of control for the TIP3055 transistor. It's a bit confusing because the pins are not numbered, so it's very easy to connect the LM317 incorrectly in your circuit this way.Forget about this circuit, because you don't need such complexity for 1A of current.Open the datasheet for LM317 and build the circuit there, see page 7 and the explanations on the page:http://www.ti.com/lit/ds/symlink/lm317.pdfDatasheets for the same chip but made by other companies may also give you more details, for example see page 7 on this datasheet :http://www.onsemi.com/pub_link/Collateral/LM317-D.PDFSo you have the transformer, the bridge rectifier, the capacitor (which can be 2200-3300uF 35-50v, if you use a transformer like the one in the circuit) and then the circuit in the datasheet.Be sure to put the LM317 on a heatsink... remember, the difference between input voltage and output voltage has to go somewhere, and it goes as heat ... for example 12v input, 5v output at 1A means (12-5)x1 = 7w wasted as heat ... LM317 can only handle about 1-1.5w by itself, without any heatsink.Even with heatsink, you really shouldn't make the regulator waste more than about 10-15w as heat .. don't put 32v input, 5v output and expect to use more than about 3-400mA before it dies. Be sensible.

author: little_carlos
author_rank: Regular Contributor
timestamp: April 15, 2014, 12:13:57 am
content: i want to use the tip 3055 so it controls most of the power, i dont want to burn the lm317t with 2 amps, in case i need it

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2014, 12:34:58 am
content: Use a LM338, thermal and over-current protection for up to 5 amps.Ditch the pass transistor and keep it simple.

author: qno
author_rank: Frequent Contributor
timestamp: April 15, 2014, 02:59:16 pm
content: There is no feedback in the control when you use the LM317 this way.There is another way possible with an PNP transistor.Check herehttp://electronics-lab.com/projects/power/021/index.html

author: qno
author_rank: Frequent Contributor
timestamp: April 15, 2014, 03:04:29 pm
content: Sorry the resistor on the input looks a bit high.What i ment was a PNP transistor not a darlington.However it could work??!!I have used the layout mentioned in the pdf below several tiimes.Is not my pdf. Found it on the web after googeling "LM317 PNP power transistor"Have fun...http://www.ee.teihal.gr/labs/electronics/web/downloads/LM317T_Variable_Voltage_Regulator.pdf

author: rdl
author_rank: Super Contributor
timestamp: April 15, 2014, 03:07:18 pm
content: You could maybe get by with just the LM317, but at low output voltage and high current the power dissipation will be a problem and you would need a large heatsink and maybe a fan. There is a circuit example in the LM317 data sheet which shows how to use a small pnp transistor to drive a larger npn which is a better way to do what you want. See figure 14http://www.ti.com/lit/ds/symlink/lm317.pdf