LM317 voltage issues

Faradave

Member

• Dec 31, 2019

• #1

Okay here's one for you guys I made a voltage regulator out of lm317 which is a voltage regulator. The weirdest thing, I supply it with a 32 volt adapter out of a printer. But the maximum I can get out of it is 20 volts. Now cording to the datasheet you'll get a couple of volts less than what, 1 or 2 less. Can you guys figure out what I'm doing wrong that I get only two-thirds the power I'm feeding it? Here's the circuit diagram that I followed. except I used a 20K pot but that shouldn't make a difference right that just makes it more sensitive

 

tvtech

Well-Known Member

Most Helpful Member

• Dec 31, 2019

• #2

How much current are you drawing at the output of the 317?

 

Faradave

Member

• Dec 31, 2019

• #3

gosh you know my multimeter is in reality a dual meter cause it only measures voltage and resistance.

 

tvtech

Well-Known Member

Most Helpful Member

• Dec 31, 2019

• #4

LM 317 will use foldback current limiting if you are trying to draw too much current from it. Try running it with no load connected. And than check the output voltage.

 

Faradave

Member

• Dec 31, 2019

• #5

Yea that 20V is just with my dual meter connected

 

Faradave

Member

• Dec 31, 2019

• #6

My 1st thought was to go in and lower the resistor value but it's all nice and packaged in box and I wanted to check with yall

 

rjenkinsgb

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #7

That is a non-standard circuit for adjusting a 317 and the results will be somewhat strange.
The pot wiper & resistor junction should also connect to the adj pin.

eg. like this; with 330 ohm and a 20K pot the adjustment range would be up to something over 70V

 

Faradave

Member

• Jan 1, 2020

• #8

Wait, so one side of the pot is connected to ground the opposite side is connected to the adjust pin the wipers connected to the adjust pin and the 330 Ohm resistor is connected to the adjust pin? like that? That diagram is unclear to me where the wiper goes

 

V

Visitor

Well-Known Member

• Jan 1, 2020

• #9

For the LM317, R1 should be 120 ohms to ensure the minimum required load current is drawn in all cases. The LM117 minimum load current is half that of the LM317, allowing the higher value for R1.

 

Faradave

Member

• Jan 1, 2020

• #10

Oh you wanna know something else? When I'm powering something, there is a high pitch whine in the pitch and volume increase as I increase the voltage output. Weird huh?

 

Faradave

Member

• Jan 1, 2020

• #11

Now that makes sense to me. Also, this thing needed a heat sink

 

dr pepper

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #12

If you put a 0.1 ohm resistor inline with the + or - output of your supply, then put your meter across it every 0.1v will be 1 amps.

 

atferrari

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #13

Faradave said:

Now cording to the datasheet you'll get a couple of volts less than what, 1 or 2 less.

Click to expand...

Not true.

 

tvtech

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #14

LM317T is good for 1.5A with a decent heatsink attached. 2.2 Amps absolute max into a short circuit. Won't self destruct. It is a beast. Well designed from word go. Thats why nothing has managed to replace it yet and it is still currently being manufactured.

I tried blowing them up with my Light Buddy. No go.

If you keep Vin - Vout less than it's specs..... you are assured of reliability for ever....heck, 8 year's now. Still going strong.

 

Faradave

Member

• Jan 1, 2020

• #15

dr pepper said:

If you put a 0.1 ohm resistor inline with the + or - output of your supply, then put your meter across it every 0.1v will be 1 amps.

Click to expand...

wait what?? is that true?!?!

 

Faradave

Member

• Jan 1, 2020

• #16

atferrari said:

Not true.

Click to expand...

Here's an example you put in 35 volts you get out 30. You can't always get out as much juice as you put in..... but if you try sometimes you just might find.... you get what you need!

 

dr pepper

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #17

Current sensing with a low ohm resistor is a very common trick, many of my circuits use the trick.

I think your mixing up the couple of volts less than the supply with the minimum output voltage from the '317, from memory it'll only go down to 1.5v or so, it may well get close to the supply but you want some headroom for it to regulate.

 

rjenkinsgb

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #18

Faradave said:

Wait, so one side of the pot is connected to ground the opposite side is connected to the adjust pin the wipers connected to the adjust pin and the 330 Ohm resistor is connected to the adjust pin? like that? That diagram is unclear to me where the wiper goes

Click to expand...

The pot is configured as a two terminal variable resistor- the wiper is linked to one end of the track.

 

crutschow

Well-Known Member

Most Helpful Member

• Jan 1, 2020

• #19

Below is the LTspice simulation of the circuit that goes from about 1.2V to 30V for pot rotation of 0 to 100% (0 to 1 on the horizontal axis).
The 6.99K ohm in parallel with the pot allows a 30v maximum output at near 100% rotation with a standard 5k pot.
This gives a slightly non-linear output voltage versus pot rotation, but gives finer voltage resolution versus rotation at the higher output voltages.

 

Faradave

Member

• Jan 2, 2020

• #20

BOOM! Somebody busts out some spice action! This is a great place for info, you guys are beyond casual! That's awesome man, I'll have to search for a 5k pot the lowest I have is 20k. My thing is recycling parts from broken electronics so I can be like "from trash came all this" but I'll save your circuit diagram.

And yea the lowest the regulator will go is about 1.25V.

Awesome guys, thanks!