lm317 adj. power supply. voltage loss on load.

Thread Starter

jgreene44

Joined Dec 8, 2016
108
hello! kind of a simple question i hope. diy adj. supply. featuring an lm317. with no load. voltage drops about 2 volts as inticipated. adjusts from 0 to whatever no problem. with a small dc motor. total dropout. with an led. reading little over 2 volts. unplug the led. right back to 12. i understand this supply has only the ability to adjust the voltage and not the current. however. i would think that an led should not be able to drop it down to over 2 volts. would think i could run a dc motor. idk. i supplied the lm317 with several sources. transformer ---> full wave bridge rectifier ---> lm317 and several manufactured power supplies to the lm317 of varying voltages. they all perform the same looks great with no load. even an led seems to draw them down... thanks in advance!


http://images.elektroda.net/56_1346052196.gif

im following this diagram.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
without an amp probe im not sure how to tell. its pretty low im sure. its an rs445 i belive thats a mubachi motor ill see if i can find the spec

operating range 24-48
@ 48v 6600 rpm, .060 amps

i did use an electrolytic cap for the .1uf on the input instead of a ceramic. guessing that does not matter.

lastly im not using a resistor. strait from the v out to the led.
 
Last edited:

#12

Joined Nov 30, 2010
18,076
Defect in the schematic. The minimum load for a 317 is 10 ma. The 240 ohm resistor should be 120 ohms.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
so you think i just need to change the resistor? trying it now.

10.93 no load
.87 with the rs445

used a 12 ohm resistor from the v out to the adjust.

voltage increases when i spin the motor in the direction it wants to spin. its at a dead stop until i spin it.
 
Last edited:

dannyf

Joined Sep 13, 2015
2,196
your description is highly non-specific and difficult to follow.

I would say in general,

1. make sure that the pin out is correct;
2. short the adj pin to ground and you should have a steady voltage on the output;
3. measuring the voltage on the 3 pins would help your readers figure out what the problems are.

no need for current measurement at this point.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
right off the pins on the lm317 no load.

supply: 13.7 from a manufactured supply rated for 12v 1 amp
adjust: 13.67
output: 10.86
input: 10.86

pin out was and is fine.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
i do? dont i? 13 in 11 out. should still be enough to run a small dc motor i assume. not sure im understanding the reason for that last post.
 

dannyf

Joined Sep 13, 2015
2,196
that tells you a lot:

1) the input at the same temperature as the output?
2) the adj. pin so above the output? it should be 1.25v lower;
3) why is the supply output so much higher than the regulator's input?
...

again, go back to the very basics, as i outlined earlier.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
ok be nice! im still new. and yes i thought that was odd too. thank you for saying it is. im going to look it over right now!

my wiring from the lm317 to a set of 3 male header pins on a proto board...
backwards! thanks so much for convincing me to look it over harder. i know this is often this case that some noob posts for help and its just a simple matter of schematics. however. sometimes thats just the push one needs. i had already looked it over 1000 times before i posted. so keep that in mind and thanks a million. mean it.

could i now add a pot. on the input ground and have the ability to control the voltage and the amperage?
 
Last edited:

Thread Starter

jgreene44

Joined Dec 8, 2016
108
i did dannyf. i did i did lol. and might have missed that edit. can i now add a pot to the negative lead into the lm317 and be able to adjust voltage and amperage simultaneously? and what would be a way one would monitor that amperage on a display?
 
Last edited:

Standisher

Joined Jan 16, 2015
125
........... can i now add a pot to the negative lead into the lm317 and be able to adjust voltage and amperage simultaneously? and what would be a way one would monitor that amperage on a display?
Not sure I'm fully understanding the first part of your question. The LM317 does not have a "negative lead"; it has input, output and adjustment pins. Connecting a potentiometer as R2 (as shown in the datasheet schematic you posted) allows you to adjust the voltage at the output. The "amperage" (current) delivered is determined by the requirements of your load (e.g LED, motor, etc.). If you want to regulate the current delivered to your load you will need another LM317 configured as a current regulator (shown in the datasheet). This second LM317 configuration would be placed before the voltage regulation part of the circuit. A cheap, ebay digital ammeter, wired in serial with the output will provide the display of current being consumed by your load.
 

Thread Starter

jgreene44

Joined Dec 8, 2016
108
seems like you understood me perfect. idk why you said that. my question was in no way vague. another guy said that too. i wouldn't mind if someone explained to me what is not clear about my question might be usefull in future posts. i see your idea to control the current is educated and yes that's exactly what i was talking about. um just wondering instead of using the second LM317 ahhhh i think i answered my own question correct me on this too. if i was to use a pot instead of the second LM317 (i assume we mean second since it was mentioned last. in series it will be first right? ya i looked back you said before.) to control current the disadvantage would be its load handling capabilities?

just to be clear, my parting question was dont use a pot? use a LM317 like you said too? a pot. might burn up or have to be huge to handle the load?? thats all im not clear about at this point! thanks so much.
 

Standisher

Joined Jan 16, 2015
125
I was not sure I had understood your question because you asked about the "negative lead of the LM317" and the LM317 does not have a negative lead. You also mentioned using a pot to vary the voltage output, but the picture the you posted in your opening post actually showed a potentiometer (R2) being used for variable output so I thought I may be misunderstanding something. On the question of load handling capability, the LM317 has a maximum current output of about 1.5A so adding current regulation as I described would allow current regulation up to or below that limit. A circuit with current and voltage regulation can be found at page 8 of the PDF here . Hope that helps.
 
Top