lm317t output stays about .7v lower than input

Thread Starter

Goinidias

Joined Dec 1, 2019
4
I've got a bunch of these and they all behave in the same way, so I'm doing something wrong.
I'm using the output calc and schematic here: https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
I'm trying to get it to near it's minimum so for R1 and R2 I'm using the same value.
However:
The output is always about .7v-1v lower than the input. So at 9v on the in pin I get 8.3v on the output.


I've used 110, 220, and 1000 ohm for resistors (r1=r2). Doesn't seem to make a difference.
Tried with and without capacitors in the datasheet: https://www.onsemi.com/pub/Collateral/LM317T-D.PDF
Switched out all the components, boards, and power supplies multiple times.

Attached image of one of my attempts. r1 = r2=220 ohm
A small van and an led with 1000ohm resistor on the output to make sure it has some load.
input pin is set to 9v and the output I'm getting is 8.3v.

What have I goofed?
 

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TeeKay6

Joined Apr 20, 2019
572
I've got a bunch of these and they all behave in the same way, so I'm doing something wrong.
I'm using the output calc and schematic here: https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
I'm trying to get it to near it's minimum so for R1 and R2 I'm using the same value.
However:
The output is always about .7v-1v lower than the input. So at 9v on the in pin I get 8.3v on the output.


I've used 110, 220, and 1000 ohm for resistors (r1=r2). Doesn't seem to make a difference.
Tried with and without capacitors in the datasheet: https://www.onsemi.com/pub/Collateral/LM317T-D.PDF
Switched out all the components, boards, and power supplies multiple times.

Attached image of one of my attempts. r1 = r2=220 ohm
A small van and an led with 1000ohm resistor on the output to make sure it has some load.
input pin is set to 9v and the output I'm getting is 8.3v.

What have I goofed?
I am not an expert on proto boards, but I do not believe you have the components connected correctly. For instance, it looks to me that you have the output load connected between the "+" power row and the output of the LM317; the load should be connected between the "-" power (i.e. ground) and the output of the LM317. Use an ohmmeter (with power to board disconnected) to verify that each component is connected as per your schematic. Post your schematic.
 

SamR

Joined Mar 19, 2019
2,386
For R1 use 240Ω and for R2 use a 5kΩ potentiometer to allow you to vary the LM317 output voltage.
 

Thread Starter

Goinidias

Joined Dec 1, 2019
4
Yea, the led lights up (with a 100 ohm resistor to keep it from burning out); so don't think it's a polarity issue.

I switched to follow the more standard setup. Since that's obviously standing out to folks.
 

Attachments

atferrari

Joined Jan 6, 2004
4,068
My suggestion: take everything out of that protoboard. Do again the wiring and prior powering up, verify with a DMM all terminals going to common. Then, check the rest. I suspect your problem is there.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
10,226
Yea, the led lights up (with a 100 ohm resistor to keep it from burning out); so don't think it's a polarity issue.
Or the LED is backwards as well.
Could you actually check the supply polarity?
If it is one of the rectangular 9 volt batteries then the small terminal is positive.
 

TeeKay6

Joined Apr 20, 2019
572
I use + as ground and - as power. Sorry I forgot that isn't standard, or I would have switched before posting.

You can find the schematic I'm going for here: https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
@Goinidias
The LM317 datasheet recommends a bypass cap to ground from the LM317 input and from the output. Without the input cap, the LM317 can malfunction due to inductance in the leads to the input power. Pls show how the 9V power is applied in your photo.
 

TeeKay6

Joined Apr 20, 2019
572
Or the LED is backwards as well.
Could you actually check the supply polarity?
If it is one of the rectangular 9 volt batteries then the small terminal is positive.
@Goinidias
You say a 100 ohm resistor is in series with the LED. However, the color code shows a 1K resistor. Just to be clear, R1=R2=330 ohms (or 220 ohms?; the color is not clear to me--orange or red). That means a properly working circuit would produce an output of 2.5V...not enough to drive the LED or the fan. This is the minimum voltage you were trying to get? (BTW, you can get the true minimum of 1.25V out by connecting the ADJ terminal to ground.) Also, in a properly functioning circuit, there will always be 1.25V between Vout and Vadj pins; have you measured that?
 
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Thread Starter

Goinidias

Joined Dec 1, 2019
4
Well folks... you guys are right on the money. I had + & - mixed up.
I also didn't correctly remember which end of the led was ground... so it was backwards and made me think things were flowing correctly since it was lighting up.
lolz. Hard to believe I wasted so many hours on this, but my sincere thanks for you guys taking a look. I wasn't challenging my assumptions and was completely stuck.
 

Audioguru again

Joined Oct 21, 2019
1,727
The datasheet for an LM317 shows R1 at 240 ohms max which is for the more expensive LM117. The max R1 for an LM317 is 120 ohms. The datasheet also shows that the input voltage should be at least 2V higher than the output voltage. Your online calculator does not read datasheets.
 

TeeKay6

Joined Apr 20, 2019
572
The datasheet for an LM317 shows R1 at 240 ohms max which is for the more expensive LM117. The max R1 for an LM317 is 120 ohms. The datasheet also shows that the input voltage should be at least 2V higher than the output voltage. Your online calculator does not read datasheets.
@Audioguru again
Viewing National, Fairchild, and TI datasheets I see no requirement that R1 have a particular value. Most of the application circuits shown in the datasheets for the LM317 show R1=240ohms. What is more important than the value of R1 is that the minimum total load--current through R1 plus whatever flows through external load--must meet the datasheet minimum of 12mA (Natl & FC) or 10mA(TI). If the external load is 0, then the max value for R1 is about 100 (1.2V/12mA) ohms or 120 (1.2V/10mA) ohms. In all cases, for good regulation the current through R1 should be much greater than the current through the Vadj terminal (100uA max). Additionally, the dropout voltage (min input-output differential) varies somewhat with load current, from about 1.5V for low current load to 2.8V for 1.5A load; a graph is provided in the datasheet. For novices, note that the dropout voltage is measured at the minimum valley voltage of any ripple that might be present in the input DC voltage.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
10,226
The bottom left resistor is connected between the output and adjust.
The problem was solved when TS connected the power and the LED with correct polarity.
 
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