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LM317A Model Not Working As Expected

 

Hi,
 
I've pulled a model of the LM317A from the Files section (LM317A_001.zip). This is a working sim.  I re-wired for constant current operation at 1.5A and the results are way off. Output is just over 1A.
 
The circuit was modified from the working schematic in the zip archive.
 
Can anyone see what's gone wrong?
 
https://i.ibb.co/xHM934j/lm317-A-sim.jpg
 
For the circuit as wired, Iout should be 1.25/R1 according to the data sheet.
The supply voltage is high enough to accommodate the 317 drop out voltage, so it's not that.
I've either misunderstood how the LM317 operates, or the model behaves differently to the datasheet.
 
Any pointers appreciated.
 
Regards.
 
 
 
 
 

On Mon, Nov 4, 2024 at 03:30 PM, KillingTime wrote:
Can anyone see what's gone wrong?
 
Well, two things for starters:
  1. Never point us to off-site web storage sites.  Upload your schematic to this group's Files area.
  2. Do not use pictures of schematics.  Upload the LTspice schematic (.asy file) itself.
 
Did you forget to read the guidelines at this group's main webpage?  Please go and do that now.  Then follow those guidelines.
 
Andy
 
 

First, read and internalize Andy’s comments…

Then try the circuit with the library LDO, LT1086.

Very similar 3-terminal reg with similar Adj voltage.

Works good in sim.

 

Dave

 

From: LTspice@groups.io <LTspice@groups.io> On Behalf Of KillingTime
Sent: Monday, November 04, 2024 12:20 PM
To: LTspice@groups.io
Subject: EXTERNAL: [LTspice] LM317A Model Not Working As Expected

 

Hi,

 

I've pulled a model of the LM317A from the Files section (LM317A_001.zip). This is a working sim.  I re-wired for constant current operation at 1.5A and the results are way off. Output is just over 1A.

 

The circuit was modified from the working schematic in the zip archive.

 

Can anyone see what's gone wrong?

 

 

For the circuit as wired, Iout should be 1.25/R1 according to the data sheet.

The supply voltage is high enough to accommodate the 317 drop out voltage, so it's not that.

I've either misunderstood how the LM317 operates, or the model behaves differently to the datasheet.

 

Any pointers appreciated.

 

Regards.

 

 

 

 
 

On 11/4/24 2:20 PM, KillingTime wrote:
Hi,
I've pulled a model of the LM317A from the Files section (LM317A_001.zip). This is a working sim.  I re-wired for constant current operation at 1.5A and the results are way off. Output is just over 1A.
The file of that name that I found includes a current regulator circuit. Which doesn't work particularly well at higher currents. The model included is unusual in that it is built up from transistors, resistor, etc. rather than a behavioral model. It includes:

.subckt LM317A IN ADJ OUT R20=25.5k
* Adjust param R20 for short circuit current limit.
* TYP = 2.2A = 48.7k
* MIN = 1.5A = 25.5k (Default)
* MAX = 3.4A = 84.5k
*

I am not going to dig into the details of this implementation but it doesn't work well at higher currents. It is fine in the 100mA example shown but is already off at 1A.

--
http://davesrocketworks.com
David Schultz
 

Apparently there is a problem with that LM317A SPICE model from 2014.  'Eetech00' is an active member of this group, and hopefully he can say something about it.
 
Why not download T.I.'s unencrypted model and use it instead?  It works.  You can even use the same symbol, but change the Value from "LM317A" to "LM317A_TRANS".
 
Andy
 
 
 

See "LM317A current_source.zip" that I uploaded to the Temp folder.  It uses T.I.'s non-encrypted  SPICE model.  It seems to work OK.
 
Andy
 
 
 

>>See "LM317A current_source.zip" that I uploaded to the Temp folder.  It uses T.I.'s non-encrypted  SPICE model.  It seems to work OK.
 
Hello Andy,
 
I didn't know T.I. had made a free model available. Learnt something new there.
I've downloaded the sim and run it. It does indeed work as expected. 1.5A output. You even put all the component values in for me from my origin al schematic - Thank You.
 
I'll use the model you uploaded from now on.
 
>> Well, two things for starters:
  1. Never point us to off-site web storage sites.  Upload your schematic to this group's Files area.
  2. Do not use pictures of schematics.  Upload the LTspice schematic (.asy file) itself.
Understood. Will do this from now on.
The .asy file you kindly uploaded for me is a copy of the circuit from my original post but with a working LM317 model, so there's no need to upload my non-working files in this instance.
 
Many Thanks.
 

Uploaded:
 
LM317A Voltage Regulator Ver. 002. includes Model file, Symbol, Test Circuit.
Corrects problems with Ver. 001
 
eT

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