current drop at lm317 const current source - Electronic Circuits Projects Diagrams Free

kermit · 29th October 2008, 08:49 PM

I googled for a constant current source and came along the lm317.
I both simulated it (in multisim) and built up the simple circuit, but I neither get a constant current when i attach a load!

I followed this circuit: http://www.roboternetz.de/bilder/konstantlm317.gif.
Everything is fine if the circuit remains open, however attaching a load let the current drop significantly.

Do you see any failures in "my" design?

Thanks

**Nigel Goodwin** · 30th October 2008, 11:36 AM

You don't give any details of what you've done, what the source is, or what the load is.

Willbe · 30th October 2008, 07:10 PM

Quote:

Originally Posted by Nigel Goodwin

You don't give any details of what you've done, what the source is, or what the load is.

Ask Boncuk, but I think your battery is backwards.

Hero999 · 30th October 2008, 07:16 PM

Yes the battery symbol and terminals don't match.

mvs sarma · 30th October 2008, 07:21 PM

Quote:

Originally Posted by kermit

I googled for a constant current source and came along the lm317.
I both simulated it (in multisim) and built up the simple circuit, but I neither get a constant current when i attach a load!

I followed this circuit: http://www.roboternetz.de/bilder/konstantlm317.gif.
Everything is fine if the circuit remains open, however attaching a load let the current drop significantly.

Do you see any failures in "my" design?

Thanks

Get to the data sheet and applications shown there. your doubts would be cleared. Study them, don't test on Multisim!

markhao · 30th October 2008, 07:26 PM

hehe, just make it in the actual... just replace parts if it is blown.

success is a series of failure!

goodluck

Boncuk · 30th October 2008, 09:31 PM

First of all, how did you make the battery symbol? Normally the long terminals are the + connections and the short ones are -.

Using Proteus ISIS the current is constant.

Have a look.

Might be MultiSim has a better supply and simulates the depleting battery as well.

Boncuk · 30th October 2008, 09:32 PM

Oops, nervous finger.

Hint: For currents below 10mA use the LM317L. Its minimum current is 1mA, but might go down to 500µA.

The LM317 requires a minimum current of 10mA. Everything below won't be stable.

Hero999 · 30th October 2008, 10:28 PM

I suspect that the problem is that the drop-out voltage is too high for his application.

The LM317 has a drop-out voltage from anywhere between 1.5V to 2.5V depending on the current and temperature, then you have to add on 1.25V for the voltage referance which gives 2.75V to 3.75V. This means that if he's doing something like powering a blue LED from a 6V supply then it won't work.

kermit · 31st October 2008, 04:36 PM

hi guys,
in fact i should have thought twice before actually posting...

first of all: i didnt create the circuit i linked to - i just linked to the resource i used.

second: you are right: i tried to build a perpetuum mobile :-) that is, the consuming power was higher than it actually could be (load was much too high in order to drive my desired current through the it)

thanks for all your help
kermit

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29th October 2008, 08:49 PM	#1
kermit	current drop at lm317 const current source I googled for a constant current source and came along the lm317. I both simulated it (in multisim) and built up the simple circuit, but I neither get a constant current when i attach a load! I followed this circuit: http://www.roboternetz.de/bilder/konstantlm317.gif. Everything is fine if the circuit remains open, however attaching a load let the current drop significantly. Do you see any failures in "my" design? Thanks

30th October 2008, 11:36 AM	#2
Nigel Goodwin	You don't give any details of what you've done, what the source is, or what the load is. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

30th October 2008, 07:16 PM	#4
Hero999	Yes the battery symbol and terminals don't match. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

30th October 2008, 07:26 PM	#6
markhao	hehe, just make it in the actual... just replace parts if it is blown. success is a series of failure! goodluck __________________ every moss and cobweb scars of a failing heart

30th October 2008, 09:31 PM	#7
Boncuk	First of all, how did you make the battery symbol? Normally the long terminals are the + connections and the short ones are -. Using Proteus ISIS the current is constant. Have a look. Might be MultiSim has a better supply and simulates the depleting battery as well. __________________ Proper Planning Prevents Piss Poor Performance

30th October 2008, 09:32 PM	#8
Boncuk	Oops, nervous finger. Hint: For currents below 10mA use the LM317L. Its minimum current is 1mA, but might go down to 500µA. The LM317 requires a minimum current of 10mA. Everything below won't be stable. Attached Thumbnails __________________ Proper Planning Prevents Piss Poor Performance Last edited by Boncuk; 30th October 2008 at 09:35 PM.

30th October 2008, 10:28 PM	#9
Hero999	I suspect that the problem is that the drop-out voltage is too high for his application. The LM317 has a drop-out voltage from anywhere between 1.5V to 2.5V depending on the current and temperature, then you have to add on 1.25V for the voltage referance which gives 2.75V to 3.75V. This means that if he's doing something like powering a blue LED from a 6V supply then it won't work. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

31st October 2008, 04:36 PM	#10
kermit	solved hi guys, in fact i should have thought twice before actually posting... first of all: i didnt create the circuit i linked to - i just linked to the resource i used. second: you are right: i tried to build a perpetuum mobile :-) that is, the consuming power was higher than it actually could be (load was much too high in order to drive my desired current through the it) thanks for all your help kermit