I tried to make a constant current source with the LM317 but i tried first to make it with a T package and it workes... but i try after this succes with a SMD(SOT223) LM317MQDCYRG4 but doesn´t work... i ordred the SMD as free sample and i got it with the name "L5 24ZEY3H" but i don´t if it´s the right device. I didn´t find the part number anywhere...
can some one help me :-(
The LM317 has a max current rating of 1.5A
The LM317MQDCYRG4 is part of the LM317M line which only has a max current rating of 500mA.
Thanks. but you doesn´t understand me. So i send you the circuit layout to have an idea. I built a constant current source with 1mA for a PT100. So at the beginning i used big devices ;-). After this succes i tried to make a SMD layout and i used instead of the LM317T the LM314L and LM314M in SOT223 Packaging. But what a surprice the both regulators doesn´t work...so i got 4mA (with the LM314M) and 2mA (LM317L) i used (with a resistance between the ADJ and OUT of 1.25KOhm).
I think that the problem is that you used huge devices for your purpose. A current source of 1mA has to be built using other building blocks. The LM317, typically, needs a minimum load current of 3.5mA. I think that that is the reason it doesn't work for you.
For your application, try the LM134.
I you need a constant current source for only 1-10mA, then use two BJTs.
See this site: http://www.vidisonic.com/2008/07/10/current-limiting-circuit/
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