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Trying to make a current limiter for my LM317 power supply. This is the complete circuit: enter image description here

I found the current limiting circuit on the web, I'am powering this circuit from a 12V 20A switching lab power supply. The problem that i have is that current regulation doesn't work, voltage regulation works fine. but when i adjust the current (above 1 amp usually) the circuit completly shuts down. the 2n3055 transistors are a bit warm but that should not matter since i only wanna test if it's capable of 2 - 3 amps for maybe 2 seconds. But the circuit just dies.

If i leave it like that (dead) but still plugged in, it comes back on again after maybe 10 seconds, then shuts down again etc.

My first thought is that the LM317 overcurrent protection is kicking in, but since the transistor is taking most of the load that shouldn't matter right?

The LM317 is very cold btw. Not hot at all. My guess is that i simply forgot to add something but i can't figure out what..

I don't have any capacitors in this circuit on the breadboard, could that be the cause of the problem?

Thanks for any help in advance!

LM317 datasheet: http://www.ti.com/lit/ds/symlink/lm317.pdf

2N3055 datasheet: http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

Update: I just noticed that current changes when i put a finger on the current regulation pot.. if it shows like 300 mA and i put my finger on it, it goes down to maybe 150 mA and jumps around until i remove the finger.. (Tried to change the pot but same problem occured)

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  • \$\begingroup\$ A link to the original web page is probably crucial. \$\endgroup\$ – Andy aka Jun 11 '16 at 19:18
  • \$\begingroup\$ Well it was a youtube video actually. this is the video: youtube.com/watch?v=xE0pzvycxPc \$\endgroup\$ – Xane Jun 11 '16 at 19:22
  • \$\begingroup\$ Try posting a screen shot of the circuit. I can see one weakness that nay not be in the original. \$\endgroup\$ – Andy aka Jun 11 '16 at 19:58
  • \$\begingroup\$ Thanks for looking, you mean a picture of my breadboard? It will just look chaos will all the wires etc. you still want one? \$\endgroup\$ – Xane Jun 11 '16 at 19:59
  • \$\begingroup\$ 2~3A through a breadboard? \$\endgroup\$ – Bruce Abbott Jun 11 '16 at 20:54
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When you see the 2-3 amps going through the load the lm317 is seeing a current overload situation and it is reacting to it with "Operation In Self Protection" mode. It is noted in the lm317 datasheet in section 8.4.4.

8.4.4 Operation In Self Protection When an overload occurs the device will shut down Darlington NPN output stage or reduce the output current to prevent device damage. The device will automatically reset from the overload. The output may be reduced or alternate between on and off until the overload is removed.

You might consider putting your voltage varying circuit before the current sensing circuit. Remember your current of the entire circuit is limited to the max current of the ml317.

You need to combine both voltage and current overload together such that they both control the 2n3055 transistor. When I find my schematic of such a circuit I will post it.

I did not see a load output of the lm317 circuit. If you are actually putting a short from the right hand 2n3055 that will be your problem.

I found that schematic:

adjustable max current and voltage power supply

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  • \$\begingroup\$ Thank u, i would really appreciate if you could find the schematic. I still don't understand how the LM317 can see an overload when the current itself doesn't go through the LM317, i thought that was the point with using power transistors. To take the load away from "other things" \$\endgroup\$ – Xane Jun 11 '16 at 20:37
  • \$\begingroup\$ Thank you for the schematic! but it uses an LM317 for the current limit and that way i can't use a pot without it heating up a lot. that's why i want a way to make the current go through a 2n3055 instead. Could you maybe give me an example of how you meant? thanks again! \$\endgroup\$ – Xane Jun 12 '16 at 0:53
  • \$\begingroup\$ What are you using for a load? All I see is a short circuit between the emiter of the 2n3055 and ground. \$\endgroup\$ – RonP Jun 12 '16 at 19:19
  • \$\begingroup\$ I'm not using anything for a load, since it's limiting the current, would it matter if i use a load or short it? but i have tried and used a load also and same problem occur = the circuit shuts down. \$\endgroup\$ – Xane Jun 12 '16 at 19:40
  • \$\begingroup\$ What I would do is separate the total circuit into 3 parts and test them. First I would disconnect the current sensing circuit and then test the voltage control side to see if it works with a load. Then test the current sensing circuit to see that it works. Lets us know what the outcome was. \$\endgroup\$ – RonP Jun 13 '16 at 0:59

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