My task is to heat a coil, with a resistance of approximately 0.8 ohm. I need to make a constant current source to make it work correctly. I want to use LM317T as a constant current source, but I could not get more than 0.2 A. Even when R1 is a 1 ohm resistor. I am using this schematic.


Can you help me please?

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    \$\begingroup\$ Welcome @Marek. What is the value of Vcc? \$\endgroup\$ – devnull Mar 13 at 10:12
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    \$\begingroup\$ (1) What is the value of Vcc? (2) Did you work out the voltage drop across the LM317? What value did you get? (3) Did you work out the power dissipation (P = VI) in the LM317? What value did you get? (4) Did you add a heatsink to the LM317? How many °C/W is the heatsink rated at? (5) What is the expected temperature rise for that heatsink with the power you have calculated? (6) If R1 is 1 \$\Omega \$ what current limit would you expect from the LM317? Try and answer all these points in your question and we can help you out. \$\endgroup\$ – Transistor Mar 13 at 10:13
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    \$\begingroup\$ I need to make a constant current source to make it work correctly. Maybe not. If it's a pure resistive load, then PWM could be made to emulate a constant current source with no dissipation in the controller. \$\endgroup\$ – Neil_UK Mar 13 at 12:23
  • \$\begingroup\$ LM317 has internal overheating protection. It restricts output current. Heatsink required. \$\endgroup\$ – user263983 Mar 13 at 13:16
  • \$\begingroup\$ What heatsinking arrangement have you provided for IC1? And add answers to the other questions before this is closed as "needs details". \$\endgroup\$ – user_1818839 Mar 13 at 14:01

You need to add up 3 voltage drops:

  1. The voltage the LM317 needs to work
  2. The voltage across the reference (1.25V for the LM317) which is also the voltage across R1.
  3. The voltage across the 0.8 ohm load.

If you are using a 1 ohm resistor, you are expecting 1.25A, so the 0.8 ohm load will drop 1 volt. The LM317 requires 3V to work. So you need a minimum input voltage of 5.25V at 1.25A.

Note that the LM317 has very poor efficiency in this application. 1.25W is getting to the load, but the input power is 6.5W so the efficiency is < 20%. With a higher than the absolute minimum input voltage it's even worse. The LM317 will need a heat sink to dissipate the several watts safely as well.

If you're making a vaping device, I suspect there are a lot of working circuits out there that will be better.


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