1

I need to make a 1.5 V voltage source for a motor with maximum current drain 1 A.

I made this circuit:

enter image description here

The output voltage without load is 1.46 V, which is fine. When I connect a load resistor which is smaller than 35 ohm, then the output voltage starts to decrease:

Load resistor - output voltage

36 ohm - 1.39 V

8 ohm - 600 mV

The maximum output current is about 200 mA while output voltage is 0 V.

Could you help me find the the problem?

The datasheet says that maximum output current should be 1.5 A.

JRE
  • 71,321
  • 10
  • 107
  • 188
Jakub Šmíd
  • 11
  • 1
  • What did you use to power the circuit (v_source) or was it just a simulation? – Andy aka Aug 20 '21 at 11:06
  • In the LtSpice it works fine. But problem is when I test it in breadboard. Source is USB charger 5 V 1 A. – Jakub Šmíd Aug 20 '21 at 11:09
  • 2
    Where are your decoupling capacitors? Is the regulator getting hot? Do you have a heatsink? Is your breadboard rated for 1 A? What is the input voltage reading when the output is loaded? – Transistor Aug 20 '21 at 11:15
  • Are you measuring the output voltage at the LM317 pin or at the motor? Have you measured the USB charger output under load? – devnull Aug 20 '21 at 11:18
  • Yes, I have heatsink. Input voltage is stable 5 V even though the regulator is loaded. I have no decopling capacitor, circuit is the same as it is in the scheme – Jakub Šmíd Aug 20 '21 at 11:19
  • Measured voltage is at the LM317 pins – Jakub Šmíd Aug 20 '21 at 11:24
  • 1
    I would say that from my experience, 1 A on a breadboard will give you troubles. The contact resistances are too high so you will get strange results and things not working. Build this in a prototype PCB with soldered connections and make sure that the LM317 has a large enough heatsink. Then this should work fine. – Bimpelrekkie Aug 20 '21 at 11:25
  • 1
    Check your datasheet for references to the decoupling capacitors. If you're leaving them out ask yourself, "What do I know that the manufacturer doesn't?". – Transistor Aug 20 '21 at 11:56
  • 2
    If you are making 1.5V 1A from 5V, the regulator needs to convert 3.5W into heat. Depending on which exact regulator package and model you use and how good the heatsink is, the part can go into thermal shutdown. – Justme Aug 20 '21 at 12:17
  • The number "200mA" occurs often enough in questions about the LM317 I suspect that there are some fakes that shut down at 200mA. https://electronics.stackexchange.com/a/632748/294393 – mabartibin Aug 27 '22 at 20:18

0 Answers0