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LM317 tripping out at 0.33 Amps Why

Eddie478

Jan 24, 2020
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Hi, have just built a small variable power supply using an lm317BTG, Its keeps tripping the thermal overload. just a basic circuit as per many on google, supply is a laptop supply at 24v @2.5a
setting regulation at 11.5 to 13.5 volts.
Its supplying a curtain, auto closer motor at 12v need adjustable for speed control all this works ok
but the lm317 is over heating yet the motor is only drawing .33 amps
r1 is a 120ohm 1/2 w resistor, and vr1 a 10k pot, c2 on output a 1uf cap, c1 on input a 100nf cap
and a protection diode 1n4001 across the input and output on the reg,
I cannot understand why the reg is getting so hot with this small amperage draw?, what am I missing?,
Tested the motor on a variable bench supply built from a computer power supply set at 12v on variable supply and it will run all day no issues, and its basically the same circuit.
 

Harald Kapp

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Nov 17, 2011
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r1 is a 120ohm 1/2 w resistor, and vr1 a 10k pot, c2 on output a 1uf cap, c1 on input a 100nf cap
I might think it were very helpful to see the schematic you are referring to. Otherwise, what the heck is R1, VR1 etc? You can upload an image by simply dropping it into the edit box (note: max. ~ 100kB size).

Chances are that the regulator overheats. An input voltage of 24 V and an output voltage of 12 V (a mean value from your numbers) results in a voltage drop of 12 V across the rgulator. 12 V × 0.3 A = 3.6 W. The device in a TO220 case (whic I assume you are using) has a thermal resistance Rth(junction-ambient) of 65 °C/W. At 3.6 W this means the junction temperature is at 3.6 × 65 °C = 234 °C above ambient temperature. No wonder it goes into current limit mode.
You will have to add a heatsink.

Or use a switch mode regulator module (cheap on the internet) which will drastically reduce your losses.
 

Eddie478

Jan 24, 2020
4
Joined
Jan 24, 2020
Messages
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I might think it were very helpful to see the schematic you are rfeferring to. Otherwise, what the heck is R1, VR1 etc? You can upload an image by simply dropping it into the edit box (note: max. ~ 100kB size).

Chances are that the regulator overheats. An input voltage of 24 V and an output voltage of 12 V (a mean value from your numbers) results in a voltage drop of 12 V across the rgulator. 12 V × 0.3 A = 3.6 W. The device in a TO220 case (whic I assume you are using) has a thermal resistance Rth(junction-ambient) of 65 °C/W. At 3.6 W this means the junction temperature is at 3.6 × 65 °C = 234 °C above ambient temperature. No wonder it goes into current limit mode.
You will have to add a heatsink.

Or use a switch mode regulator module (cheap on the internet) which will drastically reduce your losses.
thanks Harad, I added heatsink and all is well, testing right now, been running for 20 minutes tested temp at 67c is that ok?
 
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