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Old 4th April 2002, 01:12 PM   #1
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Default LM317 getting hot!

I have built a regulated supply using a LM317T, adjusted to produce 15 volts. Transformer produces a rectified voltage of 23 volts. The supply is to drive a scanner which requires a current of 1 amp. On bench testing the supply at about 600 milliamp. the 317 (mounted on a heatsink) gets very hot within about 2-3 minutes . The specs says the 317T should handle 1.5 amps!
Is it possible to use two 317's connected in parallel to share the load? I have a bunch of them in my parts bin, any ideas?

DieterD
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Old 4th April 2002, 02:16 PM   #2
tiroth is offline tiroth  United States
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Um, 23-15=8v dropped across the regulator. Your target current is 1A. P=IV, so you are disapating 8W with your TO-220 device. Assuming 10 C/W with your heatsink (probably overly generous if it is a little flag sink) you are talking an 80 degree rise above ambient.

The rated 1.5 amp assumes two things: adequate heatsinking, and a small enough voltage drop to keep the device in its safe operating area. You probably need to increase your sinking and reduce the input voltage, or consider something other than a series-drop regulator.
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Old 4th April 2002, 02:52 PM   #3
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Default LM317

Uncle Bob says to put two regulators in series with 4 volts across each one. Mount on seperate heat sinks.

H.H.
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Old 4th April 2002, 02:54 PM   #4
paulb is offline paulb  Canada
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You could put two regulators in series. The first one drops half the voltage, the second sets the final output voltage. That will halve the power dissipated by each regulator.
Oops, just noticed Harry beat me to it. Use bigger heatsinks even if you do change it to use two regulators.
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Old 4th April 2002, 02:55 PM   #5
jam is offline jam  United States
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Dieter,

You could add several diodes in series with the regulator to get the required voltage drop (0.6v per diode). No additional heatsink required.

Jam
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Old 4th April 2002, 02:58 PM   #6
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Default Bigger heatsink

Britney says to use a bigger heat sink.
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Old 4th April 2002, 03:04 PM   #7
jam is offline jam  United States
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Harry,

You got me there!

Jam
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Old 4th April 2002, 06:29 PM   #8
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Lightbulb Additonal Transistor

Just off the top of my head...

Using an appropriately sized NPN power transistor, place the LM 317 in parallel with the base to emitter junction so that the input of the LM 317 is connected to the transistor base and the output of the LM 317 is connected to the transistor emitter. Next connect a resistor with proper value and rating from collector to base. You now have a properly biased emitter follower that is controlled by the voltage that you adjust on the LM 317. Or just chuck the LM 317 all-together and use a zener on the base if you do not need adjustability.
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Old 5th April 2002, 05:57 AM   #9
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Thanks guys,
I tried the heatsink in Harry's picture but it just seemed to make things hotter!
I think I will go for the NPN solution, seems to be the better way out nd retails ajustability etc.

DieterD
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Old 7th April 2002, 07:30 PM   #10
dice45 is offline dice45  Germany
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Harry,

nice pix, really nice shape, but does she have enough CPU power to control more than her face musculature? ..
the pix doesn't look like it

Jam,
nice idea, those diodes! But don't they disspiate heat, too? They should atleast dissipate 0.6 Watt per Ampere each.
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