LM317 getting hot!

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DieterD

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Joined 2001
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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tiroth

Member
Joined 2001
anidian.com
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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HarryHaller

Banned
Joined 2002
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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paulb

Member
Joined 2001
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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stadams

Member
Joined 2002
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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dice45

Member
Joined 2002
Harry,

nice pix, really nice shape, but does she have enough CPU power to control more than her face musculature? :D ..
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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DieterD

Member
Joined 2001
I always thought the quote was

" a picture paints a thousand words" not "creates" looks like I was wrong.

To get back to the original thread ( boring) I have eventually gone for two 317's in series and it seems to have sorted the problem out.

Now where do I get that heatsink?

DieterD
 
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