Troubleshooting LM317T Heating in Variable Power Supply - Tips and Tricks

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In summary, the conversation discusses the use of a LM317T and a Power Integration IC LNK6775E to create a variable power supply with an output voltage range of 14 V to 27.6 V. However, the LM317T is not able to handle the heat generated and goes into protection mode after 5 minutes when a load of 1A is applied. Suggestions to control the heat include using a larger package for the LM317T and integrating a Buck DC-DC converter for the output regulation. The conversation also mentions using the LM317T as a switching regulator and potential circuit topologies for this approach. Additionally, it is suggested to purchase a pre-made voltage regulator for less than $2.
  • #1
Hitesh
Hello . I am trying to make power supply using LM317T. As i need Variable output voltage from 14 V to 27.6 V.
I made a power supply of 29V/1A using Power Integration IC LNK6775E. At the output of 29V i am using LM317T to vary voltage.
But after 5 minutes IC is going in protection mode if apply a load of 1A and set output voltage to 14V.
I am also using Heatsink, but there is too much heat.
How to control this heat.
And is it reliable ?
 

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  • #2
Welcome to the PF. :smile:

15W is a lot of heat to get rid of. What package is your LM317 in? When you say LM317T, does that imply you are using a TO-220 package? You probably need a larger package than that, with a bigger heatsink. Using a linear regulator at that current level and that large of a voltage drop may not be the best approach.

Have you looked at using a Buck DC-DC converter for the output regulator instead?
 
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  • #3
15 volt drop at 1 amp is 15 watts
To hold temperature rise to 100 degrees you need thermal resistance junction to ambient of 100deg/15watts = 6.67 deg/w or less
and observe 5 of those are used up internally
leaving only 1.67 for case to ambient which is your heatsink.
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00000455.pdf
upload_2017-10-11_12-31-33.png


to keep temperature rise less than 100 degrees you need a heatsink that can beat 1.67 deg/watt.
Here's a snip from
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00000455.pdf
nov-heat-sink-fig-2.jpg


about a square foot of 1/8 " aluminum should do nicely.

Alternatively you might add resistance in series with the IC when asking it to handle a lot power...
A BIG light bulb might work well.
Got an old halogen Torchiere lamp ?
upload_2017-10-11_12-44-8.jpeg
 
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  • #4
You also made a regulator that's 50% efficient, which is not good.

Why not just use the LNK6775E for your voltage regulation. I'm assuming you are using a potentiometer or similar variable resistor in the feedback path on your linear regulator. Just include the potentiometer on your LNK6775E instead. you already have feedback resistors, that how you get the 29 volts. just redo your calculations to determine the resistance swing and tolerance you need.
 
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  • #5
berkeman said:
Welcome to the PF. :smile:

15W is a lot of heat to get rid of. What package is your LM317 in? When you say LM317T, does that imply you are using a TO-220 package? You probably need a larger package than that, with a bigger heatsink. Using a linear regulator at that current level and that large of a voltage drop may not be the best approach.

Have you looked at using a Buck DC-DC converter for the output regulator instead?
Thanks for reply. But i need variable output voltage from 14 V to 28 V. By using Buck converter , it will decrease output voltage. And can i get variable output voltage using Buck DC DC converter? or what circuitry i need to be added?
 
  • #7
For just a one-of-a-kind home project,
Try a search on "LM317 as switching regulator"

the old National datasheet from before TI took them over had this image
upload_2017-10-12_10-15-35.png


i never tried it so don't know if it's well behaved.
 
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  • #8
jim hardy said:
i never tried it so don't know if it's well behaved.
I don't get it. What switches?
 
  • #9
berkeman said:
I don't get it. What switches?

If i understand the circuit,

this positive feedback path gives it enough hysteresis that Mr LM317 will oscillate between full-on and full-off.

lm317Switcher.jpg


For lower current i wonder if the inductor with a series diode could be moved down to R2's position , eliminate Q1 & R1 ?
I don't know, just speculating.

As i say i never built one. Looks like an easy introduction to switchers and inductor sizing, though.

old jim
 
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  • #10
I can't recall the exact circuit topology but I have seen a switcher built around a 317. From what I recall it was a pretty reliable setup.
 
  • #11
Why build when you can buy and have it delivered for less than $2.

http://www.ebay.com.au/itm/Voltage-Regulator-LM2596-3A-Adjustable-DC-To-DC-Step-Down-Converter-Buck-Module/232274773224?

It should handle one amp without a heatsink.
Replace the trim-pot on the PCB with an external pot of same resistance on three short twisted wires.
 
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1. Why is my LM317T heating up in my variable power supply?

There are a few potential reasons for this issue. One possibility is that the LM317T is being overloaded, meaning it is trying to provide more current than it is rated for. Another possibility is that the heatsink on the LM317T is not sufficient for dissipating the heat generated. It's also possible that there is a short circuit or other issue causing excess heat.

2. How can I prevent my LM317T from overheating in my variable power supply?

There are a few steps you can take to prevent overheating in your LM317T. First, make sure you are not overloading the regulator and that the heatsink is adequate for the amount of current being drawn. You can also improve the heat dissipation by adding a fan or using a larger heatsink. Additionally, double check all connections and components for any potential issues that may be causing excess heat.

3. Can I use a LM317T with a higher current rating to prevent heating in my variable power supply?

While using a regulator with a higher current rating may seem like a solution, it's important to note that the LM317T has a built-in thermal shutdown feature to protect it from overheating. Using a higher current rated regulator may still result in overheating and could potentially damage the other components in your circuit. It's best to address the root cause of the heating issue rather than trying to use a higher current rated regulator.

4. Are there any specific components I should use to prevent heating in my variable power supply?

The LM317T itself is designed to handle heat, so the most important component to focus on is the heatsink. Make sure you are using a heatsink that is appropriate for the amount of current being drawn. You can also use thermal paste or pads between the LM317T and the heatsink to improve heat dissipation.

5. Is there a maximum temperature that the LM317T can safely operate at?

The LM317T has a maximum junction temperature of 125°C. However, it's important to note that the datasheet recommends keeping the temperature below 100°C for optimal performance and longevity. It's best to monitor the temperature of the LM317T and make sure it stays within a safe range to avoid any potential issues.

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