Hi All,
A newb trying to build a simple variable power supply 1.2- 24 VDC. Using LM317 regulator. R1 instantly overheats and voltage soars to 36VDC.
After numerous LM317 chips, I am at a loss. My circuit is attached. I've poured over the data sheets and my math seems to be ok as are my tolerances.
Any help would be appreciated.

 

Welcome to AAC!

Check your wiring. R1 shouldn't get hot because there's only 1.25V across it.

Do you have a load on the regulator? 220 ohms isn't small enough to satisfy the minimum load requirement.

 

Double check the wiring and the LM317 pinout for the package you have.

 

I have checked and double checked pinout and wiring. No Load on regulator. Possibly have bad LM317's? Data sheet shows a 240 ohm resistor (however my Bible, The art of Electronics, says this is incorrect) which has added some confusion.

 

Where did you buy them? I've only encountered one authentic LM317 that wouldn't regulate with a 10mA load; in 40 years.

A 120 ohm set resistor should be sufficient to satisfy the minimum load requirement; 220 or 240 don't.

If you add a 5mA load with 240 ohms, you should be good.

 

29.46 VDC at pin 1 (in) 25 VDC at pin 2 (out).

 

29.46 VDC at pin 1 (in) 25 VDC at pin 2 (out).

What package?

On TO-92, pin 1 is adjust and pin 3 is in. For TO-220, pins looking from the front are adjust, out, and in.

 

Thanks di324. You have been much help. My package is TO-220. Your comments gave me insight so I put a 330 ohm resistor in parallel with
R1 (220 ohm) to give me 133 ohm load and changed R2 to 5k. Replaced LM317 and now it functions fine. What bothers me is why am I burning up 317's as I understand it is quite resilient? Again, thanks for your help. I am now also an AAC fanatic. With guys like you I can't help but learn.

 

What bothers me is why am I burning up 317's as I understand it is quite resilient?

How were they wired when they burned up?

Were they purchased through a reputable source? If you're buying from Amazon, eBay, AliExpress, or any seller who ships from China, they could be rejects or counterfeit. Since counterfeiting has become so widespread, I've started buying from reputable sources (Mouser, Newark, Jameco, etc). I noticed that Jameco has a store on eBay - that would be a reputable source on eBay; but I'd buy from their website anyway (better prices).

 

Could soldering heat damage the regulator? I run my iron at 720 f.

 

Could soldering heat damage the regulator? I run my iron at 720 f.

Not if you're making the joint within a few seconds. My iron is set to 700F and I've never damaged one from soldering.

 

Hello,

Did you also mount the decoupling capacitors as given in the datasheet?



Bertus

 

Yes I did. My schematic does not show the .1 uf I mounted at the input. The 100uf at the output was for ripple.

 

Hey there

Just out of curiousity did you use some diodes to protect against shorts? Like the ones in this diagram, lack of those have fried a few of mine in the past.

 

Thanks Marcus. I did although I don't show them on my schematic.

 

If I had to guess, which I do, I'd suspect wiring difficulties that caused the LM317 to go into oscillation.

 

Still having problems. What happens if R2 shorts to ground?

 

Still having problems. What happens if R2 shorts to ground?

The output voltage will be about 1.25V.

 

The bypass caps, C1 and C2 need to be as close to the regulator as possible. The regulator will oscillate.

Another problem may be heatsinking. These https://www.mouser.com/ProductDetail/Aavid/4880MG electrically insulate, but thermally bond the package to the metal heatsink.

You get a shoulder washer, a screw, a belleville washer and a nut as well as a thermal pad. Some thermal pads need silicone grease, other's do not.

The minimum load think is known by a lot of us.

 

317 pretty rugged -

3 Description The LM317 device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It requires only two external resistors to set the output voltage. The device features a typical line regulation of 0.01% and typical load regulation of 0.1%. It includes current limiting, thermal overload protection, and safe operating area protection. Overload protection remains functional even if the ADJUST terminal is disconnected. Device Information(

Output cap optional -

7.3.1 NPN Darlington Output Drive NPN Darlington output topology provides naturally low output impedance and an output capacitor is optional. 3-V headroom is recommended (VI – VO) to support maximum current and lowest temperature.

Note this will present as a low freq oscillation, but should still protect -

7.4.4 Operation In Self Protection When an overload occurs the device shuts down Darlington NPN output stage or reduces the output current to prevent device damage. The device will automatically reset from the overload. The output may be reduced or alternate between on and off until the overload is removed

Cap recommendations -



Regards, Dana.