LM317 - LM338, big voltage drop under load

Thread Starter

sumeryamaner

Joined May 29, 2017
114
LM317 and LM338 are easy to use and pretty straightforward devices for voltage regulation. LM338 is said to be able to supply 5A and LM317 more than 1A current.
Just as an experiment I built two regulators, one with 317 and the other with 338. I used 220 ohms / 680 ohms which gave me an output voltage of about 5.8 Volts. So far so good...
As I connected the output to my current load device, I noticed that both devices dropped the output voltage under 3 V at just 1A output!!!
I have tried with two separate LM317's and 4 LM338's with the same result.
I didn't use a breadboard nor a PCB. I mounted the IC on a small heatsink and soldered the two resistors and the capacitors (which are not mandatory as far as I know) directly at the pins. My wires were 22 AWG RC aircraft servo wires (they can handle more than 5 A without any problems).
As power source I used a lab power supply which is capable of providing 10 A. As input voltage I have set 9 V for the regulator (pretty sufficient for 5.8 V output).

Is there an explanation for this unexpected behaviour oh these voltage regulator IC's? (other than they all are defective / counterfeit products).
 

dl324

Joined Mar 30, 2015
10,748
When discussing circuits, schematics are preferred; even for simple circuits.

What type of heatsink are you using?

Where did you buy these parts? If you got them from China or some other disreputable source, they could be counterfeit or rejects.
 

Thread Starter

sumeryamaner

Joined May 29, 2017
114
I would use a 100 ohms resistor instead of 220 ohms, minimum sense current is 10mA, I've had 1.5 A from a LM317.
OK I'll give it a try. Thanks.

Did you read the PDF? In particular:
View attachment 208538
Can you explain a little bit? I am using 9 V input and 5.8 V output. That is about 3.2 V voltage differential. According to this diagramm I should be able to get about 2 A of current.

When discussing circuits, schematics are preferred; even for simple circuits.

What type of heatsink are you using?

Where did you buy these parts? If you got them from China or some other disreputable source, they could be counterfeit or rejects.
The IC's are from an authorised reseller in Turkey.
As the circuit is widely known I didn't post it but it is like this: R1 is 220 ohms and R2 is a fixed value resistor of 680 ohms. There are 10 uF electrolytics at the input and output.Ek Açıklama 2020-05-30 231630.jpg
And finally, the heatsink is this one...
pic001.jpg

The output voltage drops below 3 V as soon as I increase the current to 1 A and The IC does not heat up!
 

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SamR

Joined Mar 19, 2019
2,126
Look at it again. @~2.5V differential Vs to Vo it is ~1A. P=IE and the Watts needed will start pulling Vs down to supply the current needed to provide the power. What are you using for your voltage source to the regulator?
 

Thread Starter

sumeryamaner

Joined May 29, 2017
114
Look at it again. @~2.5V differential Vs to Vo it is ~1A. P=IE and the Watts needed will start pulling Vs down to supply the current needed to provide the power. What are you using for your voltage source to the regulator?
Do you interpret this diagram like this??? OK.
 

SamR

Joined Mar 19, 2019
2,126
Let's back up a bit. WHAT are you trying to do? What output voltage do you need from the regulator? How much current do you need? Let's see the circuit diagram, please!
 

Hamlet

Joined Jun 10, 2015
312
When discussing circuits, schematics are preferred; even for simple circuits.

What type of heatsink are you using?

Where did you buy these parts? If you got them from China or some other disreputable source, they could be counterfeit or rejects.
I have experience of these, the LM317/LM338, and an LM350. I had all sorts of problems with them compared to legacy pulls.
I sourced the LM350 from Tayda Electronics, which is pretty good, but most of their parts come from China.
The others I got from Ebay.

Problems encountered: Poor current, no thermal protection. Mystery Chinese LM317 struggles to put out 1 Amp, with 3v overhead, or 4v. Chinese LM338 was nothing more than LM317 in a to-3 case. The LM350 from Tayda was also poor, and none had thermal protection. As soon as you went over-limit the devices were fried.

I did source an LM350 from ebay, which should be a 3A part, but only had the specs of a genuine LM317: 2.2A output with thermal protection.

If postage wasn't so damn outrageous in the USA, I'd stop fooling with China/Tayda and become loyal to digikey. As it is, I've noticed that the ebay listings for electronic components from China/Hong-Kong/Taiwan/Shenzhen have dropped to 1/3rd of previous offerings. Change is coming.

I am glad that I have ALWAYS kept my China stuff labeled and separated from the stuff that is pulled or documented. Chinese resistors seem okay, but lets hope their missiles are full of LMxxx parts.
 

atferrari

Joined Jan 6, 2004
3,962
I have experience of these, the LM317/LM338, and an LM350. I had all sorts of problems with them compared to legacy pulls.
I sourced the LM350 from Tayda Electronics, which is pretty good, but most of their parts come from China.
The others I got from Ebay.

Problems encountered: Poor current, no thermal protection. Mystery Chinese LM317 struggles to put out 1 Amp, with 3v overhead, or 4v. Chinese LM338 was nothing more than LM317 in a to-3 case. The LM350 from Tayda was also poor, and none had thermal protection. As soon as you went over-limit the devices were fried.

I did source an LM350 from ebay, which should be a 3A part, but only had the specs of a genuine LM317: 2.2A output with thermal protection.

If postage wasn't so damn outrageous in the USA, I'd stop fooling with China/Tayda and become loyal to digikey. As it is, I've noticed that the ebay listings for electronic components from China/Hong-Kong/Taiwan/Shenzhen have dropped to 1/3rd of previous offerings. Change is coming.

I am glad that I have ALWAYS kept my China stuff labeled and separated from the stuff that is pulled or documented. Chinese resistors seem okay, but lets hope their missiles are full of LMxxx parts.
In the last 10 years, their last generation general cargo vessels, multipurpose tweendeckers of 25.000 to 35.000 MT dead weight, I had the chance to work with, were not only built with a high level of quality but also properly operated by their crews. Discipline is outstanding.

Notorious contrast.
 

atferrari

Joined Jan 6, 2004
3,962
As long as someone buys crap, someone will always make crap.
Not actually in the field of electronics (albeit examples do exist), in the shipbuilding industry, you can find examples of shipyards all around the world, doing what they are paid for. No more no less.
 
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SamR

Joined Mar 19, 2019
2,126
Chinese resistors seem okay, but lets hope their missiles are full of LMxxx parts.
They sell their off-spec generics to others. Their rockets use the real thing bought for them from reputable dealers of documented parts by their foreign exchange students.
 
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