LM338 Buck Bad Ends with Bad Power Supply

Thread Starter

IMP002017

Joined Jan 28, 2017
192
Hello. I have a LM338 Buck converter that is TO-3 Package. I have it setup with a Diode Reverse Polarity protection. It is setup with L7812 for Fan and Led Display power. I have the TO-3 package mounted and was covered with a ITX sized Heat Sink and Fan. So Temperature hasn't ever been very high. This is setup with a 4700 uF 50v Cap input. Output is 2200 uF 50v. This is said to be Ideal Power input of 20v. So I had it running off a 19.5v input has worked very well for some time. yesterday I wanted to test one of my 1157 bulb for a replacement of one of my Sisters front blinkers. I don't normally use used Bulbs and when I change out one that is bad I will swap them both for reasons I just like the light to be same or close to same light output on each side. Anyway I don't normally test with this PS and when I connected my Test socket with a Bulb that looks new (Still working) The Lamp lite up for a blink like it just blew. Well looking at my Supply I see that the Display for Volt out is now going down to 000 and ended and stayed there.

So I took another Buck that I have running off a different Chip only 2amp output connected. It dropped downn to 3 volts as a Protection. However I took off and put back on and powered this over 2Amp and it lit the bulb without issue. Knowing that it did put this past it limits I wanted to check what the Current draw was 2.09A but this Cheap .99c Buck from China did ok. Just not something that I use often on large loads seeing it only 2A max. Plus no Heat Sink or fan. Worked just fine. Did heat up for the 30 seconds to minute that I tested the bulbs.

I guess My question isn't the LM338 rated higher then the LM2596? I have always used the LM338 Supply as a way to have constant stable power in my shed, Never questioned the current to anything that I would power and really didn't think 1 little 1157 on the Flasher/Brake element would have been enough draw to kill the LM338? I made the mistake of Installing the Back up 338 turned it on. Did some testing over some of my normal playing around with things and didn't ever take more then 1.58a. So I thought ok, Might just been tired older one, Took the bulb and socket as it was installed before. Connected as it should have been and did the same thing again. Only thing I did different this time was turned the Volts up to 12.8 rather then 12.3 that I had it set when the First LM338 stopped working!

When I tried to clean the heatsink paste off the chip with 99% Alcohol it took off the lettering on the packages of both. One was Marked ST that was the One I was using the first time and the last one was National with the side S looking mark. However like I said both while trying to take off putty from the HS the letters came off both so I think they are Fakes. I was under the impression that they were both over 5amp able chips. So now I need to replace this and get my PS back up on the Shed bench. Until then I guess I will have to use my HP6632 that I like to keep in my Garage on the work desk.

Anything that I can do to keep this from happening again in the future? Or know of a Good source to get Real LM338 that I can get back in this. I am not sure I want to buy from China with thoughts they might not be real. Also I don't like to have to wait for things to get here from China lol

James
 

ronv

Joined Nov 12, 2008
3,770
Hello. I have a LM338 Buck converter that is TO-3 Package. I have it setup with a Diode Reverse Polarity protection. It is setup with L7812 for Fan and Led Display power. I have the TO-3 package mounted and was covered with a ITX sized Heat Sink and Fan. So Temperature hasn't ever been very high. This is setup with a 4700 uF 50v Cap input. Output is 2200 uF 50v. This is said to be Ideal Power input of 20v. So I had it running off a 19.5v input has worked very well for some time. yesterday I wanted to test one of my 1157 bulb for a replacement of one of my Sisters front blinkers. I don't normally use used Bulbs and when I change out one that is bad I will swap them both for reasons I just like the light to be same or close to same light output on each side. Anyway I don't normally test with this PS and when I connected my Test socket with a Bulb that looks new (Still working) The Lamp lite up for a blink like it just blew. Well looking at my Supply I see that the Display for Volt out is now going down to 000 and ended and stayed there.

So I took another Buck that I have running off a different Chip only 2amp output connected. It dropped downn to 3 volts as a Protection. However I took off and put back on and powered this over 2Amp and it lit the bulb without issue. Knowing that it did put this past it limits I wanted to check what the Current draw was 2.09A but this Cheap .99c Buck from China did ok. Just not something that I use often on large loads seeing it only 2A max. Plus no Heat Sink or fan. Worked just fine. Did heat up for the 30 seconds to minute that I tested the bulbs.

I guess My question isn't the LM338 rated higher then the LM2596? I have always used the LM338 Supply as a way to have constant stable power in my shed, Never questioned the current to anything that I would power and really didn't think 1 little 1157 on the Flasher/Brake element would have been enough draw to kill the LM338? I made the mistake of Installing the Back up 338 turned it on. Did some testing over some of my normal playing around with things and didn't ever take more then 1.58a. So I thought ok, Might just been tired older one, Took the bulb and socket as it was installed before. Connected as it should have been and did the same thing again. Only thing I did different this time was turned the Volts up to 12.8 rather then 12.3 that I had it set when the First LM338 stopped working!

When I tried to clean the heatsink paste off the chip with 99% Alcohol it took off the lettering on the packages of both. One was Marked ST that was the One I was using the first time and the last one was National with the side S looking mark. However like I said both while trying to take off putty from the HS the letters came off both so I think they are Fakes. I was under the impression that they were both over 5amp able chips. So now I need to replace this and get my PS back up on the Shed bench. Until then I guess I will have to use my HP6632 that I like to keep in my Garage on the work desk.

Anything that I can do to keep this from happening again in the future? Or know of a Good source to get Real LM338 that I can get back in this. I am not sure I want to buy from China with thoughts they might not be real. Also I don't like to have to wait for things to get here from China lol

James
The cold resistance of the lamp probably tripped the over current protection in the LM338. It is not unusual for the current to be 10 times the nominal current until the bulb "lights." I always thought they were almost indestructible, but a low resistance load may cause them some damage.
 

dl324

Joined Mar 30, 2015
10,002
I have a LM338 Buck converter that is TO-3 Package.
This is a linear regulator, not a buck converter (which is a switching regulator).

If you bought it from China, it's probably counterfeit. A real LM338 would have safe area protection and should be virtually indestructible.

It was a very long winded post. At least you tried to use paragraphs to organize your thoughts...:rolleyes:
 

Thread Starter

IMP002017

Joined Jan 28, 2017
192
Thanks for the Input. Sorry about the long post. I guess I try and make sure there is enough there for people to know what I am talking about or trying to Relay. Guess I don't know how I should have posted things.

Your right, Sorry I missed posted, It is a Linear Power Supply. The backup power I used was a Buck converter.

It was bought from a Local Hobbyist Market a few years ago as a Kit. Soldered it up and been using it from time to time over the years.

Again thanks for the Info hope this was short enough post this time.
 

dl324

Joined Mar 30, 2015
10,002
Thanks for the Input. Sorry about the long post. I guess I try and make sure there is enough there for people to know what I am talking about or trying to Relay. Guess I don't know how I should have posted things.
When discussing circuits, it's best to include a schematic so there's no ambiguity.

If package markings came off with isopropyl alcohol, they're probably counterfeit. It usually takes something stronger to remove markings from genuine parts. Manufacturers want markings to survive the typical cleaning chemicals used.

If you bought the kit or components from a reputable business, complain and get your money back. If you bought from China through something like AliExpress, Amazon, Ebay, you probably got what you paid for. Most of the counterfeit parts come from China and the lengths they'll go through to counterfeit inexpensive parts in mind boggling...
 

Thread Starter

IMP002017

Joined Jan 28, 2017
192
@dl324 Thank you. I sadly don't have a Schematic. Not able to find anything that I could even work with. I did find some replacements LM338K and thought buying from someone in USA would result in a good package. To find that even though they were marked with MIL specs they were fakes as well. When I contacted the seller they tell me to take a picture of the damage product so they could get to the bottom of it. There is no damage to the external package lol..

So looking over the Data for the 1157 bulb. I tested the Resistance cold and that times the Current would have been 5.02amps inrush. That is the V = I * R and within Specs for what NS says they should be able to work with. Bought 2 parts. Already destroyed both. Not my intention. After the first one, I thought I would make sure that I didn't do anything over 1amp on the supply until I could get a Good part. Then my Son while I was out of the house. Needed to test some bulbs and without talking with him about the issues he tested a 3457 that is same as 1157 in specs. Well now that supply is back to being a Shelf Queen.

James
 

dl324

Joined Mar 30, 2015
10,002
I did find some replacements LM338K and thought buying from someone in USA would result in a good package. To find that even though they were marked with MIL specs they were fakes as well.
People selling in the USA can also be victims of counterfeit parts if they don't buy from reputable sources. How are you determining that these are also fakes?

LM338 is pretty straightforward to use. From the National Semi/TI datasheet:
upload_2017-5-13_7-17-53.png
upload_2017-5-13_7-18-26.png
 

Thread Starter

IMP002017

Joined Jan 28, 2017
192
Well it is hard to say really. They look real enough however to my knowledge there is little testing that can be done. One thing. The writing on the can wipes off really easy. As stated also by another poster and in my mind as well I would think in most cases the Real deal would want to have the lettering on the device for years to come even if there was normal cleaning.
Then about the person can buy products from others that don't know or may not know it is fake. I understand that and at first I thought was the case so a few days later while looking for yet another or more LM338K to get my supply back up. I find one for a dollar more then what I paid the US seller, for lot of 10. I just wanted to know what or why so I looked at the offering and come to find out they are the same person. I don't know how one part *Same name* shows GA USA shipping from US and then the ad for the 10 has same name however ships from Hong Kong. As far as having the same store and so on.

Anyway I did look over the 3457 and tested the Cold Resistance and with the Normal Current would have been little over 5amps as in rush. As stated before the little LM 2596 buck converter can power a 1157 or 3457 after a moment of Over current protection. I would guess because it has enough time to heat the lamp some (To lower resistance) so when I take the clip off and then back on the lamp it lights. Testing with Current tester and ran stable at 2.10amps even though it is a 2amp Buck converter. I didn't leave it lit for to long the first time. However I thought I would do some more testing and found that over 2 minutes of being powered up the LM2596 started to warm up. (Not hot just warm)

So while I started to write this out, I had one of my friends that is also into Electronics Hobby. He had one LM338K by TI come by so I could test out my Supply. He said it is hard to believe that I could have fakes of the LM338K. Mind you he don't have a computer or get out much lol. So I took out my Fake 338K, Installed his. Turned on the supply and was again live and well, Able to adjust up and down voltage on the meter. Gave it 19.5V that I have been powering it. First thing he wanted to test was the ability to Kill the chip and said grab a 1157 and it had no issue lighting the bulb. He even went as far as setting up 2. With this test he powered them one at a time and then did them both and it did power them just had a little bit of delay. Did a Test again with my Amp meter in line, lit one at a time and then both for a total draw of 4.14amps on both bulbs. Not sure why testing on each was 2.10 and 2.08. Sad thing is, I now again have a Empty PS because he just didn't want to part with the LM338K. I offered enough to buy 100 fakes he still said no lol. But was nice enough to let me test the rest of my Circuit. Not that I had any doubt about it.

I am almost done trying to draw something out for diagram. The last few times it was just so bad that I got lost lol. There are just some people that it comes natural being able to draw things out looking over a PCB board. I can build things just don't draw out well lol

Thanks for all the help
Sorry my post are long. Don't know how to only write out things in 4 lines of text.
James
 

dl324

Joined Mar 30, 2015
10,002

Thread Starter

IMP002017

Joined Jan 28, 2017
192
Thank you, That will make it easier to find one that is real with hope. Not sure why they don't give you the option to pick what Brand you want. I guess they just have what brand was best rate when they picked them up. I guess the important thing is they end up being real. I will look into the 317 as well. I have been thinking about finding a 138 maybe. However I will just work on buying a real one.

In all honesty I would at this point almost pay 20 for one as long as it is real. I did offer my friend 100 for his only because it was installed and I knew it worked lol. But really that was because I was tired of de-soldering the darn things.

Thank you again for the Help and Legit place to buy some.
James
 
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