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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 6th Sep 2019, 12:18 pm   #1
Jolly 7
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Default Defective/ fake LM317T

My attempts to build a 1.2 to 37V voltage regulator have been frustrated by what I can only guess is a fake LM317T. It was part of a kit purchased from eBay. The kit did not come with the insulated washer or mica sheet either. Removing the IC has caused damage to the board and it looks like it could be a write off.
I wonder what other members' experience is in relation to LM317Ts purchased from China ?
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Old 6th Sep 2019, 12:33 pm   #2
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Default Re: Defective/ fake LM317T

You need to accept that any semiconductors from that location could be fake. Often the clue is a satin or flat looking surface and ink that cleans off really easily with mild solvents. The problem is with this part is that it is so incredibly popular, that it has been cloned and faked to death.

Just a quick scan over ebay and I notice a couple that I think are probably genuine old stock :


https://www.ebay.com/itm/Lot-Of-2-LM...kAAOSwjcVdYRyJ


or :

https://www.ebay.com/itm/ST-LM317T-T...MAAOSwds9b0X9g

You could be fairly certain these would work. I have received so many fake parts over the years myself, I now pride myself on finding ones that are not.
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Old 6th Sep 2019, 12:34 pm   #3
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Default Re: Defective/ fake LM317T

I have always had reasonable success with cheap Chinese kits may have just been lucky though.

I am wondering if it needed an insulator if the heatsink is either isolated or to the same pin as the tab you dont need one.

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Old 6th Sep 2019, 2:17 pm   #4
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Default Re: Defective/ fake LM317T

Being fairly cheap to buy anyway, I would recommend trusted supliers. I am slightly amused (or should it be bemused) as much of the genuine article of components are now produced in China too. As we know - A bit of a minefield!
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Old 6th Sep 2019, 2:20 pm   #5
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Default Re: Defective/ fake LM317T

I have bought LM317s from Chinese suppliers (not in a kit) and they've been OK, though I haven't used them in very demanding applications.
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Old 6th Sep 2019, 3:00 pm   #6
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Default Re: Defective/ fake LM317T

I only buy the TI ones. Second source chinese ones, including ST as a rule, are cost cut with smaller pass transistor and less ballasting to keep the costs down. They've even invented TO220 packages with thinner tabs now I noticed. Ugh. And yes I had an ST one blow up on me. I've not actually been able to blow a TI one up. They shut down fine. ST one let the magic smoke out in same situation.
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Old 6th Sep 2019, 10:33 pm   #7
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Default Re: Defective/ fake LM317T

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They've even invented TO220 packages with thinner tabs now I noticed. Ugh.
These were invented to be a solder down option. I have found that if you attach them with a screw, they slightly deform on the area of the screw head, so that the thermal contact is not as good as those with the thicker tab, but conductive paste helps (not a silpad) . In any case I agree with the Ugh remark, so I have bought some genuine regulators from the 1980's era for all my repairs and I never have any trouble with them at all. The best ones in the 78XX series are the Motorola brand with gold plated leads with 1970's date codes, these are never fake and always perfect.
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Old 6th Sep 2019, 10:49 pm   #8
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Default Re: Defective/ fake LM317T

That explains the it then. Thanks for that. Had exactly the same problem and decided to seek a replacement rather than risk it.

AFAIK On Semiconductor are now, by merger, selling LM317's that were made by Motorola. Not gold, not 70s but possibly a derivative of the silicon and process. Probably impossible to find out now.

On the solder down option, I was inspecting a 34401A the other day and noticed that has soldered down LM317/337 pair. On the fence on that idea at the moment.
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Old 7th Sep 2019, 12:08 am   #9
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Default Re: Defective/ fake LM317T

Long story short, got the voltage regulator kit working, well sort of. Replaced the IC with another one from my used parts box. However, I don't understand why I am getting an output of either 1.2V or 15V and nothing in between. I am using my own 10K pot, which has a spindle, rather than the screwdriver adjustable one supplied with the kit. I didn't need the mica insulation after all.
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Old 7th Sep 2019, 12:20 am   #10
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Default Re: Defective/ fake LM317T

Early transistors had gold plated leads and soldered very easily. Unfortunately, gold and tin form a brittle intermetalic which gives long term reliability problems. Recent components do not have gold plated connections for soldering.
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Old 7th Sep 2019, 1:00 am   #11
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Default Re: Defective/ fake LM317T

Let's see your circuit
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Old 7th Sep 2019, 3:35 am   #12
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Default Re: Defective/ fake LM317T

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Originally Posted by TrevorG3VLF View Post
Early transistors had gold plated leads and soldered very easily. Unfortunately, gold and tin form a brittle intermetalic which gives long term reliability problems. Recent components do not have gold plated connections for soldering.
I had heard of that but I have never come across a defective connection to a soldered (with tin-Pb) gold plated component lead yet , or a gold plated pcb pad. But seen plenty of trouble with tin plated leads on components growing whiskers, for that reason I'd prefer the gold any day of the week.

As far as I recall, many (not all) of the gold plated wires of components like IC's , transistors & top of the line resistors etc, were gold directly on steel, and if stored badly rust develops. I see this on a lot of vintage RAM & CPU IC's in my vintage computers.

Gold is just too expensive for most applications. That is why the fakers never use it. So its a handy thing to look out for avoiding fakes. For example if you want some genuine 2N2222's or BC107's get ones with gold plated wires.

Gold was still the mainstay for corrosion protection on the top of the line mil spec parts, or at least it was up until the 90's. And a lot of those parts are still perfect, like the miniature Teledyne relays etc. And Gold remains irreplaceable on many signal level switch contacts.

It a real shame the diode in the attached photo didn't have a gold plated lead.
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Old 7th Sep 2019, 8:37 am   #13
Jolly 7
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Default Re: Defective/ fake LM317T

Quote:
Originally Posted by Julesomega View Post
Let's see your circuit
Here goes. Thanks. The input is from a 12V transformer.
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Old 7th Sep 2019, 9:04 am   #14
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Default Re: Defective/ fake LM317T

Those extra long wires on the 10k adjustment pot, could they cause instability? It looks as though the pot should be mounted on the PCB.
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Old 7th Sep 2019, 9:13 am   #15
Jolly 7
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Default Re: Defective/ fake LM317T

Quote:
Originally Posted by Nuvistor View Post
Those extra long wires on the 10k adjustment pot, could they cause instability? It looks as though the pot should be mounted on the PCB.
Thanks Frank. I'll rectify and let you know if it makes a difference...
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Old 7th Sep 2019, 9:18 am   #16
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Default Re: Defective/ fake LM317T

I would recommend tacking two 0.1uF 50V monolithic ceramic capacitors each directly on the input and output pins of the regulator to ground (common, negative), just on the off chance you had an RF oscillation problem which was fouling up your DC measurements.
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Old 7th Sep 2019, 9:21 am   #17
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Default Re: Defective/ fake LM317T

Wire length shouldn’t make a difference on the LM317 referring to “troubleshooting analog circuits” by Bob please.

What might be a problem is a 12v transformer. That’s 16.8V peak minus approx 1.2v across the diodes so 15.6v. Then there’s the ripple on the capacitor which is going to be large with a load on it and the 3V or so dropout voltage on the LM317 (minimum input vs output differential). It’s probably not regulating at all.

To get 15V out you need a much higher voltage transformer. 16V AC is pushing it. 18V AC better.

Also the LM317 won’t regulate without a minimum load. The resistor chain isn’t putting enough load on it, around 2mA. Stick a resistor across the output to pull this up to 10-20mA
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Old 7th Sep 2019, 9:23 am   #18
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Default Re: Defective/ fake LM317T

It looks like the pot is just being used as a variable resistance, the bottom 2 pcb pads for the pot look joined together, in which case you only need 2 wires.
You could try putting a 0.1uF capacitor across the pot connections on the pcb, this is reccomended in the data sheet for the LM317 and would help prevent instability.

Peter
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Old 7th Sep 2019, 9:40 am   #19
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Default Re: Defective/ fake LM317T

Check that the pot is a linear law one as using a log law pot could make the adjustment a bit touchy at one end of the travel.
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Old 8th Sep 2019, 12:58 am   #20
Jolly 7
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Default Re: Defective/ fake LM317T

I guess I'll need to lay this project to rest for a while. Replacing the 10K linear pot with a 22K linear one and turning it clockwise gave acceptable voltage readings till around midway, but bizarrely the readings started to fall after that to reach 1.2V again. For some reason the LM317T did not like it, the heatsink got very hot and let off some smoke till no voltage was obtainable. ��
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