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 ? |
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. |
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. Cheers Mike T |
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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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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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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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. |
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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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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Re: Defective/ fake LM317T
Let's see your circuit
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Re: Defective/ fake LM317T
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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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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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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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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 |
Re: Defective/ fake LM317T
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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 |
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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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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