Electronics > Beginners
Lm317 vs LM338
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David Hess:
20 years ago visiting the amateur radio swap meets around cities which supported aerospace and electronics industries could turn up small lots, 10s to 100s, of various parts including IC regulators and power transistors.  I still have bags of various TO-3 bipolar, Darlington, and MOSFET power transistors, a tube full of LM337s, and about 2000 1N270 germanium diodes.  Today I would be worried about counterfeit parts but maybe this source is still viable if you look for old date codes.

If you do this though, you are likely to end up adapting your design to make use of parts which are not quite optimal which can lead to some eccentricity and personality in the result like using 4 LM337s in parallel, massively derated power transistors, or my favorite, using 2N4401s or 2N4403s in parallel for greater power dissipation because it is cheaper than a medium power part.

When I order parts from Mouser or Digi-key, if 10, 25, or whatever quantity to get the price break is not economical, then I order double the number I need or at least a couple extra which I would likely do anyway for parts like power transistors which will operate in parallel so I can make matched sets.
janoc:

--- Quote from: Mr. Scram on August 26, 2019, 06:02:01 pm ---
--- Quote from: exe on August 26, 2019, 05:33:12 pm ---That is not what I advised. It's not about spending a specific amount. It's about not going too cheap when it doesn't make sense. I'll remove the amount as other people can also trigger on it.

--- End quote ---
It's not about the amount or "triggering". It's about advising people to spend what could easily be an order of magnitude more on parts where the cheap alternative is not only functional but for a lot of people even the only real option. Your junk food and beer budget may well exceed the disposable income of others. Fakes are an issue but not hugely so. I'm not saying you'll get original parts but most are functionally equivalent enough as not to make a difference for learning purposes.

--- End quote ---

The thing is, even when on a budget and buying from Asia there are sellers and "sellers".

On AliExpress/eBay, passives like resistors and small value capacitors (not electrolytics) are rarely a problem if you can put up with larger tolerances (see the thread about the out of spec resistors). Semiconductors are risky but usually if the component is something complex like a microcontroller, you have a chance to get a genuine part.

On the other hand, power components, simple stuff like transistors, opamps and similar that are easy to substitute for a similar working part (but with often much worse specs) are almost always fake. The same for electrolytic capacitors. I don't buy those from there unless I am feeling like a masochist or I am certain I am not going to push the component in question anywhere near its limits (like the OP does).

One way to avoid this issue is to get semiconductors locally. Mouser, Farnell, DigiKey, RS etc. - the shipping fees sucks but e.g. RS does free shipping if you order on weekends, and all of these have free shipping beyond a certain value of the order, so one has to accumulate BOMs from multiple projects.

An alternative if you really want cheap but still reasonable quality parts are outfits like Tayda (https://www.taydaelectronics.com/, they have also multiple eBay accounts) or then LCSC (https://lcsc.com/). With LCSC it is very unlikely you will get a fake component.

Basically the rule of thumb for me is - if it can blow up/start a fire/cost me a lot of money in repair/lost business, I am spending the money on to get a genuine part from reputable distributor (a 40ish volt input supply would certainly fall under that "can blow up/start a fire" category). If it is just for fooling around, I will buy even a pack of transistors from AliExpress to have some stock, but then I am not going to use them for something like what the OP is building.
Mr. Scram:

--- Quote from: exe on August 26, 2019, 06:10:57 pm ---As I said before, I believe this is a wrong approach doing more harm than good. I advise buy new and original parts, esp. when it comes to jelly-bean components. Why gambling and guessing why it doesn't work? How do you know if it's "functionally equivalent" and not going to break any moment in the future? If one wants to save on delivery, buy, e.g., from lcsc.com.

You can't have original parts, fast delivery and cheap price.

--- End quote ---
It’s hard to call it a gamble if the parts usually work. Although the discussion is irrelevant to many as they simply don’t have the beer budget for original parts. Regardless of assessments it’s the “wrong” approach.
Mr. Scram:

--- Quote from: janoc on August 26, 2019, 06:19:53 pm ---The thing is, even when on a budget and buying from Asia there are sellers and "sellers".

On AliExpress/eBay, passives like resistors and small value capacitors (not electrolytics) are rarely a problem if you can put up with larger tolerances (see the thread about the out of spec resistors). Semiconductors are risky but usually if the component is something complex like a microcontroller, you have a chance to get a genuine part.

On the other hand, power components, simple stuff like transistors, opamps and similar that are easy to substitute for a similar working part (but with often much worse specs) are almost always fake. The same for electrolytic capacitors. I don't buy those from there unless I am feeling like a masochist or I am certain I am not going to push the component in question anywhere near its limits (like the OP does).

One way to avoid this issue is to get semiconductors locally. Mouser, Farnell, DigiKey, RS etc. - the shipping fees sucks but e.g. RS does free shipping if you order on weekends, and all of these have free shipping beyond a certain value of the order, so one has to accumulate BOMs from multiple projects.

An alternative if you really want cheap but still reasonable quality parts are outfits like Tayda (https://www.taydaelectronics.com/, they have also multiple eBay accounts) or then LCSC (https://lcsc.com/). With LCSC it is very unlikely you will get a fake component.

Basically the rule of thumb for me is - if it can blow up/start a fire/cost me a lot of money in repair/lost business, I am spending the money on to get a genuine part from reputable distributor (a 10A 40V input supply would certainly fall under that "can blow up/start a fire" category). If it is just for fooling around, I will buy even a pack of transistors from AliExpress to have some stock, but then I am not going to use them for something like what the OP is building.

--- End quote ---
We're obviously talking about learner and hobbyist territory here. If you buy Aliexpress components when you're doing customer work you're just an idiot. Not because it'll definitely blow up but because the risk reward proposition is completely skewed. Just charge marginally more and cover your ass.
exe:

--- Quote from: Mr. Scram on August 26, 2019, 06:22:38 pm ---the discussion is irrelevant to many as they simply don’t have the beer budget for original parts. Regardless of assessments it’s the “wrong” approach.

--- End quote ---

"The Greedy Pay Twice".
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