General > General Technical Chat
i won't buy chinise parts from aliexpress or ebay
<< < (9/22) > >>
Psi:

--- Quote from: TheDane on February 14, 2020, 01:13:22 pm ---Buying from a seller which offers tons of other (non-related/electronics) stuff is also a potentially bad sign.
If they offer a wide variety of related goods, imho they also tend to be more serious and has 'better' parts in general.

--- End quote ---

Agreed

If the aliexpress seller has listings for 16bit DAC chips, door bells, handbags and bed sheets it's a good sign they don't really know what a 16bit DAC chip actually is.  This means there is a extra high chance of mistakes in the technical listings/descriptions.
They may have added keywords that are incorrect and make you think you're getting something you're not.
And if they're that unfamiliar with the product they're more likely to buy fakes themselves without knowing.
This is why it's usually better to order from a seller that is only selling electronic items.
At least they're more likely to know something about what they are selling.

It's always good to look for inconsistencies in a listing, If the photo implies one thing but the description says something else do not buy from that seller. Find someone selling the item that seems to know what they are talking about.

When i'm looking for a seller to buy a particular item from i'm always looking for any reason to skip to the next, more expensive, seller.
It usually goes something like...

 -Way to cheap. Next.
- WTF. Why are they saying this mosfet has a Rds on of 10V. Next.
- No specs listed, just a photo.. Hell no.
- 91% feedback rating, Nope. Next.....
- This looks ok. (Clicks buy)
Ivan7enych:

--- Quote from: janoc on February 14, 2020, 09:45:46 pm ---
--- Quote from: Ivan7enych on February 14, 2020, 03:01:53 pm ---I've had opposite experience recently.

5v regulator from ST-microelectronics 78L05 officially bought in russian supplyer has failed my project - by documentation it is rated up to 30v input, but on my board it fails at 28v input, burned itself and several other ICs.

On the opposite, 78L05 from Ali with unknown marking "WS 78L05" works fine up to 35v input.

--- End quote ---

That is most likely due to the poor design of your circuit and e.g. heat caused parameter drift/manufacturing tolerances than any poor quality parts. If you are using a part rated for 30V in a circuit where it can get 28V and then you are wondering that it has eventually failed (and blew up other things), you have only yourself to blame, IMO. Never heard about component derating? Operating any component right up to its absolute maximum ratings is a recipe for disaster.

And why, for heaven's sake, are you running an 78L05 from 30V in the first place?! Even at the 5mA quiescent current you are going to dissipate around 125mW in that plastic package. Add any load and it gets much worse pretty quickly. That thing is going to be pretty toasty! Fortunately it has internal thermal protection, but it still makes one wonder what are you trying to achieve there with such setup.

--- End quote ---

My circuit was designed for 24v input, current is 1mA (3 74hc chips), so my choice of 78L05 looked just fine. Derating was done, no overheating problems. But when input voltage was accidentialy rised above 28v (below 30v, still whithin specs of this regulator), this ST78L05 at first outputs 7V, then blows itself and killed all 74HC chips...
worsthorse:
If you can buy a part for eighty cents that currently is sold by digikey for twelve dollars, there's only a few reasons for that price differential: it is a degraded copy, remarked fake, reject, or stolen.  There is just no other way to account for such price differences, at least for distributor stocked parts.  The same goes, generally, for NOS parts, especially NOS parts that weren't heavily stocked in the day outside of the United States or Europe.  Just ask yourself, why would there be a big stock of forty year old NVRAMs sitting in some warehouse in China or Vietnam?  Can you find NOS parts? Yes, and I often do, but invariably they are from individual sellers getting rid of personal stock or well established vendors that sell little else. 

I guess it depends on how you want to spend your time and money. Personally, I hate trying to track down circuit problems caused by marginal components, even when building throwaway circuits. And I don't want to have test every part that I buy to figure out if it is what it says it is on the label before I can use it.

So I've never found the money saved buying really cheap parts to be cost or time effective, whatever vetting process one uses to find a "reliable" vendor.
janoc:

--- Quote from: Ivan7enych on February 15, 2020, 04:46:45 pm ---My circuit was designed for 24v input, current is 1mA (3 74hc chips), so my choice of 78L05 looked just fine. Derating was done, no overheating problems. But when input voltage was accidentialy rised above 28v (below 30v, still whithin specs of this regulator), this ST78L05 at first outputs 7V, then blows itself and killed all 74HC chips...

--- End quote ---

That looks like you got unlucky with a dud part. However, with such high input voltage I would rather use a switching regulator instead of having such a large drop over a tiny linear regulator. If you add the quiescent current of the regulator to your load current, that's quite a bit of power being dissipated.

At least I would pick something with a higher input voltage rating, e.g. LM2936M (40V) or TPS79850QDGNRQ1 (50V). 30V rated part for a 24V input is a bit close for comfort, IMO. 
wraper:

--- Quote from: janoc on February 15, 2020, 11:24:09 pm ---That looks like you got unlucky with a dud part. However, with such high input voltage I would rather use a switching regulator instead of having such a large drop over a tiny linear regulator.

--- End quote ---
There is little point going with switcher at 1mA load. Quite likely you will have higher quiescent current than 78L05.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod