Here's why not to buy components on eBay

Thread Starter

upand_at_them

Joined May 15, 2010
519
Regarding the earlier thread about buying from cheaper sources...

I got five DS18B20 temperature sensors from eBay a year or two ago and finally got around to using them today. Each device is supposed to have a unique 64-bit (8 byte) ID. And these do, but of the 8 bytes they all have 5 bytes in common. And, more importantly, they all register a temperature that is several degrees F higher than two known good temperature sensors (everything within a couple inches of each other).

Just say no.
 

dl324

Joined Mar 30, 2015
11,564
At least your parts sort of do what they're supposed to. There was another member who bought some temperature sensors on eBay and they turned out to be transistors.

Counterfeit parts are all too common on eBay and Amazon. There are some reputable sellers on eBay; Jameco has an account, but there are far more questionable sellers. It wasn't always that way. I only consider buying on eBay if it looks like someone selling an odd lot that they picked up at a garage sale or company liquidation. I won't buy any semiconductors from Amazon.

I bought some parts on eBay from someone who appeared to be repackaging. I got some static sensitive parts packaged in florist foam in plastic bags. I put all of the parts in tubes and marked them questionable. I have some crystals from the same seller that I need to test; dropping them is enough to damage them. I'm not worried about the LEDs and axial ceramic caps I bought from them.

If you value your time, buying from reputable distributors makes sense. If you buy parts with questionable quality, you have to do 100% incoming QA and some won't be able to test all parameters.
 

Thread Starter

upand_at_them

Joined May 15, 2010
519
Could you rephrase that for me? Not sure of what you mean.
They all have the same value for byte #1.
They all have the same value for byte #2.
They all have the same value for byte #5.
They all have the same value for byte #6.
They all have the same value for byte #7.
 

Thread Starter

upand_at_them

Joined May 15, 2010
519
Oh, and the Arduino clone that I have, that I got because it has a switchable 3.3V/5V power...that just quit working today. I can get it to work if I jiggle and hold the power switch, but who the &%#! wants to do that?
 

Chris65536

Joined Nov 11, 2019
227
They all have the same value for byte #1.
They all have the same value for byte #2.
They all have the same value for byte #5.
They all have the same value for byte #6.
They all have the same value for byte #7.
Well, the 64 bits are still all unique. If they are just assigning them like serial numbers, then there will be a lot of repetition. I don't think you can assume they would be randomized. The temperature error is less forgivable though.
 

andrewmm

Joined Feb 25, 2011
574
The DS18B20 is a one wire interface chip.
Each 1Wire device has a unique address, which is assigned in sequence by maxim ( Used to be Dallas )
looking at the one wire spec, 48 bits are unique, the other 16 bits are fixed for each type of chip. A sort of device type ID.

the fact you have multiple bytes different indicates these were not all from the same batch, but that's all.

The temperature being 2 degrees F different to another sensor a few cm apart,
Looking at the data sheet , the error over domestic sort of temperature ranges is stated as +- 1/2 degree C, say 1 degree F.
so within any practical usage, they are correct.

Taking the multiple ID's and the marginal accuracy, these parts might be seconds, but they seem to be working just fine,
guess you paid significantly less than the regular USD 4 each
 

atferrari

Joined Jan 6, 2004
4,095
They all have the same value for byte #1.
They all have the same value for byte #2.
They all have the same value for byte #5.
They all have the same value for byte #6.
They all have the same value for byte #7.
The way I know the system, just by differring by one bit, that is enough to distinguish each one from the rest. What is wrong with that?


I bought four DS18S20 on the counter, most probably coming from the same batch, and they had lot of coincident groups of bits.
 

jpanhalt

Joined Jan 18, 2008
10,261
Regarding the earlier thread about buying from cheaper sources...

I got five DS18B20 temperature sensors from eBay a year or two ago and finally got around to using them today. Each device is supposed to have a unique 64-bit (8 byte) ID. And these do, but of the 8 bytes they all have 5 bytes in common. And, more importantly, they all register a temperature that is several degrees F higher than two known good temperature sensors (everything within a couple inches of each other).

Just say no.
There are also two versions, even from legitimate dealers. One will work 1-wire + gnd (parasitic mode); the other will not. If you plan to use it in that mode, be sure you get one that will work.
 

GetDeviceInfo

Joined Jun 7, 2009
1,737
Over the years, when I’ve had problems with chips, I’ll go to the errata to see if the problem is there. Some errata are numerous. When the manufacture updates, where does all that inventory go?
 

Thread Starter

upand_at_them

Joined May 15, 2010
519
Over the years, when I’ve had problems with chips, I’ll go to the errata to see if the problem is there. Some errata are numerous. When the manufacture updates, where does all that inventory go?
A couple years I bought an automotive product on Amazon that appeared to not be up to quality standards. I contacted the manufacturer, since they were the ones listed as selling it. They said "Oh, that's not us...We keep complaining but there's nothing anyone can do." Then I realized, no that doesn't make sense. If someone was falsely claiming to be the company they would get shut down pretty quick. What was happening was that the manufacturer was making money off of the otherwise total loss factory rejects by selling them on Amazon. And if someone complains, then..."No, that's not us, we swear!"
 

Thread Starter

upand_at_them

Joined May 15, 2010
519
Obvious to me: All of the fake sensors had the same value in the Scratchpad Register. So allegedly from different batches (according to the ID), but the same Scratchpad value....Ha!
 

Thread Starter

upand_at_them

Joined May 15, 2010
519
And another thing...Switches!

I got a bunch of 6x6mm tactile switches on eBay. They require significant pressing force and they don't maintain good contact. I'm trying to use them in a project needing short and long presses, and it's difficult to get the switches to stay pressed. They're going in the trash!
 

tindel

Joined Sep 16, 2012
730
I work with grad students at a local university. Every year there's a bunch of new students that purchase their parts from ebay. Every year they get to learn a valuable lesson. Buy authentic parts from reputable dealers. I'm guessing almost every part on ebay is counterfeit. Don't waste your valuable time.
 

MrAl

Joined Jun 17, 2014
7,814
Hello,

I have found that every single item i had purchased off of eBay was not as advertised in some way or another that made the item almost unusable. A CD disk for car repair was the last thing i ever bought from them years ago because it was advertised as being able to be used with my car make/model but it was not.

As far as electronic parts go, the results vary. Sometimes you get what they say sometimes you dont or the part is not functioning right or there is something unexpected about it. For example, 1/4 watt resistors with STEEL leads instead of plated or tinned copper.

One thing that worked out though was a friend purchased a huge quantity of light dependent resistors. They seem to work as expected and i have a large quantity of them now as he shared the lot with me. They were very very cheap also. They are all the same though, having 1M dark resistance and 10k light resistance which will go down to 1k with very bright direct light.
 

bertus

Joined Apr 5, 2008
20,773
Hello,

Often you can recognize the bad chinese stuff on ebay, as the english translations are sometimes very strange and incorrect.
They are only out there for mass production of poor products and profit.

Bertus
 
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