It is sometimes interesting to read how people buy expensive Opamps at eBay to very attractive prices. One IC that is sold often on eBay is the AD797. It is used a lot from people who want to “improve” their audio equipment. I will not discuss this here, why these “improvements” often make things worse, but look at the prices you pay on eBay. The retail price of a single AD797 chip at European retailers is >10$, but you find Chinese retailers on eBay that sell two of them for less than 6$. Looking at the price list from Analog, you will notice that this is even less than the price per IC you have to pay for 1000 units from Analog. How can this be? The answer is easy: fakes. Most of the buyers use these opamps in audio circuits. They never notice that it is a fake. Why? Because even the cheapest Opamps work great in audio circuits. Depending on the circuit, they might even perform better than expensive chips. The chip might not be an AD797, but it works well. So everybody is happy.
Fakes are one reason, why I would never buy parts like this on eBay. I’ve already seen fakes even for relatively inexpensive chips.
But today I found an interesting board:
This is a switching voltage regulator based on the LM2596-ADJ. 5 boards are 4.81€ including shipping from China. Are you kidding me? The chip itself costs about 2-3 US$ when you buy it in large quantities. In small quantities it is about 5 US$. There is no chance to build 5 boards like this and ship it to Europe for under 5€. How can it be?
- The chip itself is a fake or it is an old non-ROHS version that cannot be used anymore. But even with this, it will be really hard to build it cheap enough. The inductance on the board alone will cost at least 0.30-0.50 USD.
- The board itself was produced for something else and never used. Looking at the board this seems also relatively unlikely as this is a standalone PCB and input and output are designed to solder cables.
- This is an old evaluation module for this IC.
In 3-4 weeks these boards should arrive here and I will have a closer look. I will also order a sample from TI to find out if these chip on these boards look and behave different from genuine TI parts.
One final remark: Independent if these boards work well, I cannot recommend eBay to source electronic components. Ask yourself: Why should components from a 3rd party source be cheaper than from the supplier? The risk to buy fakes is very high. Sometimes suppliers have different qualities of the same chip design. Some guys buy the cheapest version and relabel it as the most expensive version.
PS: I will not post the eBay link.
I agree with Daniel: be skeptical to see prices way below the manufacturers retail price suggestion or serious distributors prices. The risk is also: you can get parts selected by “speed binning”, actually intended for the waste basket, chips failed on manufacturers ATE procedure (out of spec).
BTW: If you want to design a switching regular with less parts, I suggest to look for a chip with integrated inductor. I use for instance the LMZ14202H and I like it meanwhile (e.g. for my RPi-DAC-SPS). It can provide up to 2A, also as adjustable (any voltage out, even quite high input voltage).
It is often hard for DIY and individuals to get reasonable prices, to compete with “mass production” and “promotions”. One example:
If you use the STMF4Discovery board (a Cortex-M4 eval board with USB and audio out) as a USB sound card, you get it for USD 15.00. But if you try to get all the parts soldered on PCB as separate chips, just the micro-controller itself will consume 80% of this price.
You may find this thread, and the video attached to the OP, of interest. In the video the temperature raises rapidly to ~65 degrees once the load is increased, at which point a diode fails (the IC is intact). http://www.eevblog.com/forum/reviews/lm2596-dc-dc-step-down-modules-load-testing/?PHPSESSID=06536ef9185de6467e16686a1a58d99c
Speed binning, trashcan binning, fake… here’s another one: STOLEN.
Now by “stolen”, I don’t necessarily mean that a bunch of thugs forced a big rig off the road and unloaded its shipment, there is another way that parts are commonly stolen that makes up for a HUGE number of parts on the market (and this doesn’t just apply to electronic components!)
You see, you have these big companies OUTSOURCING their manufacturing to various places, like China. They order production of, for example, 10 million units per year. The factory runs at 15 million units per year, but only delivers 10 million units per year to the vendor (who pays for all 15 million units). The other 5 million units per year are dumped on the black market.