I bought a batch of 100 x TL431 voltage reference chips on eBay recently. It was a good price (around $6 for the lot). The are supposed to be able to generate 2.5 V reference (and with suitable resistors use that to compare an incoming voltage to it).
But when I went to test one, I got weird results (like, wiring it the way that was suggested in the datasheet, it generated 1.34 volts). I thought maybe I misread the info, and tried various adjustments (like different resistor values for the current-limit). But always about 1.34 volts.
Then I tried a second one from the same batch. This time I got about 4.8V. Odd. If the circuit was wrong wouldn't it be wrong in a consistent way?
Then a third chip. Finally, 2.501 V. At last. In the same circuit, just unplugged and replaced on a breadboard.
So I am wondering, does this happen often? Is "cheap" code for "too good to be true?".
Would possibly the eBay seller have got a batch of rejected chips and be trying to pass them off as "brand new, in working order"?