POSSIBLY FAKE TRANSISTORS

[m3vuv]

Hi all,a few months ago i brought some tip 122 transistors from ebay, (uk seller),just stuck them im my chineese component tester and the max hfe is 45,it marks them as a bipolar not darlington,could these be fake or is there a non darlinton version of the tip 122 that i dont know about? TIA

[Kleinstein]

The TIP122 has base to emitter resistors and a relatively small one for the larger transistor.  With low current this reduces the gain quite a bit and the transistor may look like a non darlington.  It may need more current (e.g. 100 mA) to test this transistor.

[m3vuv]

does that mean a 328 based tester cant measure this?,or see if its a darlington or not then,if so i guess that would explain it??,TIA.

[Kleinstein]

The µC based transistor tester likely can't do a proper test and would see it as a simple transistor with reduced gain. So showing a gain of 45 may well be OK. So chances are the transistor is OK.

Chances are the diode test between base and emitter would also show a lower reading than normal for a darlington, as the transistor includes 2 extra resistors. One may measure the resistance base to emitter, that should be some 8 K +-50%

[Kim Christensen]

Ditto to what Kleinstein said. The 328 based testers can only supply low current (a few mA) and low voltages (less than 5V) to the transistor under test. So they don't really work that well when testing higher current/voltage/power devices. For that you would use a curve tracer. But for the hobbyist on a budget, a power supply, DMM, and some resistors makes an adequate DIY test for power transistor beta.
ie: Put a few milliamps into the base and observe how much collector current flows. Do the simple math and you have your beta.

[BrokenYugo]

But for the hobbyist on a budget, a power supply, DMM, and some resistors makes an adequate DIY test for power transistor beta.
ie: Put a few milliamps into the base and observe how much collector current flows. Do the simple math and you have your beta.

The datasheet even gives the test parameters. DC gain should be 1000+ at V_CE=3.0V, I_C 0.5 and 3.0A.

https://www.onsemi.com/pdf/datasheet/tip120-d.pdf

Page 6 fig. 9 shows why the little component tester that I'll guess only puts a few mA through the part gives a low beta reading. They're a handy tool of great value, but you must keep in mind the thing evaluates everything as a small signal part.

[m3vuv]

would that explain the poor hfe readings,45 compaired to 1000 seems way off!

[Kleinstein]

With the extra resistors much of the base current would flow through the resistors and not the actual transitors. With resuclts in the low gain- So it can absolutely explain the low gain reading, when testing with so litte current. With less current the gain would drop even more, even to below 1.

[m3vuv]

so doesent mean they are fake then?,i want to use one as a driver to boost the output current fron an sg3532 vreg ic to drive 8x 2n3055 pass transistors.

[BrokenYugo]

Nobody knows. All we've concluded so far is they're probably some sort of BJT/Darlington and if you want to test them and get numbers you can compare to the datasheet, then you need to test them as the datasheet details. Because the little Atmel microcontroller based component testers can't do that.

Kleinstein suggested checking for the internal resistors, have you done that? It should read ~8k B-E, check both ways as you want the diode junctions reverse biased for this test.

You could also check the gain as Kim Christensen suggested, with a couple amp meters (this is why people say you should own 3 DMMs), power supply that can deliver 3V and over 0.5 amps (better yet 3+ so you can check both ends of the curve), and a potentiometer to set the base current. Again, it's all in the datasheet, assuming you understand the basics of how a BJT/Darlington works.

[rsjsouza]

As others have said, the tester alone will not be able to detect fake/real.

One genuine TIP122:


One genuine small signal darlington BC517:

[John B]

so doesent mean they are fake then?,i want to use one as a driver to boost the output current fron an sg3532 vreg ic to drive 8x 2n3055 pass transistors.

Do I dare ask if the 2N3055s were from ebay too?

TBH any cheap components from ebay should be considered suspect.

[Kleinstein]

2N3055 transistors are not that likely to be fakes, at least not the difficult to detect ones. The 2N3055 is still among the cheapest transistors in a TO3 case.
The parts that is prone to be fakes are those with a relatively high price with cheaper parts in the same case available. So 2N3055 or LM358 are less of a problem than higher grade parts.

The µC based tester is OK for small parts and parts are broken and if the pin_out is as expected (e.g. with TO92 transistors). It is not the right instrument to tell if a power transistor is real or fake. This test can actually be quite tricky if the fake parts are mainly lower SOA, which is a quite common problem with audio transistors.

[rpiloverbd]

I don't really consider that site a dependable source of components.

[rsjsouza]

2N3055 transistors are not that likely to be fakes, at least not the difficult to detect ones. The 2N3055 is still among the cheapest transistors in a TO3 case.

Interesting how times changed... In the 80's, where pallets of Germanium power transistors were quickly becoming obsolete, it was quite common to find fake 2N3054 (with TO-66 package) and 2N3055 ( with TO-3) rebranded from these weaker counterparts.