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PostPosted: 08 Nov 2017, 21:37 
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Joined: 23 Feb 2017, 02:02
Posts: 50
Have you been fustrated becasue fake transistors blew up many of your expensive power transistors and caused you to waste a month on fixing your amp?
Running away from aliexpress and other cheap supplier of transistors becasue your afrarid of being fake?

No need for that any more.
Build your self a transistor tester. (You can get full refund if you have sufficent evidence of fraud when buying on aliexpress)

You should stress transistors to 70% of maximum rated voltage and 70% of maximum rated power.
For to-92 transistors, a test at about 400mW is sufficent enough to prove that its real. A stress of 5-10minutes should be done.
For to-220 and bigger transistors, reasonabllly large cooling should be used, heat sink should be directly bolted on to transistor with heat sink grease. The transistor should be tested to about 70-80% max power and voltage. If the transistor withstands this power for 5 seconds or more, then its 100% geniune.
Fake trasnsitors tend to fail at high voltages and fails within 5 seconds with less than 65% of max rated power.

For saftety reasons, you should always have the test transistor facing away from you, in case of a fake transistor, non short proof supply and fuse fault, the transistor will blow up.
An approiate fuse suited to test current should be used. A short circuit proof power supply is always recomend.
For voltages of 36v or less the lm317,337 schematic is recomended as it is blow out proof and overpower proof.
For voltages of 36v or higher the discrete transistor veresion should be used.
Always mark clearly which way the pot should go for minimum or maximum current, always power up with current adjusted to minimum.
A single turn pot should be used, because you should have a piece of mind knowing your adjusted to minimum test current before powering up.

In case of fake transistor being tested, the discrete version may have its test transistor driver (to-220) damaged, this to-220 transistor along with its to-92 tranistor should have maximum ce voltage rating of at least 60% above test voltage. (the to-92 is genrally short proof and will not be damaged because of 8.2k current limiting resistor).

The emitter resistor of the test transistor should have a millivolt to volt meter connected. By monitoring the voltage and doing v=ir. you will find out the current through transistor.
Then by doing p=VI you can estimate the required test current for a 70-80% power load. The voltage of transistor can be estimated as vcc since the drop through emitter resistance is small, the votlage the transistor sees is about the supply voltage.

For example, you are using a 0.25ohm resistor and have monitored 1v accross it. I=V/R. 1/0.25=4Amps accross transistor. Your supply is 24v, this means about 96W is being disipated by test transistor (4*24v) (4*(24-1)to be 100% accurate)

Example 2 you are using a 220ohm reistor for a to-92 transistor. you monitored 0.8v accross 220ohms.
(I=V/R) 0.8/220=0.00363636363A
0.00363636363*1000=3.63636mA
You are using a supply voltage of 82v and P=IV.
The transistor is disipating about 3.636mA*82v=298mW
to be 100% accurate, it is disipating (82-0.8)*3.636mA

Feel free on asking me to design a transistor tester at 100v+ or a tester optimised for a certian operaing voltage and test load.
Should you always test transistors, you should have sockets for quick installtion of transistor and a proper case everything neatly installed.
( you can do this your self by adding collector resistances with approprate power rating to limit power to drivers. chaging zener resistor for higher volts so appropriate current flows through zener etc.)

The theory of operation for 317, is that the transistor current is dependant on its emitter voltage and resistor, the emitter voltage is dependant on the base voltage, the lm317 supplys a constant adjustable votlage to the base of transistor. A zener is in series because the minimum output voltage is 1.25V, with a zener in series the transistor can be completly turned off with voltage adjusted to minimum compared to a ~0.6v at emitter when adjusted to minimum voltage.

Theory of the discrete is virtually the same. it relies on a zener refrence, you then take a voltage divider from zener refrence with two pots. this controls the voltage into the base of test transitor.


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