Author Topic: testing transistor  (Read 546 times)

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testing transistor
« on: February 23, 2020, 05:45:19 am »
Hi all,
         I am trying to test a 2SD2102 transistor that is a darlington pair internally. The readings I get are as follows...

B-C 0.390v
B-E 0.007v

reversed...

B-C OL
B_E 0.10

naturally B-E reads short circuit but should it based on the circuit?

can't seem to upload an image or attachment :palm:
 

Offline Jwillis

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Re: testing transistor
« Reply #1 on: February 23, 2020, 10:49:44 am »
Darlington pair should have a Vbe of at least 1.2V .If you get zero or near zero ohms across any 2 points the transistor is shorted.
 

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Re: testing transistor
« Reply #2 on: February 24, 2020, 08:56:24 pm »
thanks for your response Jwillis
 

Online David Hess

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Re: testing transistor
« Reply #3 on: February 27, 2020, 02:12:34 am »
Darlington pair should have a Vbe of at least 1.2V .If you get zero or near zero ohms across any 2 points the transistor is shorted.

Internally there are likely base-emitter shunt resistors to control leakage and remove charge for faster turn off so at low currents, the base-emitter double junction may look shorted.
 
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Offline Jwillis

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Re: testing transistor
« Reply #4 on: February 27, 2020, 03:37:35 am »
Darlington pair should have a Vbe of at least 1.2V .If you get zero or near zero ohms across any 2 points the transistor is shorted.

Internally there are likely base-emitter shunt resistors to control leakage and remove charge for faster turn off so at low currents, the base-emitter double junction may look shorted.

That's good to know .Thank you.
 

Offline GerryR

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Re: testing transistor
« Reply #5 on: February 27, 2020, 01:08:30 pm »
Here is a data sheet which shows a typical schematic of darlingtons:
Still learning; good judgment comes from experience, which comes from bad judgment!!
 

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Re: testing transistor
« Reply #6 on: February 28, 2020, 01:44:28 am »
Darlington pair should have a Vbe of at least 1.2V .If you get zero or near zero ohms across any 2 points the transistor is shorted.

Internally there are likely base-emitter shunt resistors to control leakage and remove charge for faster turn off so at low currents, the base-emitter double junction may look shorted.

yes there are which is why I was curious if the reading of a short is normal. Thanks for your input.
 

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Re: testing transistor
« Reply #7 on: February 28, 2020, 01:53:54 am »
Here is a data sheet which shows a typical schematic of darlingtons:
thanks Gerry, unfortunately I can no longer upload the correct schematic for the type I have which is a To-3pf casing
 

Offline Jwillis

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Re: testing transistor
« Reply #8 on: February 28, 2020, 03:02:19 am »
I don't have any 2SD2102 or TIP140 but the forward voltage still looks low .You checked with the diode setting on your DMM?
 

Offline GerryR

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Re: testing transistor
« Reply #9 on: February 28, 2020, 12:48:14 pm »
Here is a data sheet which shows a typical schematic of darlingtons:
thanks Gerry, unfortunately I can no longer upload the correct schematic for the type I have which is a To-3pf casing

The case style will not make any difference for the schematic.  The online Hitachi schematic shows a "typical" 2.6K resistor between the bases of the two transistors and none between the base and emitter of the second transistor.  (The more typical arrangement is having two resistors and a clamping diode as shown in the TIP140 data sheet.)  (I measured the forward drop of a TIP120 that I have using the diode checker on my meter and it was 2.1 V; it is specified as 2.5 V typical on the data sheet.)
Still learning; good judgment comes from experience, which comes from bad judgment!!
 

Offline Jwillis

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Re: testing transistor
« Reply #10 on: February 28, 2020, 09:17:14 pm »
Here is a data sheet which shows a typical schematic of darlingtons:
thanks Gerry, unfortunately I can no longer upload the correct schematic for the type I have which is a To-3pf casing

The case style will not make any difference for the schematic.  The online Hitachi schematic shows a "typical" 2.6K resistor between the bases of the two transistors and none between the base and emitter of the second transistor.  (The more typical arrangement is having two resistors and a clamping diode as shown in the TIP140 data sheet.)  (I measured the forward drop of a TIP120 that I have using the diode checker on my meter and it was 2.1 V; it is specified as 2.5 V typical on the data sheet.)

The OP must be a typo for the 2SD2102 https://www.digchip.com/datasheets/download_datasheet.php?id=2959516&part-number=2SD2102he tested  .  Shows .007v but I bet that's supposed to be ohms.
The darlintons I have don't have internal resistors and read 5M - 6 M ohms one way and infinite the other way. This lead me to believe  that the resistance reading The OP got are also to low.
The data sheet shows 2.6K for the single resistor the  2SD2102  does have. He should still be reading a fairly high resistance  between the base and emitter. It should still be several Megs and not the low resistance he is reading.
Grated that the PN juction between the base and emitter won't be exactly like a common diode  but it should have a resistance of several ohms one way and infinite the other without a parallel resistor .His low readings indicate a fault between the base and emitter.
 
The TIP140 has a few datasheets with different R2 values . R1 is 8K in all   and on the TIP140D R2 is 40 Ohm . But if you look at the TIP140T R2 is 0.12 ohms . And the TIP140 is 120 Ohms. The TIP140 , regardless of type , should still read at least 8K across the base and emitter.
« Last Edit: February 28, 2020, 09:59:54 pm by Jwillis »
 


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