testing transistor - Page 1
Forum section: electronics / beginners

author: login
author_rank: Contributor
timestamp: February 23, 2020, 05:45:19 am
content: 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 0.390vB-E 0.007vreversed...B-C OLB_E 0.10naturally B-E reads short circuit but should it based on the circuit?can't seem to upload an image or attachment

author: Jwillis
author_rank: Super Contributor
timestamp: February 23, 2020, 10:49:44 am
content: Darlington pair should have a Vbeof at least 1.2V .If you get zero or near zero ohms across any 2 points the transistor is shorted.

author: login
author_rank: Contributor
timestamp: February 24, 2020, 08:56:24 pm
content: thanks for your response Jwillis

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2020, 02:12:34 am
content: Quote from: Jwillis on February 23, 2020, 10:49:44 amInternally 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.

author: Jwillis
author_rank: Super Contributor
timestamp: February 27, 2020, 03:37:35 am
content: Quote from: David Hess on February 27, 2020, 02:12:34 amThat's good to know .Thank you.

author: GerryR
author_rank: Frequent Contributor
timestamp: February 27, 2020, 01:08:30 pm
content: Here is a data sheet which shows a typical schematic of darlingtons:

author: login
author_rank: Contributor
timestamp: February 28, 2020, 01:44:28 am
content: Quote from: David Hess on February 27, 2020, 02:12:34 amyes there are which is why I was curious if the reading of a short is normal. Thanks for your input.

author: login
author_rank: Contributor
timestamp: February 28, 2020, 01:53:54 am
content: Quote from: GerryR on February 27, 2020, 01:08:30 pmthanks Gerry, unfortunately I can no longer upload the correct schematic for the type I have which is a To-3pf casing

author: Jwillis
author_rank: Super Contributor
timestamp: February 28, 2020, 03:02:19 am
content: I don't have any 2SD2102 or TIP140 but the forward voltage still looks low .You checked with the diode setting on your DMM?

author: GerryR
author_rank: Frequent Contributor
timestamp: February 28, 2020, 12:48:14 pm
content: Quote from: login on February 28, 2020, 01:53:54 amThe 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.)

author: Jwillis
author_rank: Super Contributor
timestamp: February 28, 2020, 09:17:14 pm
content: Quote from: GerryR on February 28, 2020, 12:48:14 pmThe OP must be a typo for the 2SD2102https://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.