EMRFD Message Archive 7593
Message Date From Subject 7593 2012-07-14 08:32:16 Reginald Beardsle... Identifying BJT leads w/ an ohmmeter I hope no one will take offense. But I've never seen in it print anywhere. It's my "trick question" when I meet someone really sharp. In 30+ years I've never met anyone who knew how to do this.
I'm not talking about identifying the base lead. That's widely explained. I'm talking about identifying the emitter and collector.
When I was a grad student in geology, I had a 5 MHz Heathkit recurrent sweep scope, a Micronta DMM, a copy of "Solid State Design for the Radio Amateur" and a package of unmarked 2N2222s from Radio Shack w/ lead identification on the back.
Using these I built several circuits from SSD, but none worked. At the time my day consisted of looking through a petrographic microscope and counting the occurrences of various minerals in thin slices of rock. I did this 8 hours a day, 5 days a week for 9 months.
For a break I went over to the next building 3 times a week and attended the lecture for an introductory electronics course that used Millman for a text. The course was taught by a solid state physicist the EE department had hired in large part because he'd built a computer from 7400 series logic before the 8080 appeared. As he put it, "When I finished it was obsolete."
A few weeks into the course when we got to the BJT, the instructor made a minor remark that nearly blew me out of the room.
"The forward resistance of the base-collector junction is lower than the forward resistance of the base-emitter junction despite the higher doping level of the emitter. This is caused by the larger area of the collector."
When I got home I checked the unmarked Radio Shack transistors. E & C were opposite the package label. When I swapped the leads my circuits from SSD worked.
This *needs* to be in the ARRL Handbook. The "resistance" is of no consequence. The meter simply needs to be on a range that will forward bias the junction.
7594 2012-07-14 10:04:01 Leon Heller Re: Identifying BJT leads w/ an ohmmeter 8053 2013-01-06 15:57:56 ep_mand Re: Identifying BJT leads w/ an ohmmeter Almost the same thing, but greater difference in values: Connect 100k resistor to base. Measure emitter/collector resistance by touching the other end of 100k resistor to either two connections - connection to emitter gives higher resistance and is therefore solved. This is true as almost all BJTs have larger current gain in normal connection than reversed. By the way, reverse connected BJT has lower collector to emitter saturati