BJT transistor question. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 13, 2019, 03:57:45 am
content: Might seem obvious, but I've got an application where a BJT transistor has little room for B E C wiring. Well base and collector wire diameter, the collector affords more room, and i can use a larger conductor wire. My question is, can the base and emitter wires be less diameter conductor, as the collector carries the current to the load ? I'm thinking the base junction conductor could be, but not certain about the emitter junction conductor.Any thoughts help appreciated.Thanks.

author: Nerull
author_rank: Frequent Contributor
timestamp: December 13, 2019, 04:07:25 am
content: Current doesn't magically appear in the transistor. What flows out or in through one pin must flow out or in through another. Where does the current at the collector go?

author: mcovington
author_rank: Regular Contributor
timestamp: December 13, 2019, 05:14:56 am
content: The emitter current is similar to the collector current.

author: Kirill V.
author_rank: Regular Contributor
timestamp: December 13, 2019, 01:16:05 pm
content: The emitter current is the sum of the base current and collector current. Since the base current is often very small, it is neglected in calculations, but it still exists.The transistor does not generate electrical energy - it only controls the flow of energy coming from the power supply. Because through transistor passes the flow of energy (this current) then this the flow of should enter through some pin and then withdraw through the other pin. The control current must also pass through the two pins. Therefore, the current of one of the pins of the transistor is always strictly greater than the current of the other two pins.The same conclusions can be shown with KCL, it does not matter.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: December 13, 2019, 01:19:01 pm
content: When in saturation, the base current can increase drastically, unless limited by a suitable current limiter. If this is accounted for, then only the base wire can be thinner. Collector current and emitter current shall be very nearly equal while the transistor is in the linear region.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2019, 11:22:23 pm
content: Quote from: davelectronic on December 13, 2019, 03:57:45 amYour question is too vague. Please provide more information, such as a schematic.As mentioned above, the emitter current is the sum of the base and collector currents.In most applications the base current is tiny and can be ignored. The wire for the base usually only needs to be thin, compared to the emitter and collector.Of course the above is a generalisation. In some cases, such as when the transistor is being used to switch a very high voltage, the base current can be higher than the collector current, but that's unusual.

author: Kirill V.
author_rank: Regular Contributor
timestamp: December 13, 2019, 11:30:46 pm
content: The base current may exceed the collector current if the base voltage exceeds the collector voltage. Formally, such a mode is called a saturation - all p-n junctions of the transistor are open.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 14, 2019, 02:15:11 am
content: Thanks for all your replys. Didn't mean to come across a vague, I've got some mechanical constraints with regards to rooting conductors. I'm using a heatsink where a TO3 transistor package, the base and emitter leads pass through the heatsink. I have about 2.5mm of clearance for the base and emitter soldered leads to pass through. But i have heat shrink tube to take into consideration when passing these two conductors through the heatsink. Another two holes above the TO3 package, carry the base and emitter leads back through 180° and out on to the pcb for soldering to.A circuit diagram wouldn't be of any use in my question above, if its of interest, its a psu circuit. My question was to find out if i could reduce the base, and emitter conductor there by making it easier to pass the 2.5mm hole i have to pass these wires through. So its a mechanical constraint. But, ok yes it might have implications electrically. My question was answered above, the emitter lead is fairly much equal to the collector lead. The base voltage won't exceed the emitter voltage. The holes in the heatsink are fine through solid materials, its only passing wires through that 180° turn, and between heatsink fins which are the limiting factor. And the weak link, only allowing 2.5mm space to pass leads through.I don't have a milling machine or any other means of removing material around the area of the base and emitter leads. Nothing that could do it in a tidy decent way. I know i could use an alternative transistor package, but I'm using a TO3 package.