transistor emitter and collector error - Page 1
Forum section: electronics / beginners

author: Fuldup
author_rank: Contributor
timestamp: March 24, 2024, 03:19:08 pm
content: Hi i was testing my new NPN BJT transistor (MPSA 42) and i faced some problems using it, first of all when the collector isn't connected to the power supply and base is connected to the power supply the current goes from the base to the emitter, shouldn't it go only when the collector is connected to the power supply? the second thing i noticed was that when i put the base ON the transistor let the current go to the emitter and to the collector, so are they both an output? I thought that collector takes the voltage from the power supply, the base works like an enable and the emitter is connected to the ground. Because of this issues my AND-gate doesn't work. If you need some pictures of the gate i can post them

author: Jwillis
author_rank: Super Contributor
timestamp: March 24, 2024, 03:49:10 pm
content: An applied voltage to the base will cross to the emitter regardless whether the collector is connected or not. The Base Emitter junction has a forward voltage in the range of 300mV to 700mV. Just like a Diode. When that forward voltage is reached then a small current will pass from Base to Emitter.BJT transistors are current controlled so a small current across the Base Emitter junction will create a large current across the Collector to Emitter based on the Hfe or beta of the transistor. So in the simplest terms if the transistor has a Hfe of 100 then the current from Collector to Emitter will be 100 times the current across the Base Emitter junction.Be careful not to exceed the maximum Base current as this will destroy the Base Emitter junction. This is why current is limited to the Base with a resistor.Always keep in mind that Current is never pumped or pushed through a circuit. It is only drawn or pulled. Voltage is pushed or pumped into a circuit.

author: Fuldup
author_rank: Contributor
timestamp: March 24, 2024, 04:21:55 pm
content: so what should i do in order that i can use my transistor as a switch?

author: Sredni
author_rank: Frequent Contributor
timestamp: March 24, 2024, 04:36:27 pm
content: First of all you should make sure you did not destroy it by applying supply voltage to the base-emitter junction. How did you connect it? What value of voltage, what limiting resistor?Here's a tutorial for beginnershttps://youtu.be/8DMZSxS-xVc?si=zbUGJFdJ3o2YmfGw

author: wasedadoc
author_rank: Super Contributor
timestamp: March 24, 2024, 05:01:16 pm
content: Quote from: Fuldup on March 24, 2024, 03:19:08 pmAn NPN transistor. The base to emitter is a PN junction and will function as a diode. Make the base sufficiently positive with respect to the emitter and current will flow.The base to collector is also a PN junction and will function as a diode. Make the base sufficiently positive with respect to the collector and current will flow. But usually the circuitry around an NPN transistor makes its collector more positive than its base.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 24, 2024, 05:05:12 pm
content: This will make it easy to understand:

author: Terry Bites
author_rank: Super Contributor
timestamp: March 24, 2024, 05:44:39 pm
content: The BE junction can only take so much current before it melts collector connected to the supply or not. A limiting resistor is needed in the base lead and a load or resistor in the collecor lead. Without this the transistor will die suddenly. Magic smoke and such.Here is s step by step guidehttps://www.nutsvolts.com/magazine/article/may2015_Secura

author: TimFox
author_rank: Super Contributor
timestamp: March 24, 2024, 06:28:33 pm
content: In the forward direction, the base-emitter junction looks like a normal PN diode.In the reverse direction, it looks like a Zener diode with a remarkably low breakdown voltage, typically 5 to 7 V, even with a collector-base breakdown voltage rating greater than 50 V.

author: MarkT
author_rank: Frequent Contributor
timestamp: March 24, 2024, 07:02:58 pm
content: The simple story is if you bias the collector positive w.r.t. the base (for NPN), most of the current from the emitter goes to the collector, if you don't, less so, and if you don't connect the collector then all of the emitter current goes to the base. Think of the electrons usually shooting across the base attracted by the positively charged collector, otherwise they collect around the base and find their way out of the base lead.

author: Jwillis
author_rank: Super Contributor
timestamp: March 24, 2024, 07:15:32 pm
content: Quote from: Fuldup on March 24, 2024, 04:21:55 pmCurrent can't flow unless there is a Voltage and there is No Voltage without a Current.Even when measuring with a multimeter, there is always a very small amount of current passing from the device under test and the multimeter.Every circuit requires a certain amount of voltage and be able to draw a certain amount of current to operate properly. Although there may be a voltage across the Base Emitter junction, there my not be enough current supplied from the Base Emitter junction for the circuit you wish to switch on. By altering the current that can pass the Base Emitter junction changes the amount of current from Collector to Emitter based on the Hfe of the transistor.For example If you want to switch a LED On or Off with a transistor at 12V. The LED will light brightly with 12V at 15mA. The transistors Hfe is 100.The transistors base is restricted to only 1 uA at 12V. so 1uA times the Hfe of 100 is only 100uA. Not enough current to light a LED brightly. If you increase the amount of current to cross the Base Emitter junction to 150uA then 15mA will pass across the Collector to Emitter. 150uA times the 100 Hfe of the Transistor gives 15mA that lights the LED brightly at 12V.Another way is by restricting the current base of the transistor to 150uA with a base resistor. With no voltage applied to the base no current will flow. When 12v is applied to the Base 150uA flows across the resistor to the Base Emitter junction. This turns on the transistor allowing current to pass from Collector to Emitter. This turns on the LED brightly at 15mA at 12V.Theirs literally Thousands of videos and web sites explaining how to use a transistor as a switch. Along with the math and a knowledge of Ohms Law you can predict what your transistor will do.Just remember to keep the amount of current to the base of the transistor low.

author: photomankc
author_rank: Contributor
timestamp: March 28, 2024, 07:51:52 pm
content: Quote from: Fuldup on March 24, 2024, 04:21:55 pmFor an NPN transistor:Connect a LED anode to 5V, its cathode to a 470ohm resistor and that resistor to the collector of the transistor.Connect a say 4.7K resistor from the control voltage to the base pin. This can just be a wire you swap from 5V to GND but the resistor MUST be between this wire and the base pin.Connect the emitter of the transistor to ground of the 5V supply.You now have a basic model of the transistor as a switch.Apply 5V to the base and the transistor conducts 5/4700 ~= 1mA from base to emitter. This will allow the transistor to also conduct up to ~100mA from collector to emitter as the beta is generally about 100X. The LED will turn on and it's ~10mA current will easily be accommodated by the transistor. The Transistor is current controlled as mentioned above. The base current determines the amount of current the C->E can permit. As a switch you always want to stay WELL inside that amount. So I like to select the base resistor such that the C->E current capacity is about 3 to 4x what I anticipate as the maximum load. 5-10K is generally a pretty good value to start at.This model is a good representation for most other needs. You MUST have a resistance to limit base current from the voltage controlling the switch and you MUST have a load that has resistance enough to limit current through the C->E to a level that the transistor can handle. No base resistor and the transistor will pop like a firecracker. Same if the C->E load current is a short circuit or way too much for the transistor to handle.