What is this Component? - Page 1
Forum section: electronics / beginners

author: JayMan07
author_rank: Regular Contributor
timestamp: July 16, 2015, 03:25:44 pm
content: I took apart a broken trailer light converter box (4 wire to 3 wire + GND) because I would like to make my own.I drew out the circuit board and attached it...There are two transistor type components one wasn't working but the other was so I was able to see it's operation.The one on the right (the one that worked) received 12.6V from either the "L Turn Signal" or the "Brake" to pin 3 then if the base voltage on pin 1 was equal to 12.6 the Emitter (pin 2) would be off. But for example, if the L Turn signal is on, the voltage to pin 1 drops to around 10.5V (because the 12.6V will go through the 620 Ohm resistor then through the brake bulb to GND).This voltage drop on pin 1 turns the pin 2 Emitter on.I think it's a pretty cool circuit...The component on the right was made by ST and labeled "W2M10S (or 5)" "TIP 127"The component on the left was also made by ST and appears to have been labeled "W2J10S (or 5)" "TIP 127"I can't find anything on either component on Google or ALLDATASHEET. Also I'm not sure why they were labeled differently when my testing of each side of the circuit seems to indicate they function the exact same (i.e. turn on with a voltage drop to the base).Can anybody tell me what this is, and it would be great if anyone can suggest an equivalent part number as these parts seem to be obsolete.

author: Simon
author_rank: Unknown
timestamp: July 16, 2015, 03:32:14 pm
content: https://www.fairchildsemi.com/datasheets/TI/TIP125.pdf

author: JayMan07
author_rank: Regular Contributor
timestamp: July 16, 2015, 04:19:53 pm
content: I did find a few data sheets like that for tip127 but keep in mind that my electronics knowledge is limited...so what confused me is that I didn't think a normal transistor could be setup like it was on the circuit board.So on that data sheet the pins are Base, Collector, EmitterIf that's correct then it means when Volts on the base < Volts on Emitter current starts to flow from the Emitter to the Collector. Is this a normal mode of operation for a transistor. The only mode I'm familiar with is when a voltage (or current) hits the base then it allows flow from the collector to the emitter???

author: Simon
author_rank: Unknown
timestamp: July 16, 2015, 04:33:56 pm
content: It's a PNP not an NPN, so the emitter connects to the positive supply and all voltages are negative respect to emmiter and current flows the other way

author: JayMan07
author_rank: Regular Contributor
timestamp: July 16, 2015, 04:45:57 pm
content: Ok I think I understand that...but I don't understand why the transistor would be off when the base is supplied 12.6V but then turn on when that voltage drops to ~10.5V (I'm sorry I really don't know that much about different types of transistors, I'm just used to the standard NPN).So...with an NPN the voltage to the base needs to go from 0 to 12.6 to turn it onBut with an PNP the voltage would need to drop from 12.6 to turn it on???

author: JayMan07
author_rank: Regular Contributor
timestamp: July 16, 2015, 05:04:32 pm
content: I think I may have figured it out so since it's a PNP then for it to turn on current needs to flow from the Collector to the Base. So when Collector voltage of 12.6 equals base voltage of 12.6 no current will flow, but when there's a drop in the base voltage to 10.6 then you have a 2 volt potential so current would begin to flow from the Collector to the Base and turn it on.

author: Simon
author_rank: Unknown
timestamp: July 16, 2015, 07:35:26 pm
content: Your emmiter is at 12V you base needs -0.7V with respect to emmiter to connect, that is equivalent to 11.3V obviously you need some overhead for the base resistor. you don't look at it with respect with 0V but Vcc and the voltage are all negative because with reference to Vcc.