Transistors: What am I doing wrong here? - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: April 01, 2012, 07:51:18 am
content: Today I decided to mess around with some transistors. The transistors I used were 2N2222, in the TO-92 case, which I bought from eBay. I used one multimeter and checked the current on the emitter, base and the collector. I placed a 1k resistor on the base to restrict the current going in and I used two AA cells (3v) to power the circuit. I started of by measuring the base current without the collector collected so the transistor acted like a diode. I measured that the current is the same at base as it is at the emitter and it is basically a short circuit, but with 0.7v drop, in other words a diode. I continued to investigate with the collector attached this time, I attempted to measure the currents, but didn't have enough time as the transistor got too hot for some reason (I've already overheated some transistors previously and they don't work anymore). The current in was around 300mA under the limit of 600mA for the 2N2222. I attempted to switch the emitter and collector, but the transistor got hot again. I decided to add a 51 ohm resistor the the collector in attempt to limit the current passing through the transistor, however I got the same problem.My questions are:- What should be the current in all of my experiments?- Why is it overheating? Did I get a faulty transistor?- Should I place a with the collector?- Anything else that you can say about this.

author: slateraptor
author_rank: Frequent Contributor
timestamp: April 01, 2012, 09:09:04 am
content: Quote from: Mint. on April 01, 2012, 07:51:18 amIf theory is unclear to you, simulate (see attachment); if it's possible, it's always a good starting point prior to building up a circuit for test. LTSpice predicts 241.7mA collector current. Base current can be approximated byBy KCL, emitter current is the sum of the two.Quote from: Mint. on April 01, 2012, 07:51:18 amYour transistor is dissipating too much power. I'm going to assume that you've read the datasheet and correctly identified each terminal. I'm also going to assume that your thrifty gray market purchase is not some Chinese counterfeit. Since I don't know what particular 2N2222 you're using,a quick DK searchshows max power ratings between 300-625mW. We can see that in the circuit that I assume you're testing,Quote from: Mint. on April 01, 2012, 07:51:18 amNote the number of assumptions made. A picture or schematic is worth a thousand words.

author: Rerouter
author_rank: Super Contributor
timestamp: April 01, 2012, 09:14:19 am
content: you have a dmm, so perhaps do the standard diode mode check on it,the base to emmitor should act just like a diode,here is the relevant link,http://www.allaboutcircuits.com/vol_3/chpt_4/3.htmland yes it does sound like something is iffy about it, why buy transistors off ebay?edit: i say iffy, because with th 52 ohm in series it should be well under its thermal limitedit2: are you sure its not something stupid like a 10 ohm instead of 1K resistor on the base?

author: Mint.
author_rank: Frequent Contributor
timestamp: April 01, 2012, 09:35:56 am
content: Quote from: slateraptor on April 01, 2012, 09:09:04 am2.3mA is correct, that was part of my results.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 01, 2012, 12:18:55 pm
content: At a base current (Ib) of 2.3mA,and a fairly standard current gain figure,( Ic/Ib), of about 105,you get an Ie (including Ib) of244mA,giving you, fromslateraptor'sfigures:A power dissipation of 732 mW.The device max ratings are quoted at 325mW to 625mW.Knowing the current gain,what does this suggest about the amplitude of Ib?slateraptorhas led you by the hand,it's up to you to make the logical conclusion!;D

author: amyk
author_rank: Super Contributor
timestamp: April 01, 2012, 12:21:26 pm
content: What are you trying to do with your circuit? Connecting a transistor directly across a voltage source and turning it on is just going to cause it to dissipate all the power as heat.

author: Mint.
author_rank: Frequent Contributor
timestamp: April 01, 2012, 09:31:49 pm
content: Quote from: amyk on April 01, 2012, 12:21:26 pmHow can I repeat my experiment without the transistor overheating? Do I add some LEDs bot create a voltage drop?

author: IanB
author_rank: Super Contributor
timestamp: April 01, 2012, 10:52:27 pm
content: Quote from: Mint. on April 01, 2012, 09:31:49 pmI've been watching the MIT 6.002 video lectures, where in the early lectures he spent a lot of time looking at how to analyze simple transistor circuits. In every example there was a load resistor, RL, in series with the transistor. Prof Agarwal emphasized repeatedly that this was a standard pattern. In no example was a voltage source connected directly across the transistor. Did you not notice this, or has the course not reached that point yet?In short, you must put a load resistor in series with your transistor. If it is an NPN transistor put the load resistor between the positive supply and the collector, similar to the way Prof Agarwal was doing with the MOSFET.I have attached a picture below of the relevant layout.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 02, 2012, 02:34:02 am
content: Quote from: Mint. on April 01, 2012, 09:31:49 pmYour value of Ib is too high,& is turning the transistor on too hard.If you increase the value of the resistor between the +ve supply & the base,your current will fall to a useable level.Others have pointed out that this kind of circuit is not used in any practical circuit.A load resistor is normally used in either the collector circuit or the emitter circuit in practice.The kind of circuit you havecanbe used to test for transistor operation ,with the leads from a DMM in the diode test position substituting for the 3 volt supply,as the DMM normally can only supply a limited amount of current ,so the test is safe for the transistor.Your external source,on the other hand, CAN supply enough current through a transistor turned "hard on" to cook it ,as per previous comments.