Saturday, March 8, 2008

Finding Transistor pins

Finding Transistor pins
Nice illustrations for finding pins of the transistor. Use a cheap needle multimeter with basic concept of junction semiconductor.
I got a cheap needle multimeter made from China. It costs me only 250Baht. I have been using it quite long time. The function that checks continuity helped me much for fixing many boards. One day I noticed the knob for zero adjustment could not set the needle in stable position. At first I changed the internal 1.5Vx2 battery, but the problem still exist. I noticed when I press the knob, sometime, the needle was fluctuate. Finally I found Carbon film of the POT may wear out, so after I changed a new POT, 10k, it worked very well. I saw my shelf and looked at small signal transistors. I thought I like to make a page that explains how to use the needle multimeter to find pins of the transistor. This led me to spent my year-end week to prepare this page. I hope it will be useful for young hobbyists to learn using needle meter. By the way I like to tell you one more thing, I am so happy using draw program that comes with openoffice by Sun Microsystems. It works like MACDraw, particularly the freehand tool. The truth is the latter was the main reason why I prepared this page because I like drawing picture. When I was a chid I was very happy drawing picture with 2B pencil.
The needle multimeter when set the knob to resistance measurement, the black cable will provide potential +V respect to red cable. Suppose your meter uses 1.5Vx2, the potential different across both cables will approx. +3V. We can use this EMF from battery to be biasing DC supply for semiconductor junctions.
Let's see with a rectifying diode 1N4001. The schematic symbol shows diode has two pins, Anode and Cathode. For real diode, it has a white color ring to indicate the pin is Cathode. When we test the diode with black cable tied to Anode pin and red cable to Cathode pin, forward biasing, we will see the resistance is changed to lower kOhms. If you swap the cable, reverse biasing, the needle will show higher resistance. We use this concept to check pins of the transistor

Suppose we want to find pins of a 2N2222A, TO92 plastic package transistor. We must find the base pin beforehand. Let have the black cable at the fixed position. The picture shows the black cable is at center pin. Try with red cable for both sides. Suppose we try with right side. If the resistance is lower, then do not move black one. Remember position of the needle.

Change the read cable to the left side. If the needle moved right showing lower resistance the same position for right side. Then we can conclude that this transistor is NPN.
If the transistor is PNP, then we must fix the red cable and move the black one. We will get the same result but opposite polarity.
The center pin is then base pin.
Now we know the base pin, then we will find the rest which one is collector and emitter pin.

To find the collector and emitter we must move the black cable to the left side. Let suppose the left one would be collector pin and the right would be emitter pin. The meter should show higher resistance.
Use finger touch the left pin and the center pin, if the needle moved right showing lower resistance, then the left pin will be collector and the right pin will be emitter pin.
You will notice that when release finger from center pin, the needle will move to the left.
The method can apply to PNP transistor as well, after we found the base, then similar to NPN, however just swap the cable. The red will now be collector and the black will be emitter.
We see that the equivalent structure of NPN transistor made with NPN junction. It likes two diodes with anode pin tied together. So when we have black cable (+V) tied to base pin and the red cable tied to either side, the PN junction then was forwarded biasing, lowering the resistance.
And when the black tied to collector pin and red to emitter pin, high resistance results. When we have some dc current injected to base pin (by finger). The transistor gain will multiply the base current producing higher current flow from collector to emitter.
Explanation above refers to positive charge flowing. The electron on the other hand will flow in opposite direction from emitter to collector pin!
More realistic with picture taken by Kodak CX7430.
Zero adjustment

Right side

Left side

Base biasing with finger
1. nice drawings were drawn using openoffic draw program.
Wichit Sirichote,


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