View Full Version : Transistor Confusing Me

11-13-2006, 07:16 AM
Well, that is not too surprising considering my knowledge of this stuff, but maybe some of you can help.

I have this transistor:


The plan is to use it to drive a 12VDC motor.· Before hooking it up to my BS2, I decided to try it out first to make sure I understood how it worked.· To do this, I connected the collector to Vdd, and the emitter to Vin via a LED.· Vin at the moment is 9VDC b/c I am using a BOE and the onboard 9V battery.· I origionally connected the base to Vss (wanting 5vdc to activate the transistor), but that didn't work.· So, I tried connecting the base to Vdd.· That worked.· Now, that part does not worry me too much because I was probably just confused about what polarity was needed to trigger the transistor.

However, I do not want to fry my stamp when I hook the two together.· So, I measured the voltage between Vdd and the base and it is reading ~6.25 VDC.· Not sure why it isn't 9VDC???· Also, the transistor didn't trigger when connecting Vdd to the base through my meter because the LED did not turn on.

So, I guess my questions are:· 1. Is my setup so far OK, or am I at risk of frying something.· If so, 2. How can I safely hook it to an output pin of my stamp?


11-13-2006, 07:31 AM
Putting a 220 ohm resistor between the pin and the base of the transistor should do the job.

- Stephen

Martin Hebel
11-13-2006, 07:36 AM
The link for the transistor you are using doesn't come up, but generally you place the load between the collector and +12V for your motor, the emitter to ground. The 220 ohm mentioned will be good. The motor should have a reverse-biased diode across it so that when the field collapses it will not damage the transistor.

The Process control text from Parallax illustrates how to drive a 12V fan, though due to the nature of the fan, the diode for protection is not present in all drawings.


Martin Hebel
StampPlot - Graphical Data Acquisition and Control (http://www.stampplot.com/)
AppBee -·2.4GHz Wireless Adapters & transceivers·for the BASIC Stamp & Other controllers (http://www.selmaware.com/appbee)·

11-13-2006, 07:54 AM
Thanks for the info everyone.

So, with the 220 ohm resistor, I am not running any risk of damage to the stamp, correct? If I run two or more of the transistors in parallel to keep heat down, do I use 1 resistor for all three, or a resistor for each?

Also, about the motor. How should the diode be connected? I am not really sure what you are reffering to. Also, you may need to know that I will be using the transistor(s) to provide a 0-12VDC powersource to a relay, which then goes to the motor. The relay (actually 2, 1 for + and 1 for -) will be used to reverse polarity to reverse the direction of the motor.

Thanks again for your help so far!

11-13-2006, 08:37 AM
Here is an AVI file you can play in windows media player

Right click on filename

Change name to kickback.avi

Save file

Double click to run movie

Then explore the java applet at this site to learn more about how electrons flow through all kinds of devices

This link pops up a java applet when you link to it with a default circuit running

You can change components, look at all the built in circuits, save circuits to text files and reload them.

Think outside the BOX!

Chris Savage
11-13-2006, 01:50 PM
Here is an example of using that MOSFET to drive a Relay.· I hope this helps.

Chris Savage
Parallax Tech Support