Re: My first transistor circuit


Marc Britten wrote:
I'm pretty new to electronics, I've pieced together a few circuits from
schematics and I even made a simple LED light display by hand with a do it
yourself pcb kit and a circuit of my own design (nothing terribly original).

I'm currently trying to play with transistors and I came up with a project.

I have a small heating element for coffee cups thats designed to run from
the standard power plug, so 12 - 13.8V.

It runs too hot for my tastes and would like to control it. Unscrewing it
shows that its a simple resistive heater and probably pulls a few amps.

Looking at my local radio shack I found the TIP120 Transistor which
appears to handle the load with plenty of room to spare.

Few simple problems from my beginner stand point. The base pin is powered
by 5v Max. according to the data *** I found online. So I need to
seperate the volage. I'm thinking a simple voltage divider. Would 2.4
volts be enough? I have some very high watt resistors that I bought
online back before I knew even the basics and did the math to figure that
one 8Ohm and two 1Ohm (in series) would give me 2.4 to 2.75 volts to power
the base.

Placing a simple trim pot in line should give me some control over the
current flowing through the transistor then correct? (I would have to do
the math and get the resistance correct for the H factor to give me the
window I'm looking for)

http://home.comcast.net/~marc.britten/schematic.png

Figuring that R4 is my load.

I'm asking because my first attempt made smoke and nothing else, but I
forgot to limit the voltage down to under 5V for the base.

Thanks for any help.

Marc

Hi, Marc. You've got most of what you need to get some control over
your heater.

To finish up, get a 1K potentiometer (the 5K linear taper one at Radio
Shack will work too) and hook up this control circuit with your TIP120
(view in fixed font or M$ Notepad):

|
| .-------o------.
| | | |
| | 1K or.-. |/
| +| 5K | |<--| TIP120
| --- | | |>
| 12V- '-' |
| | | .-.
| | | | | Heater
| | | | |
| | | '-'
| | | |
| '-------o------'
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


Here's the drill. The TIP120 is a darlington NPN transistor, which
means it's actually two transistors hooked up like this:

| C
| o
| |
| .---o
| | |
| B |/ |
| o---| |
| |> |
| | |/
| '-|
| |>
| |
| o
| E
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The daisy-chaining of the transistors results in a very high combined
gain (hFE(typ) = 2500 @ 4A). This means that, even at 2 amps of load
current, you should only need around a couple of mA of base current.
So you set up the 1K pot as a voltage divider, and the emitter of the
TIP120 will be about 1.4V less than the base.

That means that, for a 13.8V supply, you have an effective range of the
heater voltage with this two component voltage control from 0V to about
12.4V. That may be good enough for your needs, and is definitely
simple enough for a newbie..

Note that, if you have a heater that uses 2A at 13.8V, your maximum
transistor power dissipation will be almost 7 watts, so be sure to put
on a good heat sink with heat sink compound.

http://www.onsemi.com/pub/Collateral/TIP120-D.PDF
Radio Shack Catalog #: 271-1714, 5K linear pot, $2.89

Good luck
Chris

.

Quantcast