My first transistor circuit

The main problem with this sort of regulator is that it not only varies the power delivered to the load, it splits that power, to a large extent, between the load and the transistor.

I think you might do a search for [555 tutorial] and see if you can come up with a circuit that uses a 555 timer to pules the transistor fully on and off, with a variable duty cycle, sort of like dimming the lights by rapidly flipping the switch on and off. This method produces a lot less waste heat in the transistor, because it is either off (no heat) or fully on) only a small part of the supply voltage dropped across it while the heater drops almost all the supply).

you really should PWM it (Pulse width Modulate), because the transistor will get hot and you will need a good heat sink... look into it. you could use something simple like a 555 timer to generate the PWM..

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Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

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 | | | 12V- '-' | | | | .-. | | | | | Heater | | | | | | | | '-' | | | | | '-------o------' | (created by AACircuit v1.28.6 beta 04/19/05

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

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.

Radio Shack Catalog #: 271-1714, 5K linear pot, $2.89

Good luck Chris

Thanks for the help, seems to make sense.

One other quick question. On some circuits I see the "load" on the collector side and on others I see it on the emmiter side.

What is the difference, or more importantly is their a rule for when the load goes on one side or the other.

When the load is on the emitter side, its voltage drop is added to the input voltage requirement, because the emitter terminal is part of both the input and output sides of the transistor. When the load is in the collector leg, the input voltage is essentially fixed, regardless of the load. Sometimes you want the negative feedback effect of having the load in the emitter leg, to linearize and expand the input voltage range. Sometimes, you just want to slam the output on and off with the minimum input voltage (or are doing the negative feedback through some other route).

If you go for a pulse width modulated power control, I think you will find that having the load in the collector leg makes things work better (transistor cooler and circuit simpler). But either can be made to work.

Same idea but more efficient is using the SCR

Thanks, makes a bit of sense.

I'm also looking at PWM now that its been mentioned. I think I'll make the simple circuit first to try and cement some basic understandings, then work on PWM. I found a fairly simple PWM 555 timer circuit with a explanation of how it works at

Thanks again for all the help.

(snip)

Please, elaborate.

(snip)

• SCR

Great. Jamie and Mr. Popelish suggested PWM, and that's best, primarily because it's more efficient. The "newbie voltage regulator" with the darlington and the pot wastes a lot of power. Since this is a battery operated setup, that might be important.

Assuming 2 amps of load current, the PWM circuit will only waste a maximum of about 2 to 2.4 watts, as opposed to up to 7 watts with the simpler series voltage regulator. Throwing away battery power as waste heat is never a good thing.

If you wanted to be even more efficient, you might consider replacing the Darlington with a power MOSFET. Even in hard saturation, the darlington will have 1V to 1.2V from collector to emitter. Since power is V * I, that would mean 2W to 2.4W for a 2A load. Modern power MOSFETs have an Rds(on) of around 0.1 ohm or less, which means you'd only dissipate 0.2W max. with your 2A if you replaced the darlington. You wouldn't even need a heat sink.

Glad to help. Take the time to get a 555 from Radio Shack and try the PWM idea, too.

Cheers Chris

Yes, that's the general idea. Many variations are possible.

Thanks again for the help.

One more simple question. What would be a could test ground for this if I wanted to cobble it up on my little breadboard. I'm just guessing but I don't think its built for 2A draw.

Also, and I'm pretty sure I know this.

On that example PWM circuit I linked to replacing the 100K VR with a 50K VR and replacing the 1K R with around 51K of resistance would make the circuit start at about 50% instead of 5% correct?

I what I'm working on, adjustable is great but under a certain level it doesn't make sense.

How would you commutate it?

Huh???

You want to explain how you're goiing to turn the SCR off??

Ed

saving energy keeps the transistor cool which has the side effect of keeping the magic smoke inside it without needing a bulky heatsink.

Bye. Jasen

For a PWM circuit, you can use a 555 timer and power transistor. The link below shows a typical circuit with a pot to adjust the duty cycle from 5% to 95%. The whole circuit uses about 8 parts and controls a couple amps. Scroll down the page to the "12 Volt Lamp Dimmer" drawing.

-Bill

The little clippies on a breadboard are generally said to be good for half an amp. You can go with two if it's fairly new and still making good contact.

Be sure to solder 22AWG wires to your TO-220 transistor, so you don't have to abuse the clippies to get the leads into the board.

Good luck Chris