# Looking for a circuit design

Discussion in 'General Electronics Discussion' started by PTGA, Mar 25, 2013.

1. ### PTGA

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Mar 25, 2013
Hey all,

This is my first post here. I am planning to control a transistor as a heater using a microcontroller. The input to the transistor will be a 0V-3.3V PWM and I was wondering if anyone had a transistor circuit already designed. I will be using a Darlington PNP transistor, MJD117, as the heater and I know a bank of resistors will be required to divide the current that will need to be dropped through this transistor. Any design and calculations would be appreciated.

PTGA

2. ### Harald KappModeratorModerator

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Nov 17, 2011
When you control a transistor using PWM, it is most of the time either ON of OFF (except during the switching transients). In either state it will consume very little power - that's the principle behind using PWM.
Therefore the energy will be mostly burned in the resistors. By varying the duty cycle of the PWM you can control the average amount of energy converted to heat in the resistors.
For dimensioning the circuit, assume the resistor is always on and has zero voltage drop. Then all the power is converted in the resistors: P=V^2/R
You will now be able to control the power down from that value using the pwm signal.

Last edited: Mar 25, 2013
3. ### PTGA

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Mar 25, 2013
Well the transistor listed that I want to use is a Darlington power transistor which I would like to pass 0A-just over an amp through to give me the temperature I will need. The PWM will simulate a 0-3.3VDC to the base.

4. ### BobK

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Jan 5, 2010
Are you saying you want to the transistor to heat up?

Bob

5. ### Harald KappModeratorModerator

11,513
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Nov 17, 2011
I beg your pardon, but that is nonsense.
A transistor controlled by PWM is either on or off (neglecting switching transients).
In the off-state no current flows, I=0 -> P=V*I=0
In the on-state the transistor is saturated, meaning Vce~0V -> P=I*V=0 (approx.)
Big but: If you don't have current limiting resistors, the collector current will rise to the limit defined by the power source, since the resistance between collector and emitter is very small. This will lead to overcurrent killing the transistor.

So you need resistors.
And as explained above, little power will be converted to heat in the transistor, almost all power will be converted by the resistors. That is why PWM is used for controling etc. motors or the brightness of LEDs. Keeping the transistor cool and the resistors hot will add to the lifetime of the transistor. Some like it hot, semiconductors don't.

If you insist on using the transistor as heater, you need to operate it in the linear region. No saturation, no pwm.

6. ### BobK

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Jan 5, 2010
PWM can be used to produce an analog voltage out of a micro controller, giving you a DAC capability. You do this by running it through a low-pass filter. You could use this analog voltage to place the transistor in the linear region. However, it would be subject to thermal runaway, so you would need an emitter resistor to keep that under control, and both would heat up.

But why are you doing this anyway? Using a transistor to control PWM to a heating element is the correct way to do this.

Bob

7. ### PTGA

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Mar 25, 2013
A company is using an analog controller and the transistor listed as the oven in an oven controlled crystal oscillator. They implemented a clamper transistor to keep a voltage of 0.7V across a resistor bank and the controller indirectly controls Ic of the transistor resulting in heat dispersion. I wanted to implement a digital controller to this system and was just wondering if anyone had a transistor heater circuit already designed I could use and adjust for my needs since they were using voltages of 0-12V and mine will only be 0-3.3V.

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Jan 21, 2010