Hi,
So, I was doing a project, where I need to heat up a nichrome wire (which acts as a heating element) upto certain power for example 100mW. I have already done some computation and I need current of approximately 0.05A. I was planning on using a BJT current driver to limit the current at a constant rate. Since my current requirement is lower than 1 A, I thought of using BJT as it will be a better option than using MOSFET. But I am getting a little confused if MOSFET could be a better choice. Can anyone help me?
Also, since I am using it with MCU, should I use NPN BJT or PNP? I researched and found out that when using mcu, since mcu needs to be connected with a ground all the time, I need to use a PNP BJT. Can anyone suggest me regarding this? Thank You.
Typical way to approach this is using opamp+MOSFET like here:
https://circuitdigest.com/electronic-circuits/voltage-controlled-current-source-circuit-using-op-amp
https://wiki.analog.com/university/courses/electronics/text/chapter-4
If you want simpler solution you can use NPN based current source with circa 82 ohm resistor emitter to gnd (base to uC DAC of V=0..5V) however it will have circa 0.7V offset: the output current then
I = (V-0.7)/82
The schematics of it would be like https://secureservercdn.net/166.62.107.55/fhz.588.myftpupload.com/wp-content/uploads/2020/01/NPN-bipolar-junction-transistor-BJT-current-source-sink-circuit-schematic-diagram-by-electronzap-electronzapdotcom.png
Temperature control with relatively stable temperature output generally requires at least proportional control (PI) mechanism. This control requires analog control input rather than two state digital. You need DAC for analog control output which can be driven by microcontroller port. Since it is low power application I would suggest NPN transistor ( May be SL100) to drive the nicrome wire connected at the collector. MOSFET can also be used but the driving is critical as in general it starts conducting above 3V. BJT will start conducting above 0.7V.
npn BJT is simple method and you can control power via changing duty cycle of a output PWM. See below circuit.
PWM can be used as stated by @Hadi Kolivand. My personal suggestion is to used heating element at the collector rather than emitter. The configuration CE would be more better as it provides current as well as voltage gain.
There is no problem.
You can connect emitter to ground and replace heater to collector. Connect the heater between Vcc and collector.
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