Optimizing Microcontroller Output Using MOSFETs: Finding the Right Transistor

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In summary, the conversation discusses the use of a 2n7000 MOSFET to amplify the output current from a microcontroller. The output current needed is 120-150mA, but simulation results show that the 2n7000 can only provide 50mA when the gate voltage is 5V and 120mA when the gate voltage is 5.5V. The possibility of using a transistor as a low-side switch or a driver IC, specifically the Darlington 8 Channel, is also mentioned. The use of a driver IC would eliminate the need for multiple discrete components and a schematic for each stage is available for the IC.
  • #1
bobbbyyy1
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Hi,
I'm working with a microcontroller that outputs 5V, 25mA max from each pin. I need to source 120-150mA. I'm looking at 2n7000 MOSFET for the job. Spice simulations show the output current is 50mA when Vg = 5V (microcontroller output) and about 120mA when Vg = 5.5V. Since I'll be using over 20 pins from the microcontroller, amplifying the output for each pin is impractical. Is there another cheap and reliable MOSFET that can be substituted so this will work?

Thanks
 
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  • #2
Are you using the transistor as a low-side switch? TO-92 packaged 2n7000s should be able to handle that load quite easily. I'd try it on a breadboard to be sure.

Transistor as a switch (because I'm too lazy to mock it up in Circuit Lab):
http://www.electronics-tutorials.ws/transistor/tran_7.html
 
  • #3
Is there some reason you are looking to MOSFET? It does not seem to be very high power - and for 20 outputs - that is a lot of descreet components. Did you consider a Driver IC - like this Darlington 8 Channel in one IC... https://www.sparkfun.com/products/312
 
  • #4
Thanks for the fast replies.

MATLABdude said:
Are you using the transistor as a low-side switch? TO-92 packaged 2n7000s should be able to handle that load quite easily. I'd try it on a breadboard to be sure.

Transistor as a switch (because I'm too lazy to mock it up in Circuit Lab):
http://www.electronics-tutorials.ws/transistor/tran_7.html

The transistor is set up so that the drain is connected to a 5V supply, the source is connected to the load and the gate is connected to the microcontroller. The site you posted has interesting configurations, I'll take a look.
The way I understand how MOSFETs work is the source current depends on how much voltage is put at the gate (in this case the voltage is .5V too low to give the 120mA).

Windadct said:
Is there some reason you are looking to MOSFET? It does not seem to be very high power - and for 20 outputs - that is a lot of descreet components. Did you consider a Driver IC - like this Darlington 8 Channel in one IC... https://www.sparkfun.com/products/312
I didn't search around a lot for driver ICs. That IC you posted looks like it's just what I need, thanks! Do you know if there are PSpice models out there to run simulations with that device?
 
  • #5
The DS has a schematic for each stage.
 

1. How do I determine the right type of transistor for my project?

The first step is to identify the requirements of your project, such as voltage, current, and frequency. This will help narrow down the types of transistors that are suitable for your project. You can also consult online resources or textbooks for a guide on selecting the right transistor.

2. What are the key factors to consider when choosing a transistor?

The main factors to consider are the type of transistor (bipolar or field-effect), voltage and current ratings, gain, and frequency response. You should also consider the cost and availability of the transistor.

3. Can I use any transistor in place of the one specified in my circuit?

In some cases, you can use a similar type of transistor with similar specifications. However, it is not recommended to substitute transistors without proper knowledge and understanding of how it will affect the circuit. It is best to stick to the transistor specified in the circuit or consult an expert for guidance.

4. How can I ensure the reliability of the chosen transistor?

To ensure the reliability of the transistor, it is essential to operate it within its specified ratings and conditions. You should also consider the temperature, voltage spikes, and other external factors that could affect the performance of the transistor. Using proper heat sinking and circuit protection techniques can also improve the reliability of the transistor.

5. Are there any online tools for selecting the right transistor?

Yes, there are various online tools and software available that can assist in selecting the right transistor for your project. These tools consider the specifications and requirements of your circuit and provide a list of suitable transistors. However, it is always recommended to double-check the recommendations with a trusted source or expert.

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