Using delicate 5V IC to toggle 12V device with MOSFET, Diode, resistor

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Discussion Overview

The discussion revolves around using a 5V integrated circuit (IC) to control a 12V device through a MOSFET and NPN transistor configuration. Participants explore the safety and functionality of this setup, particularly focusing on the interaction between the Arduino Uno and the transistor switch.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the safety of using a 5V IC to control a 12V lock via an NPN transistor, expressing uncertainty about the grounding configuration.
  • Another participant seeks clarification on the emitter position of the NPN transistor, assuming it connects to ground, and questions the IC driving the transistor's base.
  • It is confirmed that the driving IC is an Arduino Uno, and the emitter position is indeed correct.
  • Participants discuss the output voltage levels of the Arduino, specifically the maximum current sourcing capability and the low output voltage (Vol) specifications.
  • Concerns are raised about the reliability of the Arduino's low output voltage, suggesting the potential need for a base pulldown resistor to ensure proper transistor operation.
  • A recommendation is made to add a diode in series with the resistor to protect the circuit, depending on the Vol specification of the Arduino.

Areas of Agreement / Disagreement

Participants express varying levels of certainty regarding the Arduino's output characteristics and their implications for the circuit design. There is no clear consensus on the necessity of additional components like the pulldown resistor or diode, indicating ongoing debate.

Contextual Notes

Participants reference specific electrical characteristics of the Arduino Uno, including its output voltage levels and current sourcing capabilities, but the exact implications of these specifications on the circuit's performance remain unresolved.

Who May Find This Useful

Individuals interested in interfacing low-voltage control circuits with higher-voltage devices, particularly those using Arduino platforms or similar microcontrollers.

NotASmurf
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Hey all, I have a 5v IC that is triggered by a sensor to unlock the 12v lock, I am using a NPN transistor as a switch with the 5v VCC as the gate, I need it to sink back to the GND of the IC without blowing it, is this safe? If not what changes would you suggest, it has been years since I've had to do this, and I'm used to using H-Bridges for such things, any help appreciated.

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You don't show the emitter position in the NPN transistor, but I assume it points to ground since you are using it as a low-side switch.

What IC is driving this transistor base through the base resistor Rb? Is it a CMOS IC (so its output swing to the 0V, 5V rails), or does it have TTL outputs (so the low output is not really to the 0V rail)?
 
It is an arduino uno, and yes to the emitter position
 
NotASmurf said:
It is an arduino uno, and yes to the emitter position
What are Vol @ Iol and Voh @ Ioh for that Arduino?
 
max 80ma and 5.5v
 
Wow, it can source 80mA? Impressive. What is the Vol @ Iol spec?
 
technically 40ma on both, but in practice runs 80ma most of the time
 
My reason for asking about Vol is that if it doesn't reliably go down to 0V (which you typically get with CMOS outputs), then you may want to add a base pulldown resistor like this:

1635527231010.png

https://www.electronics-tutorials.ws/wp-content/uploads/2014/04/switch1.gif?fit=273%2C245
 
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Likes   Reactions: DaveE, Tom.G and dlgoff
If the VOL of the Arduino is 0.5V or more, I suggest adding a diode in series with the 1k resistor of the circuit that @berkeman suggested; Anode towards the Arduino, Cathode towards the transistor.
 

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