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

In summary, the person is using a 5v IC triggered by a sensor to unlock a 12v lock. They are using an NPN transistor as a switch and need it to sink back to the IC's ground without causing damage. They are asking for suggestions and mention using H-Bridges in the past. The expert asks for clarification on the IC driving the transistor and suggests adding a base pulldown resistor if the IC does not reliably go down to 0V. They also suggest adding a diode if the VOL of the IC is 0.5V or more.
  • #1
NotASmurf
150
2
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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  • #2
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)?
 
  • #3
It is an arduino uno, and yes to the emitter position
 
  • #4
NotASmurf said:
It is an arduino uno, and yes to the emitter position
What are Vol @ Iol and Voh @ Ioh for that Arduino?
 
  • #5
max 80ma and 5.5v
 
  • #6
Wow, it can source 80mA? Impressive. What is the Vol @ Iol spec?
 
  • #7
technically 40ma on both, but in practice runs 80ma most of the time
 
  • #8
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 DaveE, Tom.G and dlgoff
  • #9
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.
 

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

1. What is the purpose of using a MOSFET, diode, and resistor in this circuit?

The MOSFET acts as a switch to control the flow of current from the 5V IC to the 12V device. The diode is used to protect the MOSFET from reverse voltage and the resistor is used to limit current and prevent damage to the components.

2. Can any MOSFET, diode, and resistor be used in this circuit?

No, it is important to choose components with appropriate voltage and current ratings to ensure the circuit functions properly and to prevent damage to the components.

3. How do I determine the appropriate values for the resistor and diode in this circuit?

The resistor value can be calculated using Ohm's law, taking into account the voltage and current requirements of the IC and the MOSFET. The diode should have a voltage rating higher than the maximum voltage in the circuit and a current rating higher than the maximum current that will flow through it.

4. Is it safe to use a 5V IC to control a 12V device in this circuit?

Yes, as long as the components are chosen appropriately and the circuit is properly designed, it is safe to use a 5V IC to control a 12V device. However, it is important to always follow safety precautions and double check all connections before testing the circuit.

5. Are there any potential issues I should be aware of when using this circuit?

One potential issue to be aware of is the possibility of voltage spikes or transients that could damage the components. It is important to include proper protection, such as a diode, to prevent this from happening. Additionally, the circuit may not function properly if the components are not chosen correctly or if there are any errors in the circuit design or assembly.

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