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switch relay VS switch Mosfet P-channel

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demnick

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Hi,

I post this because I want to correct my plan and my calcul.
I use my ATMega328 Microcontrollers on 3.3V in my Arduino at 8MHz.

Calcul transistor of the relay :

-J'ai utilisé 1 relais donc la résistance de la bobine est de 27.8 ohms.
- 1 diode 1N4007.
- 1 transistor 2N2222.
- 1 resistor 240 ohms.

The relay coil amperage is (3.3 volts / 25 ohms = 0.132 amperes) (is the intensity of the collector of transistor IC)


There are 2 operation for to find the value of the resistance between the transistor and the digital pin of Arduino.

// the intensity of the base of transistor is (IB) = IC / gain saturation.
Collector--Emitter Saturation Voltage (Note 1) VCE(sat) =0,3V (IC = 150 mAdc, IB = 15 mAdc)
gain saturation = 150/15=10
IB = 0.132 / 10 = 0.0132 amperes.

// The resistance = 3.3 volts / IB.
R = 3.3 / 0.0132 = 250 ohms (Me i use a 240 Ohms resistance)

For the transistor Mosfet P-channel:

-I use this transistor Mosfet p-channel : IRF7410GPbF
- 1 transistor 2N2222.
- 1 resistance 2200 ohms.
- 1 resistance 10K ohms.

I find this on the web about Mosfet P-channel:
Basic Transistor Drivers for Arduino Micro-Controllers
**broken link removed**

For the power dissipated in a MOSFET: P = RDSon * I²

P = 0.007 * 4² = 0.112 Watts i must dissipated

And the drop down volt: V = R*I

V = 0.007 * 4 = 28mV only

And the drop down intensity = 0

**broken link removed**

My plan and my calcul it is ok?

My resistance 2200 ohms in the Mosfet plan is ok?
I no understand how make calcul for this resistance 2200 ohms.

Thank you for your help and your correction!
 
Here is how I would do it: Note that I changed the pull-up resistor between the gate of the PFET and +7.4V. The 10K causes the PFET to turn off very slowly, while the 1K turns it off much faster.

Not clear what your load is? You imply that the load draws 4A, but because the PFET applies almost the full 7.4V from the battery to load, then I'm confused as to what the 3.3V signifies? I modeled the load as a 1.8Ω resistor, which draws 4A with 7.4V applied.

Is your load inductive, like a motor?
 

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There are some low voltage Pch MOSFET's available that you can drive the gate directly from 3.3v processor.

These Pch MOSFET's are used in cellphones with 50 milliohm Rds-ON with 3v Vgs drive. There might be some with lower Rds-ON or you can put two in parallel.
 
I think your plan is ok.
The size of the 2200 ohm resistor will detemine how fast you can turn on the mosfet. If you check the datasheet for your mosfet it will give you a number for total gate charge (nC). Let's say your nC is 10. This means your fet will turn on in 10 ns if you drive it with 1 amp. (nC/1) or in 1000 ns with .01 amps. (nC/.01). So with your setup you have base current of about 1 ma and collector current of at least 10 ma so 1 usec. turn on time. It's a little more complicated to be exact but this is close.
 
There are some low voltage Pch MOSFET's available that you can drive the gate directly from 3.3v processor.
...

No he can't. If the source of the PFet is at 7.4V, how can he drive it with a signal that swings from 0V to 3.3V?

If he can use a low-side switch (instead of high-side as he proposed), then he could use a low-threshold NFET, where the source is at 0V
 
Last edited:
No he can't. If the source of the PFet is at 7.4V, how can he drive it with a signal that swings from 0V to 3.3V?

If he can use a low-side switch (instead of high-side as he proposed), then he could use a low-threshold NFET, where the source is at 0V

Sorry, was looking at his load side voltage of 3.3v. Looks like he wants a remote enable 3.3 vdc 4 amp regulator.
 
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