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I used an IRFZ44N MOSFET to build a switching circuit to light up a bulb from a 12V car battery.

It works but it is not stable.

When the bulb is off, after a while it glows and increases.

When the bulb is on, little by little it goes dim.

First I tested it on Proteus and then I did it with real hardware.

Can anyone tell me how to make it stable?

I want the bulb to extinguish when the push button is pressed. It should then stay off.

I have a good heat sink for the MOSFET. Can't we use push button?

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  • \$\begingroup\$ 1) Does there really need to be two push buttons? 2) Is "when the push button is pressed. It should then stay off" a requirement, as this means that the circuit has to have a memory and will necessarily be more complicated than the two current answers suggest. \$\endgroup\$
    – Paul Uszak
    Mar 25, 2021 at 13:04

2 Answers 2

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You cannot leave the gate of the MOSFET floating, this results in an unstable state and may damage it.

The switch at the bottom should be replaced by a 10k resistor. This will force the gate to go to 0V when the switch is not pressed.

Also, the MOSFET should be on the lower side of the lamp like this, as the gate has to be driven to at least the Vgs threshold. If it's above the lamp, the Vgs voltage will be too low:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Your advice is valid, but "do this instead" won't help OP understand why those changes are needed. Could you add a few words to your answer to explain why R1 is necessary there, and why M1 should be moved to the low side? \$\endgroup\$
    – marcelm
    Mar 25, 2021 at 10:24
  • \$\begingroup\$ @marcelm edited \$\endgroup\$
    – Damien
    Mar 25, 2021 at 10:28
  • \$\begingroup\$ Awesome, thanks :) \$\endgroup\$
    – marcelm
    Mar 25, 2021 at 10:54
  • \$\begingroup\$ But when i put the bulb above the mosfet , the proteous gives error message.And also when I put the resistor to ground the bulb is dim. \$\endgroup\$
    – upali
    Mar 26, 2021 at 8:55
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    \$\begingroup\$ Hello Mr.Damien, your circuit is working. Thank you. Both the proteos, and real. \$\endgroup\$
    – upali
    Mar 27, 2021 at 9:28
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First of all, you have no fuses. The car battery can provide enough current to melt and burn your wires if you make a mistake, so that't dangerous.

Secondly, you can't use a N type FET as a high side switch. You need to either use a P type FET a high side switch, or move the N type FET to be a low side switch.

And third, the gate is floating so leakage currents can turn it on or off when no button is pushed.

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  • \$\begingroup\$ Thanks Mr.Justme for your effort. \$\endgroup\$
    – upali
    Mar 27, 2021 at 9:42

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