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555 driving N mosfet

MrDEB

Well-Known Member
Working on a method to have an adjustable flash on an LED neon strip.
Have attached the schematic for the test circuit, plan to use a 7805 to power the 555 but testing with a 9v battery I get 4 volts on the 555 output.
The issue is the gate has a maximum of 4v on the gate.
thinking of a voltage divider or Zener diode on the gate to avoid applying too much voltage to the gate.
I show 2 LEDs but D3 is just for testing. D2 will be a 12v LED neon strip. the 9v will be 12 v.
and yes the breadboard circuit works as showen
 

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Remove D3 & R4; that's holding the Pin 3 voltage down to some extent.

Add a 470 Ohm between pin 3 and positive supply to help the output pin reach as high a voltage as practical.

Keep the existing 555 & keep everything at 9V supply (or 12V).
 
If still determined to use a 5V supply, have you replaced the MOSFET with a logic-level one?
 
post#21 refers to the wrong schematic (see post#20 for the correct schematic.)
I will look to see If I have any other MOSFETS
I was/am considering maybe removing the 7805 or driving the MOSFET gate using a transistor.
 
I'd say to delete both resistors no matter which 555 you are using.

The IRF520 has a listed Rdson of 0.27 ohms with Vgs = 10 V. Even if you had 10 v of gate drive (which you don't), the transistor would be dissipating over 1 W, enough to get pretty warm.

If your circuit is pulling only 2 A through the LEDs when it should be 5 A, that indicates that the FET is not fully enhanced, fully on. In this case the voltage across the FET, Vds, probably is not down around 1.3 V, which is why the FET is overheating and the load current is only 2 A. You can verify this by measuring the voltage across the LEDs. If it is less than around 10.5 V, you're probably starving the FET.

ak
 
well I got this circuit connected to a 5m/16.4ft of led neon strip
It flashes as it is supposed to.
The problem is the MOSFET gets warm (stopped measuring at 130 degrees) and I show a 2amp draw.
specs are 12v .8A/meter 9.5W/meter (5m = 48W)
as my schematic shows, I added a 7805 voltage regulator.
looking back at post #15 I need to use a 7555 and/or remove the 1K and 10K resistors?

I'd say to delete both resistors no matter which 555 you are using.

The IRF520 has a listed Rdson of 0.27 ohms with Vgs = 10 V. Even if you had 10 v of gate drive (which you don't), the transistor would be dissipating over 1 W, enough to get pretty warm.

If your circuit is pulling only 2 A through the LEDs when it should be 5 A, that indicates that the FET is not fully enhanced, fully on. In this case the voltage across the FET, Vds, probably is not down around 1.3 V, which is why the FET is overheating and the load current is only 2 A. You can verify this by measuring the voltage across the LEDs. If it is less than around 10.5 V, you're probably starving the FET.

ak
 
Most Mosfet needs a gate-source voltage of at least 7V to turn on well and stay cool. An ordinary 555 when powered from 5V has an output high of only 3.6V which is too low to properly drive an ordinary Mosfet. A Cmos 555 when powered from 5V has an output that is 5V which is still too low.

MrDEB, again you are showing the schematic with the diode in the wrong place.

EDIT: Complimentary gate drivers produce high current (to switch the Mosfet on and off at a high frequency) that you do not need, but they reduce the gate voltage. A 555 already produces massive current (200mA).
A Cmos 555 produces low current and needs a complementary gate driver for high switching frequencies that you do not have.
 
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so basically need a method to apply MORE voltage to the gate.
Will investigate but thinking maybe use a p channel mosfet as in the color organ post.
 
Will investigate but thinking maybe use a p channel mosfet as in the color organ post.

It wasn't the answer there either. Reading a tutorial on mosfet switches might clear things up
 
Just run the 555 at 12 volts. That'll give you an output voltage from the 555 of about 10 Volts, which is enough to turn a standard N-channel mosfet fully on.
No need for a logic level mosfet, nor a P-channel device.
 

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