555 driving N mosfet

rjenkinsgb

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• Jun 7, 2023

• #21

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).

 

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alec_t

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• Jun 7, 2023

• #22

If still determined to use a 5V supply, have you replaced the MOSFET with a logic-level one?

 

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MrDEB

Well-Known Member

• Jun 7, 2023

• #23

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.

 

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AnalogKid

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• Jun 7, 2023

• #24

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

 

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AnalogKid

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• Jun 7, 2023

• #25

MrDEB said:

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?

Click to expand...

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

 

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MrDEB

Well-Known Member

• Jun 7, 2023

• #26

so I need more gate voltage correct?
been looking at discreet gate drivers using a 2n3904 and a 2n3906 but ?

 

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audioguru

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• Jun 7, 2023

• #27

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.

 

Last edited: Jun 7, 2023

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MrDEB

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• Jun 7, 2023

• #28

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.

 

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audioguru

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• Jun 7, 2023

• #29

You need a "logic-level" Mosfet that works well with a 5V gate voltage. Most logic-level Mosfets are N-channel.

 

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F

For The Popcorn

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Most Helpful Member

• Jun 7, 2023

• #30

MrDEB said:

Will investigate but thinking maybe use a p channel mosfet as in the color organ post.

Click to expand...

It wasn't the answer there either. Reading a tutorial on mosfet switches might clear things up

 

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MrDEB

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• Jun 7, 2023

• #31

will search for logic level mosfet

 

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MrDEB

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• Jun 8, 2023

• #32

I think I found the P channel mosfets I need?

https://www.infineon.com/dgdl/irlml6344pbf.pdf?fileId=5546d462533600a4015356689c44262c

pretty sure this is correct.

 

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alec_t

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• Jun 8, 2023

• #33

MrDEB said:

I think I found the P channel mosfets I need?

Click to expand...

That link is to an N-channel MOSFET.

 

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MrDEB

Well-Known Member

• Jun 8, 2023

• #34

https://www.panjit.com.tw/upload/datasheet/PJD16P04.pdf

pretty sure this is the p channel

 

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ChrisP58

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• Jun 8, 2023

• #35

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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MrDEB

Well-Known Member

• Jun 10, 2023

• #36

while looking for logic level mosfet I ran across this chart

Logic Level MOSFET Selector Guide

I have an unpopulated PCB that could use better mosfet choice

 

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MrDEB

Well-Known Member

• Jun 10, 2023

• #37

DANG finding an N channel logic-level mosfet seems impossible
will keep looking
the Vgs is high like 1 ohm

 

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MrDEB

Well-Known Member

• Jun 10, 2023

• #38

MAYBE but not sure
need 5A Id 12v logic level mosfet (mcu connected)

https://www.mouser.com/datasheet/2/408/TK5A65D_datasheet_en_20131101-1150934.pdf

I think I am getting some insight into the MOSFET selection?

 

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audioguru

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Most Helpful Member

• Jun 10, 2023

• #39

Why are you looking at NTE replacement Mosfets and a Japanese 650V Mosfet that has a horribly high on-resistance?

 

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MrDEB

Well-Known Member

• Jun 10, 2023

• #40

grabbing at straws and getting worried I won't find a satisfactory MOSFET
member Tony suggested Vgs/Vgs(th) (max) = 2 to 2.5 min

 

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