Simple Logic Inverter

Thread Starter

chrisw1990

Joined Oct 22, 2011
551
i need a small chip.. i made a hardware error on a project, i can fix with one mosfet, but im trying to find extremely small (passive?) solution.. so no ics
 

THE_RB

Joined Feb 11, 2008
5,438
I'm not going to another site to see your picture.

You need a NFET;
The input is the Gate (and is driven from your original logic output).
The Drain is the output and needs a pullup resistor to Vdd.
The source is ground (Vss).

The pullup resistor depends on the speed you need it to work at, if it works at higher frequencies the resistor should be smaller. Generally 470ohm or 1k will work for most apps.
 

Thread Starter

chrisw1990

Joined Oct 22, 2011
551
ah yeh ok (note to self post pictures not links) i tell other people to, dunno why i didnt think of it..
yeh i get the circuit now, just wanted to make sure it was how i thought..
ill post the image up for any googlers or future readers.
 

Thread Starter

chrisw1990

Joined Oct 22, 2011
551
just to invert the logic for a mosfet switch =] sadly i cant change the polarity of the trigger, it is active high, and my circuits switch is active low.
 

Ron H

Joined Apr 14, 2005
7,014
just to invert the logic for a mosfet switch =] sadly i cant change the polarity of the trigger, it is active high, and my circuits switch is active low.
I hope you are going to put it in front of the driver.
What is the MOSFET driver? Is it an IC, or is it discrete? Give us a part number, or a schematic (preferred).
How fast does it have to switch?
What is the minimum pulse width?
 

Thread Starter

chrisw1990

Joined Oct 22, 2011
551
I hope you are going to put it in front of the driver.
What is the MOSFET driver? Is it an IC, or is it discrete? Give us a part number, or a schematic (preferred).
How fast does it have to switch?
What is the minimum pulse width?
sorry "in front of the driver"?
no sorry when i said mosfet switch.. its a p-channel mosfet, acting as a switch, its a bodge to correct a prototype circuit, but the pin it would be driven by is active high auto controlled by the pic32 im using.
it doesnt have to switch fast, its just an on/off style switch.. power on, power off.. i just needed to invert the polarity to make sure it worked correctly.
 

Ron H

Joined Apr 14, 2005
7,014
sorry "in front of the driver"?
no sorry when i said mosfet switch.. its a p-channel mosfet, acting as a switch, its a bodge to correct a prototype circuit, but the pin it would be driven by is active high auto controlled by the pic32 im using.
it doesnt have to switch fast, its just an on/off style switch.. power on, power off.. i just needed to invert the polarity to make sure it worked correctly.
Why can't you change the polarity coming out of the PIC32? Don't you have control of the software?
I hope you are using a logic level MOSFET. Otherwise, it won't work.
 

Thread Starter

chrisw1990

Joined Oct 22, 2011
551
its a USB library function controlling the bus voltage :)
and sorry probably a terminology thing.. "logic level mosfet" i.e. triggerable by logic levels?
 

Ron H

Joined Apr 14, 2005
7,014
its a USB library function controlling the bus voltage :)
and sorry probably a terminology thing.. "logic level mosfet" i.e. triggerable by logic levels?
Yes. Rds(on) needs to be specified at or below 5V Vgs. If it isn't the MOSFET may turn on weakly, and the on resistance may be too high
 

Audioguru

Joined Dec 20, 2007
11,249
He said he looked at the "off" threshold voltage.

An IRL540 has a threshold voltage of 1V to 2V when it is almost turned off with a current of only 0.25mA. Its is spec'd to be turned on very well with a max on-resistance of only 0.058 ohms when its gate voltage is 5.0V.
 

Wendy

Joined Mar 24, 2008
22,138
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