Hi All,

Rookie mosfet question here... I am using a 555 timer and a TC4429 to switch on a VP3203N3 mosfet. The mosfet switches on as I was expecting, but the voltage is lower coming out of the drain than what is going into the source. I have 15.5v going in the source, but I'm measuring only 11.1v coming out the drain. Is this normal? Any tips on how to avoid voltage drop or calculate what it should be?

thanks!

 

hi cpyne,
Welcome to AAC.
Is the MOSFET load in the Source or Drain,?
A diagram would help.
E

 

The load will be on the drain side. Right now I'm just testing, so no actual load is connected other than my volt meter. Voltage > Mosfet > My voltmeter > Ground. I don't have a diagram handy, but can draw one up if needed

 

Hi cp,
Without a load, the MOSFET readings will be misleading, especially measuring with a voltmeter.
If you could draw up a rough sketch, I am sure we can advise.
E

 

Thanks. That is helpful just knowing that I shouldn't be measuring this way. I'll post an update soon

 

Until you show us your schematic can only guess.

 

Here's a simplified diagram of what I've got so far. I'm someone figuring this out as I go. Goal here is to eventually make a programmer that can program old PROM chips. Press the button and it will pulse 15.5v to the correct leg of the chip to be programmed.

 

hi cp.
What will the load be on the PMOS Drain other than Programming input?
I would suggest 1K resistor be permanently connected from the Drain to 0V.
Try it.
E

 

Thank you! That worked. With 1K added, the drain voltage now matches the source.

For my future knowledge, are there other general situations, other than mosfets, where a load must be present for correct measurements and function?

 

Press the button and it will pulse 15.5v to the correct leg of the chip to be programmed.

How much current does the PROM require?
Seems like you could drive it with the 555 alone at 15.5 volts.

 

They vary by model, but ~250ma for programming

 

The problem without the resistor is the the pin is connected to nothing when the MOSFET is off. You want a valid voltage on it both when on and off. The resistor makes it zero when off. This is called a pull down resistor.

 

Thank you! That worked. With 1K added, the drain voltage now matches the source.

hi cp,
The Drain needs a connection to 0V via a Load in order to create a voltage drop across the Load.
The 1K will only draw 15mA, so it should not be a problem for the PMOS

E

 

hi cp,
This is an example of the pulse from the 555 and PMOS.
I don't have a TC4429 model so have built a level shifter, the load is a 62R == 250mA.
E

 

You don't need a FET for this project,
the FET-Driver will provide way more current than You need,
and most, if not all, FET-Drivers have an Output that switches "Rail-to-Rail".
.
.
.

 

You don't need a FET for this project,

Would need to change the driver to a TC4420 non inverting or similar.
I would go with Eric's circuit and replace the driver with a transistor or FET.

 

For my future knowledge, are there other general situations, other than mosfets, where a load must be present for correct measurements and function?

Power supplies often need a load for correct regulation.