Choose mosfet/io expander combination

im trying to use an pcf8574 to drive 8 mosfets. each mosfet runs a 12v 2.5watt motor. i need to run all motors at same time. it seems like each mosfet needs at least 200-250ma to fully saturate. the pcf8674 can only source 100ma and i think i fried it when i tried to power on all the mosfets.

can someone help me choose the right parts to make this work.

pcf8574 datasheet say can source 100ma. its this combined or per output?

Something is wrong. You don't run current through the gate of a MOSFET.
Is it silicon transistors You are using?
Tell what transistor it is.

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my mosfets are irf540n. when i check the output voltage of pcf8574 it is 1.1v when trying to power the mosfet. i think maybe the mosfet require some current? more than my io expander can deliver?

it looks like there is a ps2801 optocoupler on the board

mouser optocoupler

this is the mosfet board im using.

amazon mosfet board

ahh so it is the optocoupler that is sinking all this current you think?

there is an led on the mosfet gate that indicates when its powered. with pcf8574 led is dim. if i jump it to the 5v rail it goes full brightness.

Maybe the problem is that this IRF540N transistor is not logic level. It is better to use IRLZ44N.
MOSFET is control with voltage, not current.

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No! Likely more voltage. What's the data for Vgs = Vcc?

Read the datasheet page 4. 20 Amps at Vgs =4 volt.
IRF540N pdf, IRF540N description, IRF540N datasheets, IRF540N view ::: ALLDATASHEET :::

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Nope. On page 4, figure 7 at Vgs 4 volts, drain current is below 5 amps and the transistor is just starting to conduct.

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Okey. Sloppy figure from me. 4.5 - 5 volt ought to be enough for OP wanting 200 mA.

This board you are using has a PS2801 IC in control of the 4 mosfets.
This IC is a quad photo coupler.
As its input is an LED, it cannot be connected directly to the PCF8574 output, otherwise it will "roast" the input LED or the PCF output.
I don't know if your board has serial resistors like R1 mounted on it.
Check and if not use external resistors.

This is probably the schematic of 1/4 of your board.

RV mineirin

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Maybe consider TPIC6A595?

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would i use the TPIC6A595 to drive the optpcoupler on my mosfet board or to drive the load directly? i dont think opto isolation is that important for this project.

i didnt see any TPIC6A595 "prefab boards" but i think im dealing with such low current here that i could possibly get away without opto isolators.

what i was trying to say is, what mosfet "without optocouplers" could i drive directly with the pcf8574?

also on my mosfet board, the grounds on both ends are connected together.

Are you feeding the PCF8574 with 5V?
Then any MOSFET that has a VGS < 5V.
RV mineirin

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i asked a similar question here about a different mosfet board. i might just end up buying different mosfet boards and using this solution from this thread. it seems like the irf540 may not be the ideal component for this project.

other thread,

i would be driving the mosfet directly from the pcf8574. not sure but i "THINK" i could get away without opto couplers

Hi @notsolowki

If it's a MOSFET with vgs < 5V, you can drive it.
The gate current of a MOSFET is very, very small.
Place a +-100 Ohms resistor between the gate and the PCF output just to protect any short circuits on the board.
And put a +- 100K resistor between the Gate and Source of the MOSFET.
RV mineirin

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im going to have to stock up on logic level mosfets.