MOSFET and 3.3v switching

Hi forum

I’m new to electronics......but enjoy it. I’ve played around with N mosfets and 5V arduino’s with great success. I’m trying to get it right with a 3.3v microcontroller and having issues understanding it.

I would like to do it with as little components as possible. The load is 1A at a voltage of 24V. When I try select a mosfet for the job I am concerned the Vgth won’t be enough or the Vgs max is too close to the maximum. Maybe I’m over thinking it. Anyone help out with a suggested circuit? (Trying to avoid using a transistor on the gate.....but happy to hear a suggestion if it’s the only way)

Regards

The most useful parameter to look at for a MOSFET is the voltage that Rds(on) is specified at. If it gives a resistance at 3V or less then it's good for a 3.3.V Arduino.

Then you'll see that the threshold voltage is probably a maximum of about 2V. But since Vgs(th) is the voltage the MOSFET just STARTS to conduct it's not a very useful parameter.

Steve

The threshold voltage is of no use to you, ignore it.

You need a MOSFET that works with only 3V of gate drive, which means in the datasheet there is an on-resistance quoted for Vgs <= 3.0V

Nearly all such MOSFETs are surface mount, as 3.3V logic appeared after SMT become standard, and no-one
could sell through-hole 3V capable MOSFETs. DPAK SMT packaged MOSFETs are fairly simple to solder direct
to stripboard or similar, note.

For any high power use you use a gate-driver chip and a 12V MOSFET. Gate driver chips accept 3 or 5V logic
inputs.

So you could go down this route, pick a TO220 MOSFET and a gate-driver chip. You’ll need a 12V supply
rail though.

Another approach is use a darlington transistor instead, with a 560 ohm base resistor or so - for a 24V load
the voltage drop across a Darlington is not a big deal - the darlington will get hot though at 1A, needing a
small heatsink, typically.

If you happen to have a 5V supply rail as well, 74HCT14 inverters can be used to boost a 3.3V logic
signal to 5V to drive 5V logic-level MOSFETs which are available as TO220. The 74HCT logic family
needs 5V supply, but will handle 3V or TTL logic signals reliably on the inputs.

The main thing you need are a ohm's calculator and the data sheet. Look at your circuit and determine what voltage you will deliver to the gate of the MOSFET. Then go to the data sheet and look at the RDSon at that gate voltage, be sure you check it over the device operating junction temperature, not the ambient temperature. Once you have the RDSon you can calculate the power dissipated in the MOSFET. The data sheet will tell you what the package can dissipate. Many times the rating is given using an infinite heat sink, be careful. You can always use a lower RDSon device and it will be much cooler. A darlington transistor would need to dissipate about 1.5 watts with your load. If it is a simple transistor you would need to supply about 100 mills to the gate. The hard turn rule of thumb is a gain of 10.

thanks Guys.

I have a logic level shift in the circuit already. See below link. Could I use this with a irf520N mosfet. NB its 24v load? . Would I need a resistor on the Gate?

http://www.xtcomp.co.za/index.php?route=product/product&product_id=579&search=logic+level

Does an IRF520N have Rds(on) specified at 3V or 5V or under? If not then it's not a good choice.

Steve

From the datasheet:


So, no, only 10V and above (that means nominally 12V gate drive)

MarkT:
From the datasheet:


So, no, only 10V and above (that means nominally 12V gate drive)

I know that. I was hoping to persuade the OP to read the datasheet for the device he wanted to use.

Steve