Using a transistor to switch LEDs ON and OFF

[suicidaleggroll]

Not only do you need a MOSFET with low R_DS,on, but also  - and this is important- a logic level type. The FQP30N06 is not specified for 3.3V gate-source voltage.

Datasheets says: Vgs(th) (max.) = 2,5V @ 250µA

Should be OK, no?

No.

Vgs(th) is the point at which the FET is JUST STARTING to turn on.  You typically need to drive the FET to at least double Vgs(th) to get a reasonable Rds(on).  Also, the Vgs(th) for that FET is somewhere between 2-4v (again, you need to watch tolerances), which means to guarantee that the device would be completely on with good Rds, you should be feeding it at least 8v.

For a 3.3v drive, you should be looking at devices with a maximum Vgs(th) of about 1.7v or lower.

[Tantal]

For a 3.3v drive, you should be looking at devices with a maximum Vgs(th) of about 1.7v or lower.

OK, so how about this one: NDP6060L
http://www.tme.eu/en/details/ndp6060l/tht-n-channel-transistors/fairchild-semiconductor/

Vgs(th) max. 2V
Rds(on) 25mOhm @ 5V, 24A

[Zero999]

As I mentioned earlier TimFox, you do not want to use a 3.3V supply to drive 3.2V nominal LEDs.  There's not enough buffer room and your brightness will be all over the place unless you hand pick a subset of your LEDs with matched forward voltage drops.  You also do not want to use a BJT due to the higher Vce.  You don't have enough room as it is, you need to get your transistor voltage drop to an absolute minimum if you intend to move forward with this circuit.  A very low Rds(on) Nch MOSFET is what you should be looking for.

It will work though. It may not be ideal but it'll work.

Maybe, maybe not.  He's riding the tolerance fence, not a good place to be.  If the LED Vf is a little on the high side and the "3.3v" supply is a little on the low side, it might not light up at all (at least not enough to be visible next to its neighbor running at 20mA).  I would be surprised if, with a set of 20 3.2 Vf nominal LEDs and a 3.3v nominal supply, there wasn't a strong temperature dependence and an obvious difference in brightness between at least a few of the LEDs, unless he goes through the effort to hand pick a subset that match.

It won't be anywhere near as bad as you think.

Modern LEDs give a usable light at a tiny fraction of their maximum current rating and if the LEDs are all from the same batch then their forward voltages should be pretty similar. The forward voltage also has a negative temperature coefficient which means once current starts to flow, it'll heat up and it the voltage would drop. As long as it's designed so the maximum ratings aren't exceeded then it'll work. It may not be perfect but it'll work.

[j.a.mcguire]

Why do you need the and gate if you're joining the input pins? Doesn't that override the purpose of the gate?

Plus if your switch feeds from +12V or +5V do you really have a separate input at vin? Is that to separately power the gate ic?

[suicidaleggroll]

For a 3.3v drive, you should be looking at devices with a maximum Vgs(th) of about 1.7v or lower.

OK, so how about this one: NDP6060L
http://www.tme.eu/en/details/ndp6060l/tht-n-channel-transistors/fairchild-semiconductor/

Vgs(th) max. 2V
Rds(on) 25mOhm @ 5V, 24A

It's a bit gigantic for the application, but it should be fine.

[PSR B1257]

Vgs(th) max. 2V

That's fine, but do not take to much attention of this figure.

The FQP30N06 has a maximum Vgs(th) of 4V, but that does not mean it operates properly at this voltage.
You have to refer to the graph called ON Characteristics (or similar)
For the NDP6060L it looks like this:

[LabSpokane]

When switching a fair amount of current (a few hundred mA or more), then it may be worth looking at a logic-level switching MOSFET instead.  No worries about gate current, and typically lower VDSon than the bipolar's VCEset.

Yes, I also thought about an n-channel MOSFET. But I have no idea which one to chose.

Which one would you recommend for switching 400mA @ 3.3V?

Would this one be OK? FQP30N06: www.digikey.com/product-search/en?KeyWords=FQP30N06L

Take a look at the NTD3055:  http://www.onsemi.com/pub_link/Collateral/NTD3055L170-D.PDF

They are about $.50/ea in low quantity and will switch up to 9A.  It's still extreme overkill, but it's hard to get a MOSFET that's much less expensive. 

[Tantal]

Take a look at the NTD3055:  http://www.onsemi.com/pub_link/Collateral/NTD3055L170-D.PDF

They are about $.50/ea in low quantity and will switch up to 9A.  It's still extreme overkill, but it's hard to get a MOSFET that's much less expensive.

I found two more:
STP55NF06L: http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=STP55NF06L
STU95N2LH5: http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=STU95N2LH5

Both with very low Rds On and Vgs(th) only 1V (1.7V). And both under $1.

[PSR B1257]

STU95N2LH5:

Drain-source breakdown Voltage=25V
Therefore not suited for your application.

[suicidaleggroll]

STU95N2LH5:

Drain-source breakdown Voltage=25V
Therefore not suited for your application.

His maximum Vds is 3.3v, where's the problem?

[PSR B1257]

Had another thread in mind, where a 36V LED is to be controlled 

[LukeW]

Take a look at the NTD3055:  http://www.onsemi.com/pub_link/Collateral/NTD3055L170-D.PDF

They are about $.50/ea in low quantity and will switch up to 9A.  It's still extreme overkill, but it's hard to get a MOSFET that's much less expensive.

I found two more:
STP55NF06L: http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=STP55NF06L
STU95N2LH5: http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=STU95N2LH5

Both with very low Rds On and Vgs(th) only 1V (1.7V). And both under $1.

As was mentioned above, you may not want to run the FET gate close to Vgs(th). Parameters like the effective Rds can change as a function of Vgs, and they are often quoted for relatively high Vgs voltages (say 10V). It's not a binary "all or nothing" thing once you cross the threshold voltage.

Particularly in applications where the drain current is high, or super-low on state resistance is needed, you may need to look at driving the gate from a higher voltage, say 10V, and interfacing that to the 3.3V or 5V microcontroller domain. A gate driver IC like a MCP1402 may be considered.

But in this 400mA application, that's fairly small, so Vgs probably doesn't need to be that high.