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Topic: IRF740 MOSET question (Read 188 times) previous topic - next topic


Sep 21, 2019, 07:06 am Last Edit: Sep 21, 2019, 07:19 am by xl97
I was asked to help someone with a halloween costume.... (vape thing into fog for FX purposes)

It uses a fish pump and a vape 'machine'?  (I dont know official term..I dont smoke)..  and some non-toxic fog oil...

I soldered everything up.. and using my bench top PSU at around 3v (it was a few days ago now.. but I believe it was just at 3v).. and it worked fine.

He wanted to add in the RF switching module so his wife could trigger it remotely..

Specifically this one:

In the tutorial he sent me to follow..   it uses this N-channel power MOSFET:

It was out of stock.. and I had an IRF740 laying around..

However.. when I tried to use it..  it doesnt turn on.

I thought perhaps the 3.7v battery (also used in this odd tutorial) wasnt powerful enough.. so I removed and powered it from PSU again.. (nothing)..  I see the RF module V+ pin says 5v  (not sure why they use a 3.7v battery?)

so I thought if I turn up the PSU a bit.. it will work.. it started to 'buzz'.. and spurt a bit..  but ultimately didnt work.

My initial thought was the IRF740 MOSFET wasnt opening fully?


I'm not always sure what to look at.. but does it say opens @2v and fully open @ 4v?

if so.. then what is my problem?



Now I'm confused even more seeing this:


Is that saying it needs 20v to be fully open??

Vgs - Gate-Source Voltage:   20 V

Then I also read this:

The IRF740 is an N-Channel Power MOSFET which can switch loads upto 400V. The Mosfet could switch loads that consume upto 10A, it can turned on by provide a gate threshold voltage of 10V across the Gate and Source pin
I guess both allude to not having enough voltage to fully open it?  (hence wrong MOSFET).  :(


Is that saying it needs 20v to be fully open??

Vgs - Gate-Source Voltage:   20 V
No it is saying any more than 20 V and you fry it.

But any more than 10V doesn't turn the FET on any harder, that is the on resistance is as low as it is going to get.


You're looking at the Gate Threshold voltage Vgs(th). That tells you that it will START to conduct somewhere between 2 and 4V.

The more interesting parameter is Rds(on), the resistance when fully on. If that is only stated at 10V then the chances of it being fully on at 3.7V or even 5V are remote. Logic level MOSFETs usually give this parameter at 4.5V or even 2.8V. They're the ones that are useful with Arduinos.



Sep 21, 2019, 02:51 pm Last Edit: Sep 21, 2019, 08:27 pm by ReverseEMF
Another thing to look at is the Output Characteristics curve.

Notice how there are no voltages below 4.5V, on the VGS list.  That's a clear sign this MOSFET was not designed to operate at Gate voltages that low.  And, even at 4.5V, yer not getting it's full capability!

BUT, in a one-off pinch [i.e. MacGyver style], you can learn a lot about how the device will perform, by studying this graph.  Never use it with a gate voltage below 4.5 Volts, but at 5V, it can be relied on to manage just shy of 3A at the Drain.  Kind of ridiculous for this device, but hey -- if the bad guys broke your radio, and stranded you on a desert island, and you need to MacGyver together a signal light, and the IRF1902 MOSFET is bad, and all you have is an IRF740, well then, you might just be all set!!

Also, notice what a HUGE difference half a volt makes! 

@ 4.5V on the gate, the Channel limits current to 700mA max.
@ 5.0V on the gate, the Channel opens to a whopping [by comparison] 2.9ish Amps.
@ 5.5V on the gate, we're talkin' more like 6.5A!!
BTW: @ 8V on up to the MAX safe Gate Voltage the channel behaves more like a sloppy half ohm resistor that decreases to around at the top of the curve [hence the "sloppiness"].

Of course, all this changes if the MOSFET heats up, which is what this graph is all about:

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Thank you everyone.. (karma for all!)  LOL

Drain-Source On-State Resistance RDS(on) VGS = 10 V

^ thank you for that bit of info as well.

I'll order a proper MOSFET for this project...


As a rule of thumb, IRF MOSFETs are not logic level; IRL MOSFETs are.

Indeed, look at the data sheet, see if there's an RDS(ON) value for VGS < your intended voltage and you're good.

There are MOSFETs that open quite well at 1.8V, though for full performance they need 2.5V or even a little more at the gate.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.


The IRF740 is a 400 volt part, completely inappropriate for low-voltage circuitry, as is has 550 milliohms on resistance, which is massive.  9 milliohms and 30V was the spec of the device from Adafruit.  You chose a device
different by one or two orders of magnitude in important parameters, as well as not logic-level.
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