The gate threshold is the voltage at which the MOSFET begins to conduct. If you look at the datasheet, you will see that the "on" state resistance is characterized for VGS as high as 10V, with an absolute maximum rating of 12V.
Prop output pin @ 3.3V, but isn't the Gate Threshold Voltage VGS(th) 2V maximum? ...
Phil Pilgrim is correct. And, in this case, what the data sheet is talking about when it states "2V maximum" has to do with the statistical distribution of the manufacturing process. Most of the mosfets will come out of manufacturing with the VGS(th) being significantly less than 2V, but what they're telling you is that they guarantee that none of the devices will need anymore than 2V before they start conducting.
Don't feel bad, pacman, that particular spec burned me in the very beginning, too, and I got the wrong mosfet... then Phil came to my rescue with the IRF3708. God bless his heart.
OK, I think I see the on coming light (or is it a train?)
I just get concerned when I see things like Maximum voltage. To me a maximum is just that - many more units higher and the magic smoke comes out...
Can you explain a bit further....
Looking at the graphs again, Figure 13 gives me a on resistance of about 0.011A at 3.3V VGS. Now if I was running a 12VDC system (and switching on/off slowly - like about 0.2Hz) and, from your example, running a solenoid with a resistance of typically 200 ohms; Would I be dissipating somewhere in the order of 59 u watts across the fet? And thus not _really_ have to worry about a heatsink?
It's been so long since I've had to read component data sheets I'm _real_ confused...
And would 1K resistances be suitable for both the resistors on the Prop side? (I know that's a pretty open question - but a general clue would be appreciated)
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The future is in our hands.
Which way to the future?
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Threshold is not a very useful figure beyond telling you if it's a logic level mosfet or not. The real gate switching point is more like twice the typical threshold voltage.
Reading the IRF3708 datasheet indicates that it's switching level is around 2.5 volts. There is indications of large variability in the production batches. I haven't played with enough mofets to know if this is a typical issue or not though.
pacman said...
Looking at the graphs again, Figure 13 gives me a on resistance of about 0.011A at 3.3V VGS. Now if I was running a 12VDC system (and switching on/off slowly - like about 0.2Hz) and, from your example, running a solenoid with a resistance of typically 200 ohms; Would I be dissipating somewhere in the order of 59 u watts across the fet? And thus not _really_ have to worry about a heatsink?
Correct. Well, I get more like 40uW. And you can subtract the off-time off that also.
pacman said...
And would 1K resistances be suitable for both the resistors on the Prop side? (I know that's a pretty open question - but a general clue would be appreciated)
It depends. If you only want a basic on-off switch then yep, 1kR is fine.
The value of the series resistance affects the amount of heat in the mosfet. The higher the resistance is and the higher the switching frequency is the more the mosfet will heat up as it will spend less actually turned fully on or fully off. The mosfet will end up burning the power that should have gone to the actuator.
Thanks evanh. i suspected it would, but didn't _really_ want to cook one if i could avoid it.
Getting IRF3708's in Aust seems to be a bit problematic. Hence my reluctance.
Does someone know of a good substitute for the 3708?
I've had a few searches and most of the sites are either in Asian (which I don't have the skills to read) or give me a bunch of 'options' - unfortunatley I don't know enough to be able to seperate the wheat from the chaff.
What are the important datasheet 'bits' to look for?
as always,
thanks in advance.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
=================
The future is in our hands.
Which way to the future?
=================
Comments
Prop output pin @ 3.3V, but isn't the Gate Threshold Voltage VGS(th) 2V maximum?
Therefore, don't we exceed the maximum and thus damage the mosfet?
Thanks...
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=================
The future is in our hands.
Which way to the future?
=================
-Phil
Phil Pilgrim is correct. And, in this case, what the data sheet is talking about when it states "2V maximum" has to do with the statistical distribution of the manufacturing process. Most of the mosfets will come out of manufacturing with the VGS(th) being significantly less than 2V, but what they're telling you is that they guarantee that none of the devices will need anymore than 2V before they start conducting.
Don't feel bad, pacman, that particular spec burned me in the very beginning, too, and I got the wrong mosfet... then Phil came to my rescue with the IRF3708. God bless his heart.
I just get concerned when I see things like Maximum voltage. To me a maximum is just that - many more units higher and the magic smoke comes out...
Can you explain a bit further....
Looking at the graphs again, Figure 13 gives me a on resistance of about 0.011A at 3.3V VGS. Now if I was running a 12VDC system (and switching on/off slowly - like about 0.2Hz) and, from your example, running a solenoid with a resistance of typically 200 ohms; Would I be dissipating somewhere in the order of 59 u watts across the fet? And thus not _really_ have to worry about a heatsink?
It's been so long since I've had to read component data sheets I'm _real_ confused...
And would 1K resistances be suitable for both the resistors on the Prop side? (I know that's a pretty open question - but a general clue would be appreciated)
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
=================
The future is in our hands.
Which way to the future?
=================
Hey, nice looking board! What are those red connectors and where can I buy some?
The smaller one is Deans Micro and they're rated for 15 amps.
You can get them at almost any RC hobby vendor. Amazon and Tower Hobbies are some of the more convenient.
Reading the IRF3708 datasheet indicates that it's switching level is around 2.5 volts. There is indications of large variability in the production batches. I haven't played with enough mofets to know if this is a typical issue or not though.
Correct. Well, I get more like 40uW. And you can subtract the off-time off that also.
It depends. If you only want a basic on-off switch then yep, 1kR is fine.
The value of the series resistance affects the amount of heat in the mosfet. The higher the resistance is and the higher the switching frequency is the more the mosfet will heat up as it will spend less actually turned fully on or fully off. The mosfet will end up burning the power that should have gone to the actuator.
Getting IRF3708's in Aust seems to be a bit problematic. Hence my reluctance.
Does someone know of a good substitute for the 3708?
I've had a few searches and most of the sites are either in Asian (which I don't have the skills to read) or give me a bunch of 'options' - unfortunatley I don't know enough to be able to seperate the wheat from the chaff.
What are the important datasheet 'bits' to look for?
as always,
thanks in advance.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
=================
The future is in our hands.
Which way to the future?
=================
Hmm, good bits ... spend time understanding the graphs. You will be able to see many of the listed specs in the graphs.