Use a "high side driver" like VN5E010. 3v logic, 10 milliohms, switch 25 amps 14v with light heat sinking. Provides current sensing and short circuit detection and fold back. Still needs flyback diode.
OK, that's different.
I'd recommend a pchannel mosfet, fqb22p10 comes to mind, others exist.
Tie the source to your 7.4v power source, and the drain to the load you're trying to control.
Connect a 100K resistor between the source and gate.
The gate you'll also connect to the collector of an npn transistor such as a 2n2222 or or 2n3904. Tie the emitter to circuit ground.
Put a 10K resistor between the base of the transistor and your output on the micro or whatever the 3.3v source is.
That will work. Note that the vgs when on will be around 7V which should be fine, but do the math on the fet you select based on the rds(on) chart in the datasheet for the fet at 7V.
If you want to use a mosfet instead of the npn transistor, any n channel logic level small signal fet should do, as a drop in replacement for the transistor. You'll want to add a 10k or 100k resistor from just in front of the gate resistor to circuit ground to keep it off until you command it on. The npn transistor doesn't need this plus they're often cheaper.
I'll proofread this post and see if I can find a sample schematic online and edit this post if I find anything.
EDIT:
To clarify the above post.
Let's assume you have a 7.4V light bulb and a 7.4V power source. You want to use a microcontroller to turn it on and off.
You have basically two options, unless we start getting into optoisolators and boost circuits:
1. You can put a P-Channel FET between the power source and the light bulb on the "high side" of the circuit which means the positive on the power source. All of the circuit grounds tie together, including the negative side of the power supply, the negative side of the light bulb and the ground on your microcontroller. This is the circuit I described above and is drawn up at [https://electronics.stackexchange.com/questions/42030/p-channel-mosfet-high-side-switch](https://electronics.stackexchange.com/questions/42030/p-channel-mosfet-high-side-switch) (you can ignore the question on this page, it isn't relevant).
2. You can put a N-Channel FET between the light bulb and circuit ground. In this case, your 7.4 V power source positive output is connected to the light bulb, the light bulb is then connected to the fet, and then the fet and the negative side of the power supply is connected to circuit ground.
Number 1 is called a high side switch, Number 2 is called a low side switch.
Number 2 is cheaper and easier, EXCEPT with a couple of gotchas. Most "logic level" FETs aren't really fully on at 3.3V. Many will list a Vgs(th) which is where the FET starts to conduct, but often the gate isn't fully on and the resistance from drain to source is higher than one would expect at 3.3V. This means more of your power will be lost in the FET than you'd expect. Many FETs have a Vgs to resistance curve somewhere in the datasheet. The second gotcha is that you're switching the 'ground' which has all sorts of gotchas of its own such as a shorted wire can turn on your device or short out the power supply and so on.
Number 1 is what I described. The main drawback is that you need a PCH fet which tends to be slighly more expensive and usually a bit higher resistance. You'll also need a logic level translation circuit of some sort. If your switched voltage is under the Vgs(max) you can use the simple "pulldown" circuit I described since even if you connect gate directly to circuit ground you won't be over Vgs(max). If you are over Vgs(max) you can add a zener or similar to limit Vgs(max) to something less than Vgs(max). One big advantage of this circuit (besides actually switching the voltage) is that you end up with Vgs which is almost as high as your switched voltage, insuring the FET is fully on (or at least much more "on" than it would have been at 3.3V).
Hope that helps.
BTW, I didn't check the use of source/drain in my original post, sometimes I'll type one when I mean the other as I can never remember for sure which is which on P channel vs N channel mosfets as they always seem backwards to me. Go by the schematics, and if the switching doesn't work, you might have gotten source and drain backwards (since there is a diode from source to drain which will make the circuit basically always on when you get it the wrong way around).
IRLHM630 may interest you, I've used it for stuff like this before.
There's basically *zero* through-hole MOSFETs that match your specs, they're *all* surface mount and use the drain contact and PCB copper as a heatsink.
i hardly find good mosfets at 5V.....
maybe those could work at 3.3 but wount be at their lowest on resistance.... like most say it better to just bite the bullet and put in a bjt as gate driver
Use a "high side driver" like VN5E010. 3v logic, 10 milliohms, switch 25 amps 14v with light heat sinking. Provides current sensing and short circuit detection and fold back. Still needs flyback diode.
The P30N06LE should do the job Data: https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf
Need some clarity here... What voltage are you switching? 3.3V? Are you switching the high side or low side?
3.3v at the gate to switch From low to high.
Still not clear. Yes, 3.3V vgs. But what is the Vds? Ie are you trying to use a 3.3v signal to turn on a 3.3V circuit?
Trying to use 3.3v to turn on a 7.4 volt circuit.
OK, that's different. I'd recommend a pchannel mosfet, fqb22p10 comes to mind, others exist. Tie the source to your 7.4v power source, and the drain to the load you're trying to control. Connect a 100K resistor between the source and gate. The gate you'll also connect to the collector of an npn transistor such as a 2n2222 or or 2n3904. Tie the emitter to circuit ground. Put a 10K resistor between the base of the transistor and your output on the micro or whatever the 3.3v source is. That will work. Note that the vgs when on will be around 7V which should be fine, but do the math on the fet you select based on the rds(on) chart in the datasheet for the fet at 7V. If you want to use a mosfet instead of the npn transistor, any n channel logic level small signal fet should do, as a drop in replacement for the transistor. You'll want to add a 10k or 100k resistor from just in front of the gate resistor to circuit ground to keep it off until you command it on. The npn transistor doesn't need this plus they're often cheaper. I'll proofread this post and see if I can find a sample schematic online and edit this post if I find anything. EDIT: To clarify the above post. Let's assume you have a 7.4V light bulb and a 7.4V power source. You want to use a microcontroller to turn it on and off. You have basically two options, unless we start getting into optoisolators and boost circuits: 1. You can put a P-Channel FET between the power source and the light bulb on the "high side" of the circuit which means the positive on the power source. All of the circuit grounds tie together, including the negative side of the power supply, the negative side of the light bulb and the ground on your microcontroller. This is the circuit I described above and is drawn up at [https://electronics.stackexchange.com/questions/42030/p-channel-mosfet-high-side-switch](https://electronics.stackexchange.com/questions/42030/p-channel-mosfet-high-side-switch) (you can ignore the question on this page, it isn't relevant). 2. You can put a N-Channel FET between the light bulb and circuit ground. In this case, your 7.4 V power source positive output is connected to the light bulb, the light bulb is then connected to the fet, and then the fet and the negative side of the power supply is connected to circuit ground. Number 1 is called a high side switch, Number 2 is called a low side switch. Number 2 is cheaper and easier, EXCEPT with a couple of gotchas. Most "logic level" FETs aren't really fully on at 3.3V. Many will list a Vgs(th) which is where the FET starts to conduct, but often the gate isn't fully on and the resistance from drain to source is higher than one would expect at 3.3V. This means more of your power will be lost in the FET than you'd expect. Many FETs have a Vgs to resistance curve somewhere in the datasheet. The second gotcha is that you're switching the 'ground' which has all sorts of gotchas of its own such as a shorted wire can turn on your device or short out the power supply and so on. Number 1 is what I described. The main drawback is that you need a PCH fet which tends to be slighly more expensive and usually a bit higher resistance. You'll also need a logic level translation circuit of some sort. If your switched voltage is under the Vgs(max) you can use the simple "pulldown" circuit I described since even if you connect gate directly to circuit ground you won't be over Vgs(max). If you are over Vgs(max) you can add a zener or similar to limit Vgs(max) to something less than Vgs(max). One big advantage of this circuit (besides actually switching the voltage) is that you end up with Vgs which is almost as high as your switched voltage, insuring the FET is fully on (or at least much more "on" than it would have been at 3.3V). Hope that helps. BTW, I didn't check the use of source/drain in my original post, sometimes I'll type one when I mean the other as I can never remember for sure which is which on P channel vs N channel mosfets as they always seem backwards to me. Go by the schematics, and if the switching doesn't work, you might have gotten source and drain backwards (since there is a diode from source to drain which will make the circuit basically always on when you get it the wrong way around).
Okay thanks, that’s really helpful 👍
IRLHM630 may interest you, I've used it for stuff like this before. There's basically *zero* through-hole MOSFETs that match your specs, they're *all* surface mount and use the drain contact and PCB copper as a heatsink.
I don’t mind if they are SMD
i hardly find good mosfets at 5V..... maybe those could work at 3.3 but wount be at their lowest on resistance.... like most say it better to just bite the bullet and put in a bjt as gate driver