First Time Modder Question - Do I need a Mosfet?

[amurphy]

I have been vaping for around a year now, and I've decided to build my first box mod. I've ordered everything I need, but I was watching a video tutorial on how to build one, and the guy doing the video used a mosfet in his. I ordered a 60A switch, so do I really need a Mosfet as well?

Also, I plan to wire in two 15A resettable fuses in parallel (so, 30A total, right?) for safety. If I'm using two 26650s that are rated for 32A continuous discharge, will the fuses cut off the power, even if I build my rda at, let's say 0.2?

Thanks in advance!

 

[K3nn0Q]

While you don't necessarily need a MOSFET, they are a good idea if the switch is not capable of handling the load.
I would double check the switch you ordered, as a lot of allegedly 60A switches are actually 2-5A switches.
They aren't a safety measure like the fuses are, it can still fail, but it's far less likely to fail than an under-rated switch.
The resettable fuses (PTC, I assume) don't completely cut the power, but actually trip into a very high resistance.
They're for catastrophic failure of the circuit, so they may or may not trip right at 30A.
Are you building a series, or a parallel box mod? A 0.2 load should be fine for a parallel (~21A), but should trip the fuses with a series battery configuration (~42A).
Good luck!

 

[amurphy]

While you don't necessarily need a MOSFET, they are a good idea if the switch is not capable of handling the load.
I would double check the switch you ordered, as a lot of allegedly 60A switches are actually 2-5A switches.
They aren't a safety measure like the fuses are, it can still fail, but it's far less likely to fail than an under-rated switch.
The resettable fuses (PTC, I assume) don't completely cut the power, but actually trip into a very high resistance.
They're for catastrophic failure of the circuit, so they may or may not trip right at 30A.
Are you building a series, or a parallel box mod? A 0.2 load should be fine for a parallel (~21A), but should trip the fuses with a series battery configuration (~42A).
Good luck!

The fire switch is a momentary push button, usually used for car ignitions apparently, haha. i got it off ebay, but it seems legit. here's the link

Does that look good to use?

and yes, the fuses are PPTC. I plan on building a parallel box. I'm looking to make a mod that I don't have to charge the batteries for several days, as opposed to insane voltage haha. I would have tried to do a regulated box, but I figured I would start with something that (technically) would be a little more basic.

Thanks again!

 

[david4500]

The fire switch is a momentary push button, usually used for car ignitions apparently, haha. i got it off ebay, but it seems legit. here's the link

Does that look good to use?

It is no where near 60 amps.

from the ebay descirption:

"3. Panel CUT SIZE: 19mm DIAMETER.

for more information, please visit http://www.APEM.com AV series PDF"

Website: AV Series APEM, industrial and professional switches, joysticks, switch panels, LED indicators

Link to the pdf: http://www.apem.com/files/apem/brochures/switch-pushbutton-AV-ENG.pdf

Look at the 19mm momentary push buttons.

Depending on the contact material, that switch is rated for 2 or 4 amps.

 

[K3nn0Q]

Had a feeling it was a Chinese seller on ebay when I heard the spec
60A switches exist, but they're huge and expensive

You'll definitely need a MOSFET to carry the current load for that switch - but it also lets you use pretty much any other switch you may want to use

I'd stick to reputable sellers, like Digikey or Mouser - it'll also generally be cheaper, and significantly faster shipping

Plus both those sites have extensive search capabilities, so you can fine tune the results to find exactly what you want. Just remember to get a few resistors for the MOSFET (4.7K~15Kohms, 1/4W seems to be the norm)

 

[amurphy]

It is no where near 60 amps.

from the ebay descirption:

"3. Panel CUT SIZE: 19mm DIAMETER.

for more information, please visit APEM, industrial and professional switches, joysticks, switch panels, LED indicators AV series PDF"

Website: AV Series APEM, industrial and professional switches, joysticks, switch panels, LED indicators

Link to the pdf: http://www.apem.com/files/apem/brochures/switch-pushbutton-AV-ENG.pdf

Look at the 19mm momentary push buttons.

Depending on the contact material, that switch is rated for 2 or 4 amps.

so basically, i must get a mosfet? haha oh well. is there a specific one that i should get?

 

[david4500]

At 2 amps & 4.2 volts, you could only fire a 2.1 ohm coil or higher. If you would like to build lower resistance than that, you'd certainly need a mosfet.

As mentioned in the diagram, I parallel the mosfets. This will halve their on resistance & split the current load. You can use a single mosfet if you like... but I recommend two.

 

[amurphy]

At 2 amps & 4.2 volts, you could only fire a 2.1 ohm coil or higher. If you would like to build lower resistance than that, you'd certainly need a mosfet.

As mentioned in the diagram, I parallel the mosfets. This will halve their on resistance & split the current load. You can use a single mosfet if you like... but I recommend two.

so if i use two mosfets, it will halve the resistance of the 15K resistor? so i could build a lower ohm coil? trying to make sure i understand before i attempt to build haha.

 

[drmarble]

Nope, it halves the on resistance of the mosfet. The resistance is about 1.4 milliOhm for the IRLB3034. This would halve it to 0.7milliOhms. Thus, for a 20 amp build the drop across the mosfet would fall from 0.028 volt to 0.014 volts. The power dissipated by the pair of mosfets would drop by a factor of four, and the power dissipated in each single mosfet would drop by a factor of eight. This improves the longevity of the mosfets.
The resistor is just their for bias. No major current flows through it, hence the 1/4w size.

 

[edwinoey]

IMO, mosfet is important to keep the amp load from the switch. Meaning that the switch will degrade over time without mosfet due to the huge amp load thru the switch. Some mosfet can take bigger amp load up lo 100 amp more. A indie box makers told me that i can build up to 0.05 and even 0.03 but not recommended. I mean 0.2 is low enough for me. So yeah mosfet is important too [emoji1]