Better undesrstanding of MOSFETS

[jonnychadootz]

From what I have been seeing/hearing MOSFETS are used in the situation of inadequately rated switches (like if you had a switch rated for 5V - 3A draw) yet you needed 1 that was capable of 7.4V - 25 A draw. What I don't understand is why you wouldn't just get a switch that's rated for the power draw you actually require? Could some1 please clarify this for me? I did a search but most of it came back with how to wire the MOSFET etc, which for the most part I already understand. Much appreciated in advance

 

[drmarble]

It is hard to find switches with a 30 amp rating. The numbers listed on Ebay are not correct. They may state 30 amps at 5 volts but they are incorrectly changing the rating for 120 volts. Others just flat out lie to sell more. Most are 3 amps at most. Look at the manufacturers spec sheet before you believe the rating.

 

[tmassey]

If you tried searching for a 25amp switch you might understand. Limited selection, kind of pricey and for the most part they are huge. At least that's why I use a mosfet, beats trying to rig up a mech switch and saves your battery from the pitting and arcing


Sent from my iPhone using Tapatalk

 

[edyle]

From what I have been seeing/hearing MOSFETS are used in the situation of inadequately rated switches (like if you had a switch rated for 5V - 3A draw) yet you needed 1 that was capable of 7.4V - 25 A draw. What I don't understand is why you wouldn't just get a switch that's rated for the power draw you actually require? Could some1 please clarify this for me? I did a search but most of it came back with how to wire the MOSFET etc, which for the most part I already understand. Much appreciated in advance

Because the requirement is extreme.



It's like your car starter switch at the steering wheel; when you make the last click with your car key that activates the starter motor, the way it does that is by activating another switch (the 'solenoid') next to the starter motor, and very high current then goes to the starter motor and not up to the dashboard where your hand is.

So in your own car, when you turn the key, it is a small switch that controls another switch that turns on the big current.
And in the mosfet mods, you have the same thing, a small switch controls another switch that turns on the big current.

 

[Robert Cromwell]

A real 25 amp switch would be hard to operate and BIG. Much easier to simply control a Mosfet with a smaller current rated switch. A Mosfet is simply a high current low forward voltage drop transistor.

 

[jonnychadootz]

I guess I see the relevance (a lower V and A switch + mosfet would be less expensive/smaller. At the same time I have about (6) 12V - 25-30 amp switches sitting in my tool box atm. Even a 125V switch with an amp rating of 25-30 would still work in these particular applications (I've used them) It's only when you exceed the switches max rating that you run into problems from what I have seen seen. At the same I do have alot of smaller momentary switches with lower amp rating than what I am using. Gonna have to grab a few MOSFETS and have some fun. Thanks for the input good people's

So with this in mind, where should I get (or look for) a mosfet to power a 5V - 4A PS. At the same time putting out 10 amps? Are there MOSFETS that can be adjustable?

 

[Robert Cromwell]

Jonny you are getting too deep into electronics for a beginner. Google mosfet power circuits and see what I mean. Easy to turn a mosfet on and off but adjustable is another matter. Yes it can be done but is not simple to explain or figure out the details.
Many of the high current mods just turn the mosfet on and off really fast to control the wattage going thru the coil. PWM is the term I believe. But actually more just pulsed DC since nothing is being modulated. But I digress, just complicated to explain to a beginner in electronics.

 

[jonnychadootz]

Your entitled to your opinion (which is simply wrong (Never mind I'll figure it out on my own....Thanks any

 

[david4500]

Even if you had a switch that was properly rated to handle the current, it would by physically large and have more resistance (more power dissipation and voltage drop) than using a suitable mosfet.

 

[jonnychadootz]

Even if you had a switch that was properly rated to handle the current, it would by physically large and have more resistance (more more dissipation and voltage drop) than using a suitable mosfet.

I understand more than a few of the people here think (am I an electronics wiz, no)....At the same time I am not a beginner (if I was I wouldn't build a box myself) and was trying to further my knowledge of certain resistors/mosfets and the like. I have plethora of momentary and other switches that are smaller in size and rating than what I need to use them for which is why I asked the question about MOSFETS.. I have since ordered 2 - 18650 batteries/charger, a sled to hold/wire them and the hobby box I have a momentary switch rated at 3amps that will be powered by 2 - 18650 batteries with a 20amp capability (per battery) What mosfet should I be using so that switch/mosfet will put out 10 amps being that is all I need? An adjustable output would be ideal if possible. And yes. I plan on wiring in a cutoff switch for when the mod is on the move for safety

 

[drmarble]

The mosfet doesn't give you adjustability. We normally use them as switches. They are on or off. The device works like a mech but with a reliable switch. The voltage isn't adustable. If you want adjustable voltage you need a buck chip like the raptor. Lots more wiring is required.
I use an N-fet to switch the low side. I think it is an IRLB3034PbF but am not sure.
Here is a nice diagram:

.... luck.

 

[jonnychadootz]

The mosfet doesn't give you adjustability. We normally use them as switches. They are on or off. The device works like a mech but with a reliable switch. The voltage isn't adustable. If you want adjustable voltage you need a buck chip like the raptor. Lots more wiring is required.
I use an N-fet to switch the low side. I think it is an IRLB3034PbF but am not sure.
Here is a nice diagram:
View attachment 419803
.... luck.

I think you are right. What I am looking for is less a switch and more a regulator. Need to find 1 that works with 7.4 volts that the 18650's will output at the same time I want to limit it to a 10 amp draw (with the availability to up the amps slightly if needed) It appears I'm looking for something along the lines of potentiometer/rheostat. Which I'm having a hard time find as they are all seem to be rated in ohms/resistance. Much like what you would find on the bottom of an adjustable voltage EGO battery except in the having the ability to go slightly higher in the voltage output range. I could easily gauge it from there.

 

[Visus]

pots which are rheostats are rated in watts.

What you want is a switching regulator. There is no way around using a switching regulator if you want adjust ability such as the ego twist, it also uses a switching regulator not just a pot. The pot or rheostat only changes the field in the inductor and at very low wattage. A 50watt pot would be an absolutely monstrosity and its efficiency would be horrible..

look up okr or "evercool mod" they both are awesome mods and will let ya shoot a bit above rated 10 amps like you want.
If you want huge power go for a raptor build. @ 20 amps rated and can do up to 30 amps

 

[Dougiestyle]

digikey.com

Search okr-t 10

Search the same here and look for tutorials by Mamu

This will give you what you're asking for

 

[jonnychadootz]

pots which are rheostats are rated in watts.

What you want is a switching regulator. There is no way around using a switching regulator if you want adjust ability such as the ego twist, it also uses a switching regulator not just a pot. The pot or rheostat only changes the field in the inductor and at very low wattage. A 50watt pot would be an absolutely monstrosity and its efficiency would be horrible..

look up okr or "evercool mod" they both are awesome mods and will let ya shoot a bit above rated 10 amps like you want.
If you want huge power go for a raptor build. @ 20 amps rated and can do up to 30 amps

digikey.com

Search okr-t 10

Search the same here and look for tutorials by Mamu

This will give you what you're asking for

Very nice, this looks along the lines of what I have been searching for. Thanks you

 

[edyle]

I think you are right. What I am looking for is less a switch and more a regulator. Need to find 1 that works with 7.4 volts that the 18650's will output at the same time I want to limit it to a 10 amp draw (with the availability to up the amps slightly if needed) It appears I'm looking for something along the lines of potentiometer/rheostat. Which I'm having a hard time find as they are all seem to be rated in ohms/resistance. Much like what you would find on the bottom of an adjustable voltage EGO battery except in the having the ability to go slightly higher in the voltage output range. I could easily gauge it from there.

Maybe you should get something like this:
https://www.fasttech.com/p/2135407

 

[jonnychadootz]

Maybe you should get something like this:
https://www.fasttech.com/p/2135407

That looks beautiful. The only problem I can see with it is that it only fires @ 1.0 ohm the lowest. If it went slightly lower in the resistance dept (0.6) and above I would have been sold. I usually rock out somewhere in the 0.7 - 0.8 range. Damn, that is so close to what I am looking for.

 

[opensec]

you could also take the easy (more expensive) road and just get a DNA or Yihi chip

 

[opensec]

or
https://www.fasttech.com/products/0/10011134/1735400-modz-pack-d-na-30-mod-vw-modules

 

[edyle]

That looks beautiful. The only problem I can see with it is that it only fires @ 1.0 ohm the lowest. If it went slightly lower in the resistance dept (0.6) and above I would have been sold. I usually rock out somewhere in the 0.7 - 0.8 range. Damn, that is so close to what I am looking for.

If you usually use 0.7 - 0.8 ohms at 4 volts, then you just reconfigure your coil to run at 8 volts.
Example if you use a 0.8 ohm dual coil at 4 volts, you could use the same wire to make a 1.6 ohm single coil running at 8 volts.