When do you need a MOSFET?

[Rogerborg]

volts give you the oomph

Voltage determines how fast your motor wants to spin.  The current that your battery can supply limits how fast it can spin. 

It's a bit peculiar that batteries don't quote an amperage, and you have to work it out from capacity x C rating.  Either way, I reckon that current is where 7.4V lithiums shine over 7.2-8.4V nimh.

 

[bitofanidiot42]

I’m confused and my head’s spinning, my grasp on electricity is not good! 

Why do amps matter and not watts? Isn’t current just watts or amps? 
I’m probably making a fool of myself but everything I search up doesn’t seem to make sense, I just know that volts is the amount electricity that can get in and amps are how fast the electricity is flowing.

 

[EDcase]

So 7.2 Nimh would give lower fps than a 7.2 lipo?

Nope,  think what the motor is doing.  Its pulling back a piston against a spring until it releases at a certain point.

If it does it quickly or slowly doesn't matter much as it will release at the same point so the BB will be pushed with the same amount of air.

The only thing that will change is the time to next shot as the system cycles faster with more juice.

The 7.2v NiMh may struggle to actually pull the piston back against the spring so much more likely to get a locked gearbox where its partially pulled back but doesn't have enough juice to finish the cycle. Major pain...

Li-Po's have more current so even though its 7.4v it still has more power.  Think of it like a garden hose.  The voltage is the diameter of the pipe and the current is the water pressure.

 

[bitofanidiot42]

Thanks so much, I get it now

 

[Dan Robinson]

Watts is the amount of energy over time, amps is how much energy is being supplied. 

Ignore Watts for this purpose.  Its volts and mah. 

Another way to look at it is to compare to a gas gun.... 

Volts equal your pressure. 

Mah equal your storage capacity. 

Watts would be how much gas is discharged per shot so equal to the piston of an aeg. 

 

[bitofanidiot42]

I think I’ve learned what I came here for.

My conclusion is that a 7.4 Lipo will give a higher rpm than a 7.2 but the fps is always the same, I should probably get an m90 and hack away at the gearbox. It doesn’t matter too much for the rpm as my local site seldom allows full-auto.

As for MOSFET’s, they are for high power batteries to stop the carbon hurting the gun so quickly. They can be expensive and fancy and programmable or cheaper but not absolutely necessary for a cheaper gun as replacing the trigger contacts isn’t the end of the world (although they are becoming harder to find in the U.K.).

 

[Dan Robinson]

Seems fair, although, partly because of my job, id consider a basic mosfet an essential upgrade. 

If only to reduce unnecessary/easily preventable repairs going forward. 

 

[EDcase]

Yeah a MOSFET can be advantageous if you want fancy stuff like Burst, ActiveBreaking, Pre-Cocking etc.

If you don't need all that and you're using 7.4v batteries, something else will fail in the gearbox LONG before the trigger contacts become a problem so better to upgrade everything when you have to get in there to fix something.

 

[Skara]

Depends on the application.

If you play cqb, where you need a snappy gun, then a programmable mosfet is a nice, semi mandatory upgrade.

If you don't play cqb and you use full auto quite a bit (e.g. woodland games) then a mosfet becomes less important, it's still s nice thing to have onboard but not strictly mandatory.

These two apply to 11.1v batteries as well, and while it is true that a 11.1v will accelerate wear and tear on trigger contacts, it will still take an awful lot of time to render them unserviceable.

 

[bitofanidiot42]

Thanks lads,

I think I’m getting a bit off topic but toy guns are so complicated…

Shimming a gearbox. 
I know what it is but how necessary, should I do it as soon as I get a gun or only if I notice an issue with the gears’ contacts?

 

[Adolf Hamster]

 I know what it is but how necessary, should I do it as soon as I get a gun or only if I notice an issue with the gears’ contacts?

it's generally best to leave a gun alone especially if it's your first/only pew.

whilst a lot of ootb pews have less than perfect shimming, they're generally good enough for purpose. the risk is opening up the gun first time is fraught with the potential for making mistakes on reassembly and messing something up. whilst that's just part of the learning process i can say from experience (having not listened when being told this back in the day) that the walk of shame gets real old the tenth time you've done it when your latest build/experiment hasn't gone to plan and left you with a pew that's either performing terribly or straight up not working at all.

 

[EDcase]

Yep, as Hamster said, best not to tinker at first and just enjoy how it comes (unless it performs really badly then inform retailer)

Changing plugs to Deans is good if you know how but not critical.

Wait until you have a second toy as backup or if something breaks before exploring the wonderful world of teching.

 

[Rogerborg]

So 7.2 Nimh would give lower fps than a 7.2 lipo?

Oh, crikey, I've just noticed that you said fps rather than rps.

The battery and motor are irrelevant to the fps / kinetic energy of the BB.  They'll either pull the piston and spring back until it releases, or they wont.  You'll get the same energy each shot[*], all that will change by shoving more electrons around is the rate of fire and trigger response.

[*]Technically, if you're running the gun so fast that the sector gear pre-engages with the piston before it's all the way forward, you'll get lower energy, but that's the least of your concerns at that point since your piston and/or gears are going to shred.  If you ever put together a gun that can do that, then that's a case where you could use a smartish mosfet to reduce the rate of fire, while still retaining snappy semi-auto response.

Shimming your is satisfying, but there's always the possibility that you'll make it worse than the Chinese sweatshop child that did it in 30 seconds.  I'd echo the sentiment that it's something to do once you have two usable guns.  A quick-change spring makes the process of getting the gearbox apart and back together a lot less fraught. Some strong flat magnets are handy to stick under it to hold everything in place.

As usual, Luke at Negative cuts through the woo and just does it pragmatically, while tripping balls on red wine and gabapentin.

 

[Nick G]

Never, no one ever needs one of the things. Granted I am an absolute technophobe and see them as pointless stupid electronic crap that will fail at some point ! ?