Polyswitch Instead of Fuse

[emit]

I just finished installing an XCORTECH XET304U mosfet, 11.1V lipo, deans connectors and 16 gauge wiring in my Cyma cm028. The rate of fire has gone up from 11 rps to 19 rps and the trigger is much more responsive so a win so far.

 

I've been reading that some people remove their fuse to reduce the resistance in the wiring.  However I wanted piece of mind and in my search for an alternative I came across polyswitches as low resistance and resettable alternative to  fuses. I connected two 16V 10A polyswitches https://www.ebay.co.uk/itm/153536855033 in parallel and the gun seems to work fine however on stress testing it after about 15-20 seconds on full auto they get very hot and the rate of fire drops dramatically.

 

Using an IR thermometer I noticed that the temperature of the polyswitches went as high as 85 degrees Celsius and I then stopped testing as I was worried I'd burn them out. The wiring, battery and mosfet remained cool throught.

 

What worries me is the fact that they are stored alongside the battery in the plastic stock where out of sight during a game they could melt the cables, stock etc of they got too hot?

 

Has any got experience with polyswitches? Am I worried for nothing? Can it melt plastic/cables? Any way to lessen the risk? Add a third polyswitch in parallel for example?

[Davegolf]

TBH you are adding a potential risk by running them.

You are also adding to resistance by creating more connections, and again as such more points of failure.

 

I mean look at the cross sectional area of your 16g cable VS the fuse legs!

 

You have a lot of room with a full stock, a fuse is a SHTF failsafe, which should not need to be replaced regularly.

Safeguarding against shorts or overloading.

 

Semi auto can pull upto or over 30A, but not for long enough to bother a normal fuse.

Full auto anywhere between 12-20A

All based on a healthy setup.

 

The best setup without nuisance tripping or induced heat or other factors is a conventional automotive or glass fuse.

 

Just run a regular fuse holder, with whichever fuse rating that is stable on Semi spamming. Normally 30A.

 

Simplicity always wins!

 

[Sitting Duck]

Blade automotive type fuses are quite common...

 

There is the regular type which has aprox 4.3~4.5mm connectors

&

There is the mini/micro type fuse which has 2.8~3mm connectors like on motor

(the highest the mini/micro rating is about 30amp max afaik

the larger regular fuses go up much higher)

 

stock guns usually come with 20a or 25a as standard

and 30amp micro fuse "should" be OK for most moderately tweaked guns

 

pic of regular blade fuse install with mosfet...

 

 

Now if you use a mini fuse installed in a similar way

it will take up less room...

 

 

the purple "mini" fuse goes up to 30a max, not all places stock 30a mini

but they exist up to 30a without needing to go to larger size fuses with larger connectors

(basically just need to keep same 2.8mm motor connectors to hand for mini fuses)

 

When you first pull the trigger from stationary - there is a MASSIVE SURGE of say 80 amps

for a very brief (milliseconds) moment every time you fire say on semi or start of burst

 

Then the current draw settles down a bit and you get an "average draw" of say 11a to 15a on auto

 

Grab a test meter...

 

 

something like these - say £7:50 - ish

 

plug it in between gun & battery...

fire on auto for say 3~5 secs

(if worried - do it say 7.4v)

 

read off the amperage on auto burst

 

effectively DOUBLE that reading at least - so really a 30a fuse will suffice most instances

 

You should check the current/amp/wattage draw on your gun - it tells you a lot...

 

if in stock build you are much over say 12 amp your shimming/motor height is shit on a 350fps gun

I said STOCK BUILD - with a lame 24~27tpa ferrite motor pulling a m100 spring on 18:1 etc...

 

If you you use a HIGH SPEED motor then expect the amps to rise to 20a+

(yeah HS motors are not that great & this high amp/current draw will drain your battery quickly)

 

School science crap - AMPS x VOLTS = WATTS, - it is all relevent but amps is a good thing to monitor

The AMPS will generally stay the same - or there abouts on 7.4v or 11.1v, in same gun

just the watts increase from higher volts being used (is same gun I mean)

Wattage does matter to say the spec of the Mosfet & stuff but we generally focus on amps

 

TL-DR crap

buy a power meter thingy, test to see what your AMP DRAW is on auto

if it is higher than 12a on a basic std build then that is a bit high imho

this higher amp draw is what blows the fuses (rated in amps of course)

so check to ensure your gun isn't badly assembled/shimmed & drawing more amps than is should

 

if done correctly I'd say the CM.028a on say a m100 max spring, 27 tpa cyma motor etc... will draw 10a

(I'd still just shove in a 30a mini micro fuse)

 

Polyfuses require careful planning & anything a little more "electronic" from fleabay can be a gamble

You buy cheap 3034 mosfets from fleabay & is quite likely they are snide mosfets

and snide 3034 will not handle the whopping current draw, nor will they have same internal spike protection

(basically a cheap 1404 type mosfet only capable of handling half the current & will go tits up with voltage spikes)

You bought polyfuse, I'd bet nowhere near the so called advertised spec

(like buying a mobile phone battery or power bank from ebay/China - will only have half the so called capacity)

Your fuses might have been rated at XX amps or a load of XXX watts

but in real tests I'd lay it is perhaps more near half of the advertised load capacity

 

Naaaahhhhhh - just go with a mini auto blade fuse & be done with it

though the AK stock you use the regular one, heck even buy a fuse holder

(just if space is tight, create your own with 2.8mm connectors installed on its side so to speak)

[emit]

I will play it safe and replace them with the glass fuse that came with the gun.

[emit]

16 hours ago, Sitting Duck said:

 

 

if it is higher than 12a on a basic std build then that is a bit high imho

this higher amp draw is what blows the fuses (rated in amps of course)

so check to ensure your gun isn't badly assembled/shimmed & drawing more amps than is should

 

if done correctly I'd say the CM.028a on say a m100 max spring, 27 tpa cyma motor etc... will draw 10a

 

 

 

 

I just ordered a watt meter to see what the load is. The 12a and 10a mentioned - is this on single shots or under auto?

[Sitting Duck]

48 minutes ago, emit said:

 

I just ordered a watt meter to see what the load is. The 12a and 10a mentioned - is this on single shots or under auto?

 

Auto say 3~5 sec burst

takes a second or two to stabilise an "average" draw...

 

it initially starts off at 80a from still,

then lowers to say 5a on piston release (no load at all)

then as it picks up piston, spring tension load increases to say 40a

then releases 5a etc...

after a few seconds the meter calculates an "average" draw rate as it cycles again again

(about 11a - maybe a little less as a m95/100 with good compression on 455mm barrel = 350fps

in other words, if good seals, on a 455mm AK the spring will be a m95 say so less like 10a)

It is hard to accurately quantify the draw as a high speed motor really ramps up the amp draw

ferrite motors suffer stalling on higher springs than neodym etc...

so "load" on motor varies shitloads depending on build - which is basically what you are checking

(The amp draw on/from the motor itself)

 

You can lightly tweak the motor height if it sounds a bit rough

and together with the amp draw and your ears detecting the less shrilly whiney sound...

you can fine tune the motor's height/position to best sound/amp draw etc...

 

as said if worried do it on 7.4v than on 11.1v

note the amps

8.4v x 10a = 84 Watts (7.4v LiPo fully charged)

12.6v x 10a = 126 Watts (11.1v fully charged)

as each 3.7v can hold a max 4.2v

 

amps stay same, but Wattage changes due to different voltage

like I said Wattage is not what we are looking for, you are checking the Amps

(but the mosfet might have a max Wattage in its spec, though you'll be fine)

 

Just a few seconds on Auto - note the amps

should be around 11a give or take 1a

(personally think it should 10a or less, but old motor,

carbon build up increases resistance & less efficiency etc...)

 

a nicely tweaked gun should be 15a to 17a

(using a 25k~30k neodym motor 13:1 etc...)

BUT REALLY DEPENDS ON MOTOR/BUILD ITSELF

 

The higher the amps, the quicker the battery drains

and also usually more amps = more heat generated at motor

so it is wise to try to keep amps low or lower if possible if gun was built correctly

 

doubt if you got high amp draw above 15a like a tweaked gun

but the amp draw will tell you a lot & what fuse to use

(as said a 30a mini will be plenty unless draw is high or you have a short at motor cage)

[emit]

I just measured it with a 7.4V battery and the current peaked at just under 8A on full auto. The wattmeter doesn't  seem to show a summary or peaks after the testing stops so 7.9A was the highest number I observed during the test. Does this make it a very understressed setup indicating a weak spring or perhaps not much compression in the cylinder?

 

The gearbox is stock v3 and it chronos at 345 to 350 fps with 0.2g. It used to chrono at 335 before I replaced the stock   inner barrel with a 6.02 430mm ZCI and the Maple Leaf Hop bucking and nub.

 

Not sure how good the shimming is as I haven't taken the gearbox apart but tried to push on the gears through the  bushings' holes and there didn't appear to be any vertical play in the gears.

 

Seems to shoot well enough from what I can see at 30m in my garden with 0.3g with an ocassional  bb flying a few inches off  from the other ones but will have to take it skirmishing to see how it performs over longer distances.

[Davegolf]

You are porbably on an M100 spring, in which case your FPS / air seals are all very good

 

I was going to say, but held my tongue... dont get hung up on measuring amp/current, unless you have very expensive test kit - as you have found - it does not show peak draw all that well etc

The best test for basic kit is just do a sustained full auto burst (3 secs) and get the constant/settled amp draw.

 

The amp draw you are reading is showing you power consumed, this is affected mostly by higher or lower effort required to turn over/cycle the gearbox.

A more poweful spring, binding/tight shimming, sticky piston, gear meshing, etc, will all increase effort.

 

There are so many variables, but get a base reading.

Your gears should have a very small amount of vertical play, clearance for lube and a tiny amount of heat expansion. Talking a 3 thou 0.003" or 0.75mm.

 

Provided your motor height is in the ball park, you may find you can perfect it by doing the basic amp draw test above, adjust the height like 1/8 turn, re-test etc to get the lowest reading.

You should not be more than 1/4 turn either way than your start point really if mesh is set correctly.

 

You can use your base reading against lots of little tweaks - shimming / piston rails / gear setup etc

PS make sure your battery is always in the same state of charge for apples to apples test results!

[Sitting Duck]

To be honest I've never really bothered to test a stock gun.

 

Just a tweaked gun usually starts at around 11a to 17a.

 

The Cyma motor is 27 tpa but weak ferrite, has some torque from 27tpa that keeps amps low, but moment the weak magnets go under load these ferrite motors can stall on a m110/120 spring.

 

Due to long barrel, bit of creep, you probably only need m95/100 to hit 350fps.

 

The gun is shooting then leave it as is, it was to get a rough idea to see if a large draw was blowing the polyfuses. As such, it seems the polyfuse is shit and a simple 25a to 30a mini fuse will be easily enough to provide protection without tripping/blowing regularly.

 

All it was, to get a very basic benchmark, to study the amperage rising & falling requires more equipment & in the end matters little how detailed it all is for most of us. It starts at 80a at first then drops/rises etc....

 

If it blows a fuse, ergh it blows the fuse, end of the day we are just checking the average draw to compare against. Bit like studying the kinetic energy & bollox on joules/weights, in the end we want it near the site's limit but just under the chrono in most cases, than an Open University course in the study of BB mass & Magnus bollox. If it shoots straight, stays airborne, hits the target most times that will do with the actual scientific study crap.

 

Gun is not drawing silly amps, so the fleabay polyfuse is shit it seems, fit the blade fuse & you are golden

[emit]

Thank you both, its been very educational 🙂

[Sitting Duck]

5 hours ago, emit said:

Thank you both, its been very educational 🙂

 

sir we are all STILL LEARNING - with every bastard issue with these bastard toy guns present

yup at times these guns are bastards, bastard coated bastards