Electrical contacts melting plastic

[kuchervano]

On a new ICS AK74. Using a custom 8.4V NiMH battery at 4300 mAh large. Sad part is that it worked fine for a day or so, then gun stopped working. Opened it all up, noticed that the contact inside the gearbox that the trigger mechanism engages is loose, used a screw driver to bend it into place, made sure the insert doesnt get stuck between two, put it all back together and fired some more. After a dozen or so shots on semi/full auto the weapon ceised to function again. Repeated the disassembly process and noticed melted plastic marks around one of the contacts (the one immediately from the battery). Pictures attached



Im not very good with electricities and so have no idea why this happens or how to remedy it. Help please?



edit: pics redone

[mcguyver]

Linky no worky

[hattrick]

*radio* pictures down, need backup

[mcguyver]

Are those contacts dry or was there some grease in there? I have seen grease contamination of the contacts on Ver 2 mechboxes lead to damaged switches, but usually there are tell-tale signs of repeated arc formation. This shows up as corroded or vapourized metal. I have not seen any switch contacts themselves melt like what your pics show.

Was your fuse still intact (unblown) and did the gun behave normally during firing? Was the rate of fire slow or normal?

It takes alot of current to melt switch contacts like that.

[Kokanee]

why a 4300mah battery? 3000mah would be more than overkill for that aeg

[kuchervano]

2macguyver - rate of fire was normal as far as i can tell, steady tra-ta-ta-ta on full auto, im sure the p90 i got with 9.4v 1100mah is shooting much faster than this AK did.
There is melted/burnt parts on the center wedge that closes this switch, on the side of contact that melted. There are also spark marks on all three pieces of this assembly, but as i noted only that piece melted.
As far as contact clenliness is concerned, should have been fine since it wasnt opened, dipped in water or anything like that. The fuse is not blown but the strand inside also shows signs of heating up, a lot.

2kokanee - because i managed to get one made for a bit over $100, and since thats the best $/mAh i saw just went for it. Never suspected it could lead so such damage.

[mcguyver]

What have you done to this gun and what is your fuse rating?

[kuchervano]

I have done nothing to the gun except what i already mentioned, not sure about previous owner.
The fuse is rated *all gasp and point fingers* 30A 250V.
We found the weak link, and that link is me...

[Scarecrow]

Originally Posted by kuchervano

We found the weak link, and that link is me...

Admiting it to yourself is your first step towards recovery.

I'm so-so with eletricities too, and methinks that 4000mah with 30A 250V fuse is waaaaaaay too much, hence your switch cooking.

[kuchervano]

So what do I have to do? I think i can repair the trigger box with some slow curing epoxy and a lot of swearing. But the fuse, plugs, contacts, change them to what?

I read over this post however it deals with plugs heating up and not the trigger contacts. I re-checked the contacts throughout the length of the wire and all of them seem fine and dandy

[mcguyver]

A battery will only discharge the current that a load demands. This means that the motor was drawing enough current (or an added load of a slight short circuit) to melt that switch. It doesn't matter whether you have a 2000 mah battery or a 10000000 mah battery. The battery will only discharge current as the load demands. Batteries don't "push" out current, the have current pulled out of them.

Think of your problem in those terms and go from there.

If your fuse was showing signs of thermal damage as you say, then the current was definately high and not that is was a defective switch. I would look into your motor and gearbox shimming, spring load, etc.

The more load the motor is under, the more current it will draw. An overshimmed gearbox adds load. A heavy spring adds alot of load, an over-engaged motor adds load. All these things can add up to way more current draw than is needed or than the switch can handle.

[kuchervano]

Roger that, thanks.

edit: any other contributions/suggestions are still welcome!

[Greylocks]

I know it's a bit extreme, but I would double check the motor to find out why it draws so much current. Is this a stock spring? Is the mechbox properly assembled?

Then I'd go back to a 8.4 2000mah battery (or close to it).

That fuse is insane. Bring it down to a 20 or 25a.

[kuchervano]

Thank you for the input Greylocks.
And update for those who are following this exciting drama: 9.6V 1100 mAh battery does not heat/melt/burn the contacts with the motor turning free. The 8.4V 4300 mAh battery does. Going to Canadian Tire now to get a multimeter to measure the resistance/voltage/amperage of the circuit.
Reference fuse, my P90 has a 15A one, this had 30A; 20A sounds like a compromise?

[mcguyver]

If you know someone who has a clamp-on current meter that will read DC amps, try that. Resistance is subjective as it can and will fluctuate with current, so any reading you make on the system (without motor) will be useless.

You can't really measure the motor either and get a useful result. The DCR will change depending on commutator engagement with the poles.

You can get DMM's that will read DC amps if you hook it in series with the gun circuit (doesn't matter where), but they are limited to 10 amps usually and that is not sufficient.

A good digital clamp-on meter is still the best, but the cheap crap ones are $200 and up. Mine was around $600 and I use it for work everyday. That's why I said to borrow one from a friendly electrician.

The reason that the smaller battery doesn't melt the contacts is because it is simply not capable of discharging the same amount of current that the large one is. A multimeter will show a larger volyage drop with the smaller battery than with the large one.

Remember heat is a factor based on power. Current times voltage = power. The small battery will drop voltage as it trys to discharge current at the same rate as the large one. The large one will not drop it's voltage nearly as easily and will deliver the bigger current while manitaining it's voltage.

This still does not solve the obviously excessive current consumption of your gun. The large battery did it's job, but the motor and it's connected load were in excess of the switch capacity.