Archive for BRIT NERF Home of the British NERF Community
 


       BRIT NERF Forum Index -> General Nerf Discussion
Minky

PSA: Don't be stupid. Check, check.. Then check again.

Been meaning to put this out for awhile..  This is a story of how Not to do things...

So I had wired 4 LEDs in series in my magwell...



...4 in series proved too much for my 6v test pack to light but I wanted to test that circuit before moving on. My low voltage circuit is running off a second Mosfet,  but I'd tested it's use. It seemed to be adding a load of resistance into the circuit but when I operated the switch it worked so I was happy it was all kosher. So after talking to folk I decided to plug in my Lipo. Connected up..  No problem.. But nothing was working..  After I couple of seconds I was spooked and began to disconnect...

FWISHHH!

The wire on the JST connectors right next to the XT60 I was disconnecting went up in flame and smoke.  For a second I thought it was the LiPo itself... In this moment...



The wire burnt through and..



The JST desoldered itself.. I don't mind saying it sh*t me up. Before the smoke had cleared my missus had bought a fire extinguisher. In honesty if the low awg wire hadn't acted as a fuse it could have been worse. A room full of LiPo cloud worse.
LiPo got a new home..



But what went wrong!?! The circuit worked! I'd tested the bleeding thing! It worked! At this stage I hadn't finished the other end of the low power loom so I tested the loom from that end.

First thing I found...



...I hadn't even wired the damn LEDs. But this wasn't what had caused the 'FWISHHH' so I carried on back along the circuit.



...No problems there and finally we get to...



...the Mosfet. Note the melted up negative from the JST. Note the wire colours..  Those of you who have wired Mosfets previously may not notice it straight away..  Yeah.. Yeah,  I'd wired it back to front. By doing this I'd put a positive on the Source the battery negative on the Drain and the low power negative on the gate. This somehow allowed the low power loom to work across the 10k Resistor.. I can't remember exactly the sequence of events,  it could be that I flipped the killswitch before going to unplug the XT60.. Whatever, the voltage fluctuated across the Gate and the Mosfet plugged the positive of the LiPo into it's negative and bringing the 'FWISHH'.

Moral of the story is even when you've done something successfully once, even when you recite the order of the pins to yourself as you're walking along,  even when the circuit works! Check, CHECK, CHECK!!
blindgeekuk

Wiring a mosfet back to front is easy to do, I did it the same night as you!
Boff

This is why I don't wire my FETs with the resistor across the top like that. It becomes very easy to be able to create shorts and back circuits like this. Put it out of the way on the gate pin and it'll make life much, much easier when doing checks. Smile

I'm impressed there was that much flame and smoke, however. The worst I've had from a backwards FET was that it just exploded. I still have the pieces pinned to a notice board somewhere. Or was that a diode malfunction, I can't remember... Too much pixie smoke.
OldNoob

Also kids- An object lesson in why we do standard wiring colours, fixed connector choices like XT60 for high current and why checking and double checking is important.
See that JST? That's what happens in stalls to thin nasty wire. It would have been fine with signal current, as designed.
Minky- Great to own your mistakes, too many people act like they get it right every time. Boff can attest to my penchant for smoking FETs and UKNerfwar can tell you about the blaster I sent put that shot backwards.
UKNerfWar

OldNoob wrote:
...UKNerfwar can tell you about the blaster I sent put that shot backwards.


Ah, the famous dart hoover you built for me. That was fun.

We've all done it at one time or another but owning your mistakes is a rare thing these days. Hats off to you for that.

I find that if you look at a thing for too long you become blind. At work I always walk away and have a cuppa. Check one last time and then push the button. I've not killed anyone yet. I do the same with Nerf.
Boff

Let me think about all the switches, FETs, LED, microcontrollers, motors and other electronics I've blown up in my career. Oh wait, don't... The existential dread about the cost will set in and we're all fucked. Razz
Minky

@BGUK -  It doesn't help the majority of us hit this stuff after a full days work with tired eyes..  Of course I only had the 3 pins to choose from. I don't know how you do how well you do!

@ON - The Modulus mod is there to teach me what to do,  the skills for it,  and the pitfalls..  Though I could of done without that one lol It really does show the difference in wire quality though doesn't it? The same current must have been passing through that 22awg silicone wire and it isn't marked!

@UKNW -  I really like the idea of the strategic brew. Walk away,  have a I think,  maybe make a list of what to check when you walk back..  Same with the BGUK comment it's probably not a great idea to test last thing at night when you're tired and just finished putting a lot of work into this thing that you Want to work.

@Boff -  I was fortunate that the Mosfet and the battery came out the other side unscathed,  I didn't realise eyepro was a good idea for Mosfet testing!

@ON and Boff -  thanks with your advice on how to test the LiPo post short and what to look out for on that.
OldNoob

I totally forgot about the pvc insulation and higher quality wire! There's another thing well demonstrated, remember people pvc likes to do the burny burny dance at a much lower temperature and a few more pence per meter spent on a good quality wire will always be good value for money.

       BRIT NERF Forum Index -> General Nerf Discussion
Page 1 of 1
Create your own free forum | Buy a domain to use with your forum