No more sparks! - RC Groups
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Nov 06, 2017, 10:34 PM
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Mini-HowTo

No more sparks!


Here is a design for a very capable MOSFET on/off switch that can be built DYI and used with LiPos from 2s to 8s. Rated for 50 A continuous and 180 A peak.
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Nov 06, 2017, 11:26 PM
fools? at least i'm honest :)
korakora's Avatar
interesting, but 1st can you tell me what the dissadvantages of sparks when we plug the battery? if just only sparks i can deal with it
2nd if that mosfet burn (let say over current or getting hot) in a mid of flight it will get disconnected to the battery? if so it will more risk than w/o this
Nov 07, 2017, 12:44 AM
just look at it smokin'
z-matrix's Avatar
Quote:
Originally Posted by korakora
interesting, but 1st can you tell me what the dissadvantages of sparks when we plug the battery? if just only sparks i can deal with it
2nd if that mosfet burn (let say over current or getting hot) in a mid of flight it will get disconnected to the battery? if so it will more risk than w/o this
Hi!
Sparks destroy relay contacts, cause combustable gases to ignite, and might cause a fire
Mosfet needs to heat up over 100C (case tomperature) to start screaming
If battery voltage drops below minimum required gate-source voltage at current, mosfet will destroy itself
If there is no freewheel catch diode, the avalanche at turn-off can destroy the mosfet.
If the mosfet is not turned on-off fast enough it will explode if loaded heavily.
I would go for the mosfet instead of a relay anytime, tuning the performance if required.


Z
Nov 07, 2017, 01:11 AM
fools? at least i'm honest :)
korakora's Avatar
Hi Z, thanks for that detail information, but on the other hand if i'm not use Relay nor use switch (just plug the battery manually) i guess i'm not get any advantages with this method ? maybe a little bit of course no parts happy with the spike of voltage but compare with the risk (if something happen with the mosfet)

so yes i'm appreciate with this method, but not this time for me to implement this..maybe with my next project when it need it something like this
Nov 07, 2017, 02:06 AM
Registered User
Aren't you worried to drop from the sky having at least 6 (SIX!) FETs that control every motor of your platform, thus 24 such "time-bombs" in a quadcopter ?
Nov 07, 2017, 02:35 AM
fools? at least i'm honest :)
korakora's Avatar
Hahaha...yes you got my point, this is what i'm concerned of
is the risk is comparable with what we get advantages from this method or not, but if we get advantages just for to reduce sparks only i think still this is a good idea, but for my different purpose of course
Nov 07, 2017, 09:34 AM
Registered User
pilotpete2's Avatar
The sparking issue with higher cell count power systems is so easy to get around.
MPI makes a real good arming switch with a simple resistor and momentary switch bypass of the arming plug. Connect the battery, then when you're ready to arm, just push the button to charge the caps in the esc, then insert arming plug,,,,no sparky
Pete
Nov 10, 2017, 05:18 PM
Registered User
Agree with pilotpete, seems overkill... Sparks are usually generated by the inrush current required to charge the big capacitors. There already are many connectors with integrated resistors, or easy to build solutions where you first connect a smaller plug with a series resistor, then the bigger plug in parallel (bypassing the resistor). Cheap, reliable and do not add another failure point

While it's true that there are many points of failure on a multicopter, having a single point of failure handling 50A continuously seems to be a bad idea. Any vibration can short the thick copper wires, and any cold solder joint can spell disaster, plus the extra switch that can cause problems over time with in-flight vibrations. Not to mention the Mosfet itself having problems if overheating...
Nov 11, 2017, 01:24 AM
just look at it smokin'
z-matrix's Avatar
Quote:
Originally Posted by robca
Agree with pilotpete, seems overkill... Sparks are usually generated by the inrush current required to charge the big capacitors. There already are many connectors with integrated resistors, or easy to build solutions where you first connect a smaller plug with a series resistor, then the bigger plug in parallel (bypassing the resistor). Cheap, reliable and do not add another failure point

While it's true that there are many points of failure on a multicopter, having a single point of failure handling 50A continuously seems to be a bad idea. Any vibration can short the thick copper wires, and any cold solder joint can spell disaster, plus the extra switch that can cause problems over time with in-flight vibrations. Not to mention the Mosfet itself having problems if overheating...
The mosfet itself is perfect, if it is protected from over current, over voltage, and proporly sized then it will not overheat, the only downside i see is cost, size, weight, and additional resistance, the best mosfets now have 0.5 milliOhm R_ds_on, and are rated 700-1000 A peak (@30-60V)
A mosfet switch can be made to turn off at over-current, over-temperature, undervoltage, or via servo signal, and it can be made to turn back on with a hysteresis...


Z
Nov 11, 2017, 01:24 PM
Registered User
Quote:
Originally Posted by z-matrix
The mosfet itself is perfect, if it is protected from over current, over voltage, and proporly sized then it will not overheat, the only downside i see is cost, size, weight, and additional resistance
... and overkill

I mean, yes, there are times when a Mosfet is perfect as a high current switch, and I can think of many.

To avoid sparks, though, a Mosfet is overkill. A simple connector with a resistor (that can be switched out or in a parallel connector) is all you need. Light and safe, no additional points of failure compared to a standard LiPo connector (Deans or XT30/60/90, makes no difference).

Here's how (from: http://www.rc-warbirds.com/blog.php/tag/anti-spark/). Connect the smaller connector with a resistor, wait 5 seconds, connect the bigger one. No sparks, no extra circuitry
Nov 12, 2017, 12:06 AM
fools? at least i'm honest :)
korakora's Avatar
got it !!
now i remember how i need this "DIY" for

ok, i have timer to turn the (220v 600w) water pump every day with tottaly 4 times it turn on & off (2 ON & 2 OFF) but it wont take long, only 3-4 months i have to replace that timer (already bought 4 times), and i found relay get burn (even it can handle 2000w on the spec )

the questions is, can i use this method for my purpose? what parts on that scheme should i replaced?
Nov 12, 2017, 05:11 AM
Registered User
No you can't, because the switch is DC and your pump is AC.
The 2000W rating is for resistive load, your pump is inductive and the rating for this typically is like 15%.
Just get an industrial relay (contactor) with 10A AC3 rating and you're good.
Maybe use two contacts and have it interrupt L and N simultaneously, usually these have 3 or 4 working contacts.
Nov 12, 2017, 05:33 AM
fools? at least i'm honest :)
korakora's Avatar
Quote:
Originally Posted by learningarduino
..Just get an industrial relay (contactor) with 10A AC3 rating and you're good.
thanks for the link, but that was not a relay but just a connector (manual connector) that will back to stone age :lol i want automatically with timer, i cant find relay with AC3 grade
i think it would be great if i have to replace just a relay from that timer with bigger capacity or thats why i wondering a same way with this DIY, how to make that relay safe from burn because of spark when relay get connected (same idea but different scheme of course i'm understand)
Nov 12, 2017, 06:15 AM
Registered User
You need to control the contactor with the clock, as an amplifier for the relay contact in the clock.

It may be possible to replace the clocks relay with a tungsten based 16A relay, but even those may weld together.
I've seen stuck contacts from 8x58W flourescent lamps on these, even with Finder industry quality relays.
Nov 13, 2017, 10:10 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
"The spark is your friend" ( CastleCreations).
No spark, then input caps are dead and rest of controller will soon follow. Preferably taking plane with it
Explanation
www.rcgroups.com/forums/showthread.php?952523-too-long-battery-wires-will-kill-ESC-over-time-precautions-solutions-workarounds


On the other hand, too big a spark is bad for connectors. Not to mention my nerves.
Solution: use a cheap extra sacrificial connector.
The extra/very thin wire will reduce current, no need for resistor. You'll still hear a (weaker) spark as an ok signal/indication.

from
www.mgm-compro.com



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