High current circuit breaker design

[toomas]

Hi,
I'm desiging a remotely operated circuit breaker for local robowars competition. It's a safety feature for cases when the power mosfets or relays fail and people don't want to go close the robot to cut wires. It doesn't have to be resettable, but it'd be nice. What are your ideas how to do it best?

I was told that the most powerful robot draws 350A 12V continous with 1kA peaks (although it may be overestimation).

First idea was to use some high current DC contactor with a series fuse to protect contactor from overcurrent, but now I'm thinking it may not be enough, because it's meant to be a safety device. There's a chance that the contactor may weld short and fuse won't blow while the robot still draws a kW or two. We can't afford to overrate contator much, because these things will be expensive.

So it got me thinking that maybe there are fuses that can be tripped with separate control lines. A DIY solution would be to severely overheat a normal fuse to make it trip. I think I can't trip the fuse (maybe 300A) by shorting main battery to ground because of safety

[M. András]

may not be suitable, or would be oversized, but look at compact  circuit breakers and air operated circuit breakers for distribution cabinets

[nctnico]

There has to be some kind of fuse between the battery and the power supply so I'd go crowbar. A big fat SCR will do nicely to short the supply and blow the fuse.

[H.O]

There are circuit breakers with remote trip feature. The ETA 437 series has an optional solenoid for example.
Search for remote trip and/or shunt release and you might find something COTS(ish) instead of DIY. A 300A unit will be both big and expensive though - I'd think.

For emergency stop of machinery and such you generally put two contactors in series, with a safety relay monitoring the NC auxillary contacts of the contactors. If either of the contactors gets stuck the monitoring system detects that and won't enable the system and it's not very likely that both contactors fail at the same time so even if one contactor gets stuck the other one will cut the power. For a battle robot you could probably get away without a proper safety relay but TWO 300A contactors is going to set you back a pretty penny - and take up space.

[T3sl4co1l]

Easy: turn the contactor sideways, so it crowbars the fuse.  Now contact welding is a feature, not a bug!

Tim

[Ian.M]

If you are in a country that legally permits D.I.Y. pyrotechnics, *AND* you can get it past the competition safety rules, an electrically triggered explosive disconnect would be possible.  Otherwise crowbarring a fuse is almost certainly the best option.

[toomas]

I forgot to mention that cost is actually quite important, because this would be mandatory for all robots who enter arena. There may be 20 of them.

I'm starting to like an idea of blowing the main fuse with a beefy SRC. It's smaller, lighter and cheaper and than using two contactors or even one. Each contestant should use a fuse that matches their battery pack (blowing up battery pack would be bad), the rest of circuit is the same.

This fuse seems like a good choice. It's available from 23A to 200A and is 1$ piece. (I'm assuming 350A robot doesn't really continously draw 350A and can use 200A fuse)
http://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/bolt-down-fuses/littelfuse_midi_datasheet.pdf

From my research max burst discharge rate for typical 1Ah+ li-ion RC car battery is 50C or better (10-20 seconds). I should come up with a rule of thumb to contestants for choosing a right fuse. Previously mentioned 23A fuse will trip at 100A in 1 second, 200A will trip at 500A in 1 second. Something like FuseRating = 10 x BatteryAh, rounded down

Also how beefy SCR do I need? I'm thinking about putting 2 or more of these n-ch mosfets in parallel (rated 429A silicon, 240A package, 24V) https://www.tme.eu/en/Document/44a3ce3419cd1a3f6e1cf6952a438f0f/auirf1324s-7p.pdf

[T3sl4co1l]

SCR is not a MOSFET...

Also, transistors blow in microseconds, fuses blow in milliseconds.  Guess which one will win!

SCRs have the unique distinction of handling just enough fault current that it is practical to fuse them.  In industrial applications, special super-fast fuses are needed to protect them: "semiconductor" fuses.  At lower voltages, they are robust enough that they can be used to blow conventional fuses.

The important parameters are:
Surge current rating -- can it withstand the fault current needed to blow the fuse?
I^2*t for fusing -- can it melt the fuse before it does?

Offhand, you may need a stud type SCR to do this, but I don't think you'll have any problem finding a part that can.  Whether it's cheap and available, may vary...

Automotive relays/contactors may be more attractive; they're often quite cheap for their ratings.

Tim

[rstofer]

The starter solenoid for a car can handle several hundred amps for a short period of time.
Shorting the battery isn't going to do anything to improve battery life.

[Ian.M]

OTOH better to crowbar and risk tripping the battery protection rendering the battery pack useless, than risk serious personal injury trying to shut down a runaway flailbot or thwackbot.

You'd have a two stage emergency shutdown: first level would shut off power by the bot's normal method and also hold all ESCs and CPU boards in reset, and the second level would, as a last resort trigger the crowbar.

Its also probably worth having a rule that the main battery connector must be pluggable, with a stout loop that can be hooked onto to disconnect it, and easily accessable from the underside.  Then, worst case, the bot can be flipped with a spade, and the connector unplugged with a button hook.

[max_torque]

Most robot wars type competitions have both a Permissive Action Link (PAL) and a remote Emergency Stop Contactor (ESC).

The PAL is normally a manually installed component, without which no battery power can be drawn.  (often a larger sized Anderson Plug wired as a loop).  This is only fitted once the Robot is in the arena and away from the majority of humans.  This is the first line of defense against un-expected operation, and is manually fitted / removed for a good reason!


The ESC is an entirely separate device, that can remotely cut battery power.  This is intended to disable / immobilise the robot should a control system be damaged or run out of control.  It can also be used to prevent further damage from Fires etc following electrical short circuits caused by battle damage etc.

However, once the PAL is fitted the Robot is considered UNSAFE at all times, and a malfunctioning ESC cannot result in human injury as the arena is expressly designed to contain the worst case robot failure / energy release.

The PAL therefore is the critical link and means the ESC does not have to be 100% fail safe (which would require multiple contactors etc )

[duak]

Automotive starter solenoids are probably capable and are tough and inexpensive.

Gigavac (gigavac.com) makes some small, high current contactors that are more completely specified but are more expensive than starter solenoids.

Littelfuse makes the Polyswitch resetable fuse.  I believe they use conductive particles that are separated at high temperatures.  I see they have an automotive series although the maximum current ratings are not in the 350 A range.  http://www.littelfuse.com/products/resettable-ptcs/bladed-devices.aspx

Cheers,