I thought I remember reading in years past that the general recommendation was to replace the standard 20A automotive fuse on the Spikes with the auto-resetting fuses we use everywhere else. I don’t remember this violating any of the rules, yet <R65> and the Q&A this year say differently… was I incorrect in past years? I don’t remember this only applying to the compressor.
-Rob
Yes, you were incorrect in past years. At least for the past 6 seasons I’ve been involved with, this same rule has been in place.
The only allowed use of a 20-amp breaker instead of a fuse has been for the air compressor.
This rule has been the same since at least 2006, and is only in place because the inrush current of the compressor can occasionally surge over 20 amps for a brief moment causing them to trip.
R60 in 2010 allowed the breaker on any Spike. We used window motors on Spikes with breakers without issue.
R55 in 2011 banned it again.
This is another rule that really makes no sense. But, it is a rule.
The inrush current profile of a compressor is very different from that of a potentially stalled motor. The protection profile of a self reseting breaker is very different from that of a one time fuse. This isn’t an arbitrary rule.
Actually, it makes a lot of sense. If you put a motor on the Spike and it pulls too much current under load, the fuse will blow, end of story.
If you have a reset breaker in there, it will snap, reset, snap again, reset, over and over again, not providing proper circuit protection and causing something else more important to become the fuse, sometimes with frightening consequences.
The use of the resetting breaker in the compressor is a concession suggested by, I believe, the manufacturer of the compressor. It has a high in-rush current on startup, but a much lower running current. Other motors don’t have that characteristic.
Dr. Bob
Chairman’s Award is not about building the robot. Every team builds a robot.
To me, it’s like saying you can’t use a CIM on a Victor because the stall current of a CIM is greater than the current rating of the Victor. The Victor can handle significantly more current for quite a while, longer than it will take for the breaker to trip.
The stall current for all of the Spike-legal motors is below or very slightly above 20a anyway, so it’s irrelevant since the Spike can supply stall current indefinitely. The resetting breaker gives me more peace of mind that a one-time fuse, in terms of vehicle reliability and maintenance.
I have never understood why spikes have fuses. They are already current protected by the PD board with an auto resetting breaker. Furthermore, I have (outside of FIRST) used a spike to switch 40A+ with no issue. I personally like the idea of downgrading the auto resetter in the PD board to a 10A or 15A. At least this way the system will give you warning that a mechanism is drawing too much power with a temporary shut down rather than a “oh too bad, no more using that function for the rest of the match, better luck next time!” blown fuse.
Stop me if you’ve heard this before…
The breakers are there and sized to protect the wiring from melting and bursting into flame, not the electrical components along that wiring.
Electrical components must protect themselves, as their circuit designers are the only ones who know what the design limitations are.
Wires are slow to burn components, but electronics are quick to fry components.
The Snap-Action circuit breakers we use are slow to respond components to match wiring characteristics of slow to burn.
That’s not true of the RS775-12. Per the datasheet on the breakers, you’d very likely be able to run it at +20A for extended periods of time if not continuously. http://www.snapaction.net/pdf/vb3.pdf
Stall for that motor is 30a (spec).
At stall, the breaker will trip between 3.9 and 47 seconds.
The motor will certainly not last 47 seconds at stall.
Spike specs: According to Vex Pro site
-20a continuous
-100a surge for <2 seconds
-Design will accept 20a auto-resetting circuit breaker
100a is 500% over the rated current of 20a for 2s. The surge current rating of the Victor is only half that, 250% for the same time period. We frequently run motors rated for 200% rated stall current through the Victors without issue.
It’s still like saying a Victor can’t ever handle more than 60a (rated continuous current) so we can’t connect a possibly larger load to it. The Spike will live with a 20a breaker and any Spike-legal motors. The 20a breaker will still trip if the wire shorts, which could already happen with a 20a breaker in a Spike for the pump.
Electrical components must protect themselves, as their circuit designers are the only ones who know what the design limitations are.
That’s an excellent point but then how come we don’t stick with the same logic that was used to design the spikes and see fuses on all electrical components? Albeit the jags have an over temperature shutdown but what of the Talons and Victors? The Victor manual specifically states:
Ensure there is a circuit breaker either inline with
the 12V power input to the speed controller, or inline
with the motor. Use an appropriate circuit breaker for
your application to ensure that long term exposure to
a stalled motor (high currents) will not overheat the
Victor.
Right, but the motor can operate between 20A-<30A, above the spikes continuous rating. Potentially up to 47 seconds with the snap action breaker.
The Jaguar is rated as follows:
-40a continuous
-60a for up to 2s
-100a for up to 0.2s
At 60a, the 40a snap action MX5 will trip between 3.9 and 47 seconds.
According to the Jaguar’s spec, it has exceeded its spec limit before the best-case breaker trip.
So maybe we shouldn’t use Jaguars, because we can pull more current with a CIM than they are rated for before the breaker will trip?
The point is that, in our use-case, it is reasonable to exceed the continuous current ratings for a short period of time. The Spike also has the highest relative current rating of any FRC-legal controller.
The inclusion of fuses on electrical components is a rather interesting topic. For the most part, fuses are used for safety. When examining the failure mode of a particular circuit, you need to consider the safety implications of such a failure. A few examples:
So, what about Spikes and speed controllers?
Well, with Spikes the failure mode can be pretty severe - the contacts inside a Spike are pretty large, and the plastic holding the whole thing together can melt. As a result, a Spike can short circuit, causing your breaker on the PDB to trip, or it could short the outputs to the inputs, causing the motor hooked up to it to be permanently on - a potentially much more dangerous situation, depending on what the motor is doing.
What about a speed controller? Well, speed controllers are much more complex. Generally speaking, failures in these end up with an open circuit - the motor simply won’t work. This is a much safer failure mode.
Fuses can also be used to protect expensive equipment. For example, many surge protectors have fuses in them - the fuse will blow before your computer or TV is destroyed.
In this case I think it’s more analogous to our spike scenario to say that we shouldn’t bypass/decrease the over temp protection on the jaguar since the fuse seems to be for internal protection of the spike.
I agree with that but unfortunately the rules are the rules. I’m also curious to know how those 20A snap actions typically behave above 20A. The data sheet gives a huge range…
the robot moves with the safety enabled. And when the joystick moves forward and back one light is green the other red. When turning left both lights are red and when turning right both lights are green. They were a solid Orange when enabled but not moving. The lights still flash orange when not enabled. Is that okay?
Otherwise I would like to thank everyone for there help in fixing the problem!
That’s perfect. I think you might be in the wrong thread, though.
I, too, have always been puzzled by why the auto-resetting circuit breakers are permitted in the PDB, but not in the Spike. In all honesty, I don’t recall ever hearing an explanation that satisfied my curiosity.
Does anybody really know why the rule is there, rather than just speculating as to why they think it is there?
I’ve heard all kinds of rumors, but still don’t feel that I’ve really heard why from somebody who had inside knowledge of the reasoning used by the authors of the rule.
One “rumor” I have heard is that snap-action auto-reset breakers are not allowed in the Spike as the wider terminals of the auto-reset breaker (as compared to a standard automotive fuse) bend the Spike terminals out slightly, leading to problems when a standard automotive fuse is later used.
I do understand the difference in the two above cases. Clearly, the “single trip” breaker provides better protection.
However, if better protection is the goal, why are auto-reset breakers allowed in the PDB? Does the PDB respond to the presence of an auto-reset breaker differently than the Spike does?
I am particularly seeking to understand why it is okay to use auto-reset breakers in the PDB but not in the Spike.
Not really, the self resetting circuit breaker in effect becomes a current limiting device.
Helping other teams troubleshoot problems I’ve seen circuits that were shorted and were causing rapid cycling of the breaker and I’ve not seen damage to the wire, which is the purpose of a circuit protection device.
On automobiles self resetting circuit breakers are often used for items that are considered mission critical like headlights and wipers. While I’ve seen shorts in those circuits and people driving them for extended periods of time with the breaker cycling I’ve not seen damage to the wire other than what was causing the short.