We are looking at adding several LED indicators to our robot, and I’m leaning toward putting together a small custom circuit to control them. It would consist of a FET for each LED, with the drain and source connected to 12V and GND and the gate connected to a DIO. I was going to breadboard it with all of the FETs, LEDs and current-limiting resistors on one board, with 12V and GND connected to the PD with a 20A breaker. Total current draw will be <100mA.
Reading through the rules, R40 talks about wire gauging - specifically stating that you must use 18AWG wire for the 20A circuits. It also has the following note:
“The branch circuit may include intermediate elements such as COTS connectors, splices, COTS flexible/rolling/sliding contacts, and COTS slip rings, as long as the entire electrical pathway is via appropriately gauged conductors.”
I’m not really sure how to interpret the last statement - ‘appropriately gauged conductors’. Do they mean that the entire circuit must be able to withstand 20A, or that your conductors must be appropriate for the calculated current draw of your circuit? Obviously there is a big difference between 100mA and 20A, and making the entire custom circuit out of 18AWG wire would be a pain. I would like to run the 18AWG wire to my breadboard and use much smaller wires on the board itself, but I don’t want to have that rejected by the inspector at the competition. I’m sure this issue has been dealt with before by other teams, can anyone provide some insight into this?
Thanks
Jeffrey Hill
Mentor, Team 2930 - Glacier Peak Taterbotz
First, I am not an inspector. Do not quote me in an attempt to justify your wiring to an inspector who takes issue with it. However…
It is my interpretation that the power wires leading to a custom circuit must be sized appropriately for the circuit breaker protecting them, but the circuit itself need not be able to survive a fault which causes more current to flow than it was designed for. I believe that the wiring within a custom circuit is at the discretion of the team using that circuit.
We were thinking about doing something similar. My first thought was to create a PCB for the LEDs and just use the signal port off of the digital side car to power and control the lights with a common ground. We were able to bread board it quickly and if you put the LEDs in parallel you can have quite a few without them dimming considerably. Would that work for you?
We’re only planning on a few LEDs, individually controlled. My first thought was to use the DIOs directly, but they aren’t rated to handle the current - IIRC they can only do 5mA. The solenoid bumper might be an option, I think they can handle more current. The circuit itself isn’t really a problem, I have a simple design drawn up - I was mainly just wondering if I’d run into problems during the inspection. To be safe, I may put together an alternate set that runs off the Spike relays that we could swap out quickly if needed. Thanks everyone for your advice!
Keith,
There are a few other threads were this is discussed. The DIO comes directly from the NI digital module that is speced at +/- 2ma and is pulled up to +5 on the DSC through a 10k resistor.
This should be acceptable to inspectors. At the FL regional last year, we dealt with a few custom circuits, and as long as the initial wire was 18g, we weren’t too concerned, as long as it followed all of the other custom circuit rules. Just be prepared to explain the circuit to the inspectors and the judges, as they will likely take an interest.
Jeffrey,
I should have answered this earlier. The 20 amp breaker rule requires that you use #18 to feed your custom circuit. However, you may add a smaller fuse to the custom circuit if you feel it is appropriate.
F. Custom circuits and sensors powered via the cRIO-FRC or the Digital Sidecar are protected by the breaker on the circuit(s) supplying those devices. Power feeds to all other custom circuits must be protected with a dedicated 20-amp circuit breaker on the PD Board.
In addition to the required branch power circuit breakers, smaller value fuses or breakers may be incorporated into custom circuits for additional protection.
No one expects that wiring on a custom circuit board be #18. Common electrical practice would suggest that a 1 amp fuse would protect anything on the board that is wired with smaller gauge wire. LEDs for instance are rarely wired with #18 gauge wire.
so if what I am reading is correct my design for a display is legal:
3 displays (Triangle, Circle, Square), each controlled by a spike.
2 have 3.1 v dissipation per LED (red and white), so they will be in 3 strings of 4 (to get approx 12v total). the 3 strands of 4 per display would be spliced into 18 gauge wire. each individual strand would use 22 gauge wire (what we have waay to much of in stock).
the blue would be the same only using 4 strands of 3 (4.x dissipation).
is my design legal or do i need fuses or bigger wire somewhere?
Larry,
If I understand you right, you are saying the LEDs you are planning on using drop 3.1 volt per LED? Can you let us know the part number for these? If you connect 4 3.1 volt drops in series, that will be 12.4 volts. If the battery is fully charged when you turn them on, they will draw lot’s of current. When the battery voltage drops, they will shut off or be very dim. What you want is two or three in series with an appropriate sized resistor in series. It then sound like you are going to wire two or three of these series strings in parallel, is that correct? How big are these displays going to be? Are the LEDs mounted to something? A circuit board perhaps?
I question the 3.1 volt spec. Normally I see a drop of less than that, but these may have a built in resistor or current limit built in. Working with LEDs it is good practice to add the series resistor to prevent over current. Can you provide a link to where you bought them.
OK,
You are buying “Super Bright” LEDs. They do have a higher voltage drop but the same rule applies for current limits. You do want to have a resistor in the string to keep the current at specified level, usually in the 20-40 ma range. If you are making each shape with parallel strings, then the wiring within each shape can be #22 with the resistors wired right at the string. If you wire each shape back to a Spike, make that wiring #18. Each shape is then the custom circuit and all rules are satisfied.
The only caveat I can supply at this point is the relative brightness will be a subjective thing. If the inspector, ref or FTA think they are too bright or if a team complains they are too bright, it will be up to you to correct the problem. That will mean, make them dimmer, or remove the breakers that feed them.
**Do not try to avoid using a resistor. ** LEDs are not linear devices, meaning that when they get a little warm, their current consumption increases. More current leads to more heat leading to more current - this is known as thermal runaway. It ends in smoke.
Go ahead and use your four LEDs in series, but then add a small-value resistor - say, 20 Ohms or so (do the math) - in series. This will prevent thermal runaway.
The voltage drop across an LED is determined by its chemistry (and therefore the color it produces). Blue & White run at about 3.5 volts, red runs at about 1.8 volts. This is a physical quantity, not generally subject to change. Keep this in mind.
My advice is to use a milliammeter to measure the circuit current, and adjust your circuit to reach the necessary current, ignoring voltage.
Oh, and if you want, you can use ONE spike to power 2 colors. Hint: what’s the “D” in LED stand for??
if the LEDs were not ordered yet, i will see about getting the kind with the resistors pre-installed (ebay has ones like that for 12v). if they were ordered, i will toss in a resistor.
I forgot about diodes not being linear… the flu must have rotted my brain.