Custom LED Circuit - Wire Gauge?

[jhill0914]

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

[Alan Anderson]

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 Do not quote me in an attempt to justify your wiring to an inspector who takes issue with it.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.

[MrForbes]

Your custom circuit could include a smaller fuse that would protect the smaller wiring in your custom circuit, right?

[Sh1ine]

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?

[drakesword]

How many lights are we talking?

Cant they be powered and controlled directly by the digital side car or solenoid bumper?

[jhill0914]

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!

[Al Skierkiewicz]

Alan is correct. The breaker and the wire it protects are sized accordingly in the rules. 20 amp breaker protects #18 AWG or larger wire.

[wireties]

You could use high-efficiency LEDs straight from the DIO channels - or run them from solenoid channels.

[Al Skierkiewicz]

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.

[wireties]

Originally Posted by Al Skierkiewicz

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 is a fine point I reckon (and a very understandable mis-interpretation) but my advice comes from a detailed reading of the NI 9403 spec. The DSC is NOT strictly spec-ed for 2mA per DIO output. The DIO channels on the DSC are directly connected to the 9403. The 9403 is spec-ed for 64mA total on 32 channels and TEST data shows 4.4V out when driving a 2mA load (on a single channel). It does NOT say that sinking more than 2mA will drive a single channel into over-current protection mode. And this type of spec is pretty typical. Page 8 of the NI 9403 Operating Instructions and Specifications shows an example driving an LED. So one can drive a few very high-efficiency LEDs (2-5mA) and be careful not to exceed the total 64mA. I have a few chip LEDs on a board laying around, tried this real quick and it works fine.

DIO is not the best way to go though. I would not recommend a team w/o an EE mentor use the DIO channels for LEDs. The solenoid outputs or relays or custom circuits are better choices. For example, it would be easy to drive scores of LEDS with some I2C high-current latches.

HTH

[Ken Streeter]

Originally Posted by jhill0914

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. ...

Originally Posted by wireties

You could use high-efficiency LEDs straight from the DIO channels - or run them from solenoid channels.

Just FYI, as of this time, it is against the rules (specifically R60) to drive LEDs (or any custom curcuits) from the solenoid breakout:

Originally Posted by TheRobotRevH.pdf

<R60> Solenoid Breakout outputs shall be connected to pneumatic valve solenoids or photoelectric sensors, PN 42EF-D1MNAK-A2, only. No other devices shall be connected to these outputs.

[ratdude747]

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?

[Al Skierkiewicz]

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?

[ratdude747]

they were cheapies from ebay in china... the price was irrestably low and a mentor's grown up son had bought some and had no issues.

they are not here yet. for now, i made a test array out of 4 ultra brights with 3.1v. they are powered by the 22 gauge wire. it has been working fine.

the size has not been determined yet. each display will be about 5-6" diagonally. they are meant to be visible half a field away.

the led's would be mounted in a plastic panel, with either hot glue or a second panel to insulate the solder joints.

i was trying to avoid resistors... i figured that the correct number of LED's in series would be the best solution.

[Al Skierkiewicz]

Originally Posted by ratdude747

i was trying to avoid resistors... i figured that the correct number of LED's in series would be the best solution.

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.