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Unread 02-02-2011, 09:48 AM
jhill0914 jhill0914 is offline
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Custom LED Circuit - Wire Gauge?

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?

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Unread 02-02-2011, 09:55 AM
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Re: Custom LED Circuit - Wire Gauge?

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 Do not quote me in an attempt to justify your wiring to an inspector who takes issue with it.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.
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Unread 02-02-2011, 09:59 AM
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Re: Custom LED Circuit - Wire Gauge?

Your custom circuit could include a smaller fuse that would protect the smaller wiring in your custom circuit, right?
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Unread 02-02-2011, 11:06 AM
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Re: Custom LED Circuit - Wire Gauge?

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?
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Unread 02-02-2011, 11:14 AM
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Re: Custom LED Circuit - Wire Gauge?

How many lights are we talking?

Cant they be powered and controlled directly by the digital side car or solenoid bumper?
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Unread 02-02-2011, 11:35 AM
jhill0914 jhill0914 is offline
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Re: Custom LED Circuit - Wire Gauge?

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!
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Unread 02-03-2011, 07:49 AM
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Re: Custom LED Circuit - Wire Gauge?

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.
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Unread 02-09-2011, 01:06 PM
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Re: Custom LED Circuit - Wire Gauge?

You could use high-efficiency LEDs straight from the DIO channels - or run them from solenoid channels.
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Unread 02-09-2011, 04:39 PM
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Re: Custom LED Circuit - Wire Gauge?

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.
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Unread 02-09-2011, 05:06 PM
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Re: Custom LED Circuit - Wire Gauge?

Quote:
Originally Posted by jhill0914 View Post
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?
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.
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Unread 02-09-2011, 05:19 PM
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Re: Custom LED Circuit - Wire Gauge?

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.
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All is better now, NOS parts are working fine. Why does this year's game remind me of Violet in Willie Wonka? Hmmmm, I see blueberries!
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Unread 02-09-2011, 05:21 PM
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Re: Custom LED Circuit - Wire Gauge?

Quote:
Originally Posted by jhill0914 View Post
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. ...
Quote:
Originally Posted by wireties View Post
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:

Quote:
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.
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Unread 02-09-2011, 06:47 PM
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Re: Custom LED Circuit - Wire Gauge?

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?
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Unread 02-09-2011, 09:26 PM
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Re: Custom LED Circuit - Wire Gauge?

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?
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All is better now, NOS parts are working fine. Why does this year's game remind me of Violet in Willie Wonka? Hmmmm, I see blueberries!
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Unread 02-09-2011, 11:06 PM
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Re: Custom LED Circuit - Wire Gauge?

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.
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