Fans from the PCM?

So just to test a fan today my team connected a fan to one of the ports on the PCM, it worked perfectly. My question is would this be competition legal?

Check rule R42. Custom circuits may be controlled by the PCM.

Is there a reason you don’t want to hook it into the PDP to have it run continuously?

Strictly speaking, looking at the manual, I can only seem to find fans being powered off motor controller input leads as being explicitly legal (unless I’m blind).

However, the PCM is allowed to power Custom Circuits, so I should think a fan can be legally powered from there. (insert usual caveat that CD isn’t definitive)

That is not the intended purpose of the ports, but I don’t see anything that makes it illegal.

Remember, the maximum combined output of all the ports is 500 mA. A small fan could draw 90 mA. A large fan can draw 1000 mA or more.

The purpose of that is to make it legal to cool the motor controller with a fan, and to take the power from the motor controller power leads. I don’t think it was intended to be limiting.

First off, check that your fan is in the list of allowed fans in the next-to-last row of table 4-1, in R18.

Hard drive motors or fans that are included in the 2015 Kickoff Kit, 2015 FIRST Choice, are a part of a legal motor controller (including manufacturer provided accessories), or COTS computing device
So, fans are considered motors.

Then, check R42 for which loads can be controlled from the PCB. I see solenoid valves, solenoids, and custom circuits, but no motors (other than the compressor).

From a practical standpoint, all of the solenoid ports put together are only rated for 500mA (See the last line of the table at 1.2 of the PCM manual)]('s%20Guide.pdf). Only the smallest fans are going to draw less than this and leave you enough capacity to safely use the solenoids.

A CUSTOM CIRCUIT is defined as follows:

As a motor, a fan would not be considered a custom circuit, so it cannot be powered off the PCM.

Fans aren’t CUSTOM CIRCUITS per R30. (Fans are listed in R18, since they have integrated motors.)

The easiest way to get an answer would be to ask Q&A: “Are fans considered CUSTOM CIRCUITS for the purposes of R42?”

Per Table 4-1 under R18, it’s not clear if a fan is considered a custom circuit or a motor (please check that table to ensure the fan is, in fact, legal).

R55 part D states “Fans may be attached to motor controllers and may be powered from the power input terminals.”. As those motor controller input terminals are correctly connected to the PDP terminals, this ruling would imply that connecting fans to the terminals is legal. I’m not aware of any rule that would make it illegal. As a historical note, this hasn’t always been in the rules. A long, long time ago it wasn’t strictly legal to hook fans up to the inputs of the speed controllers they were on, and the ONLY way to do it was to go through a dedicated 20A breaker (you could have multiple fans per breaker).

For what it’s worth, my team has legal fans cooling our compressor hooked up to a 20A breaker.

We have a 120mm fan connected directly to the PCM to cool our compressor as well. The fan does run as long as the robot is turned on. We don’t think this is bad because the compressor doesn’t stop being hot just because the robot is disabled.

But what if we get to the competition and the inspector interprets the rules that we have to have this on a controller? Then what controller? It seems silly to waste a Talon on something like this. Perhaps a relay instead? Still it has to be turned on via the program so it isn’t running when the robot it disabled.

I’d just like to know so I know to have the parts we need at the competition.


Read the whole paragraph, especially the first sentence, beginning with “But what if”.

The bottom line is that the rules really don’t provide clear, unambiguous situations on fans. Some places they’re in the list of motors, other places, they’re not on the list of motors. If I were of such a mind, I could probably put together a rather tight case that there is no legal way to power an electrical fan that is not part of a COTS computing device on an FRC robot, either switched or unswitched.

Perhaps the best solution is to use a paper fan, and drive it with pneumatics.

Maybe we will look into using a spike.

Can you direct connect the fan to the PDB? Do you have to go through a motor controller?

That’s the question that I want to do a little more research to see if I can figure out an answer. Since the fan is a MOTOR from the list of allowable MOTORS

Hard drive motors or fans that are included in the 2015 Kickoff Kit

then I assume that they are not CUSTOM CIRCUITS.

And “Table 4-4: Legal Power Regulating Device Use” does not list fans, so we don’t know what we are allowed to use to power them, whether it’s a Motor Controller, Relay Module, direct connection to the PDB, or the PCM or VRM.

A quick search of the Q&A does not show that this question has been asked or answered.

Consider the intent of the ‘motors must be connected through controllers’ rule. It is there to stop mechanisms from moving when the robot is disabled. It is a safety concern and a game-play concern. Is it dangerous or unfair to allow a cooling fan to remain on when the robot is disabled? No, unless you jam your finger into one, then it stings like crazy. Fans used to cool motor controllers stay on when the robot is disabled, I see no intent in the rules that you prevent a fan from cooling a compressor from running while the robot is disabled.

FWIW we used a fan hard-wired to the PDB last year with no comments from any inspectors.

We did too. But I get to play inspector this year, too…so I’m looking at the rules a bit differently.

Question submitted on directly connecting a fan to the PDB.

What number? Can you link please?

I can agree with this first hand. That hand got into the fan and broke a blade off the fan. Miraculously my hand was fine, but stung a little. The new fan has heavy duty blades so we put hard nylon grill on both sides. Now it is noisy.

FWIW we used a fan hard-wired to the PDB last year with no comments from any inspectors.

Good points above… yet this. Since the fan is noisy I think I will put it on a relay as I had planned. Whatever we do (or don’t), we will get the inspector that makes us do it the opposite way we have done it. If yours is opposite us, and you get passed, we will not tell on you. This has been our way for the last few years (and it has indeed happened), we will be ****ed but stoic.