Is a dimmer switch on the mini bot legal?

[ADTRavenger3585]

Hi, we withheld our minibot so we could finish putting it together and fixing it and someone had the idea to use a dimmer swith on the robot. I'm not going to assume that everyone who reads this knows what I mean by a dimmer switch. The one that my teammates picked out is one of the round ones that when you ouch it down it will turn off the light. Thinking the way I do I thought about the legality of the switch. Legal or not?

[jvriezen]

Hi, we withheld our minibot so we could finish putting it together and fixing it and someone had the idea to use a dimmer swith on the robot. I'm not going to assume that everyone who reads this knows what I mean by a dimmer switch. The one that my teammates picked out is one of the round ones that when you ouch it down it will turn off the light. Thinking the way I do I thought about the legality of the switch. Legal or not?

I see no reason why it would not be legal. I'm assuming from your description you are using a dimmer not for its dimming characteristics, but the push button on/off nature of it. Nice thinking. Might want to lock it somehow into the full on setting so it doesn't rotate on you.

[GaryVoshol]

Well, it's a light switch common in household applications, so why not?

[Taylor]

Remember that officially, Chief Delphi is not the official source of official information. It may be a good idea to submit this to the Q&A just to be sure.

[SuperBK]

I'd say its legal - its a common household light switch. But have you tried one yet? They work by controlling the AC so it might not work with DC even in full on mode.

[Tom Line]

SuperBK nailed it on the head. Dimmers are designed for AC, and they generally use an inductor and a neat little circuit board to handle their operation (they'd get awfully hot if they acted as a simple resistor).

The ones I've played with don't work with 12V DC at all.

[nitneylion452]

Hi, we withheld our minibot...

Also, remember that the minibot does not count toward you withholding allowance.

[Al Skierkiewicz]

Guys,
A dimmer is not a switch. It might have a switch inside but is classed as a dimmer.
<R92>
N. no more than two common household light switches,

And from TU #12
anything sold as a “light switch” for household use is allowed.

BTW, the dimmer also contains a variety of non-legal components including SCR or Triac, resistors, pot, inductors and capacitors.

[davidfv]

Anyone have any issues this weekend with Mini-bot switches during inspection?

We are looking for a switch so when the mini-bot hits the top, we can then back drive the motors. Anyone know of one that would work?

[Mr V]

Anyone have any issues this weekend with Mini-bot switches during inspection?

We are looking for a switch so when the mini-bot hits the top, we can then back drive the motors. Anyone know of one that would work?

The basic "3 way" household and the common "limit switch" with 3 terminals can be used to put the motors in "brake" mode to slow its' fall.

[Doc Wu]

There are a number of kinds of "Light Switches" that are sold that would work and have a push in snap action. Think of the light switches found on bathroom fixtures, or in the base of a table lamp. You may even be able to find them in a 3-way or SPDT configuration.

We were told for inspections this weekend to be liberal in the interpretation of the light switch rule. As long as it is called a "light" switch where sold it was allowed. I saw one that was like an old fashioned car headlight switch.

But the vast majority used either the conventional wall-switch toggle style, a "limit switch" such as the microswitch in the KOP, or the Tetrix switch.

Whether a dimmer switch would be allowed is a matter of interpretation and probably needs clarification. Since the dimmer part of it would do you no good on a DC motor, using it as only a switch seems like overkill, but I think would be within the spirit of the rule. Modifying the guts to provide some kind of brake or speed regulator would probably be over the line.

This is just my opinion, of course...

[xisybyl]

Re response by Mr. V: How would one put a motor in "brake' mode without using a resistor? Wouldn't that damage the motor?

[jvriezen]

Re response by Mr. V: How would one put a motor in "brake' mode without using a resistor? Wouldn't that damage the motor?

I'm a different Mr. V, but our minibot used only one switch total-- a simple household 3-way wall light switch. Battery leads were connected to the non-common terminals, motors connected to Common and to one of the other terminals.

Motor acts as brake when motor leads are shorted together -- no damage observed.

John Vriezen
FIRST team 2530 "Inconceivable"
Mentor, Drive Coach, Inspector

[Mr V]

You really asked 2 questions so I'll break it up.

The reason you don't need the load (resistor) to make the motor into a brake. Spinning the motor "backwards" turns it into a generator. The polarity of the output is such that it makes the motor want to turn in the "forward" direction or provide resistance to being back driven. This is the exact same method method used by the motor controllers in the brake mode, no resistance is involved other than that of the motor windings and the nominal connection resistance.

The reason it won't burn up the motor is that the output voltage and thus current is proportional to the speed of the "generator". The resistive or braking force is proportional to the input voltage. So the faster it turns the more braking force it generates, meaning that it will reach an equilibrium and fall at a constant velocity. The caveat is that if you were to spin the motor faster than it's free speed at its rated voltage, for any length of time you could harm the motor. However to do so would require a load greater than the motor's capacity.

On our minibot in brake mode it takes about 3X as long to come down the pole as to go up.

We initially used the three way "common household" sw but in the interest of weight savings we went to the limit sw. It required a little more polycarb to make a lever system that allowed the wheels/motors closing to turn it on but still latch in the off/brake position. The extra sq" or 2 of pc was more than offset by the weight savings of the micro sw.

The wiring is very similar to the three way. The com term goes to the motor terminal you want (+) for up. If you want the sw depressed to be the "on" state connect the NC terminal to the motor up (-) and B-, the NO terminal to B+.