How much current and voltage are actually considered a lot?

Hey guys, I am new to robotics and I’m not sure how much current and voltage are actually considered a lot. Like for motors, for example, how much is a lot? I don’t really understand…

Depends on the situation, for charging phones 5v 2a is perfect; however 240v 40a may release magic smoke and teach you what accidental SPOT welding can do, but is perfect for charging a Tesla.

Usually robots take 12v 20a-40a from the pdp to motors. The roboRIO is 45W nax, which is usually 12v 3.75a in theory. Everything on a FRC robot is powered by one 12v battery.

Oh wait so does the pdp take in voltage and output current? And that output current is between 20-40 amps per motor? What happens if the current utilized exceeds to available current? Wait is current proportional to the battery voltage?

Voltage and current are related but are physically different things. The water analogy is often used here–voltage is like water pressure, current is the actual volume flow of the liquid. The PDP is a distributor–it takes in current (one big pipe) and splits it out into smaller pipes. The voltage is (mostly) unchanged.

The battery is not an ideal source–it (and the wiring) have resistance, which causes the voltage to drop when more current is flowing (the relationship is called Ohm’s Law-- V=I*R). Technically the PDP also has a bit of internal resistance, so the voltage into the PDP is a tiny bit higher than the voltage out, similarly the input and output voltage of wires is slightly different due to the current flowing through them (the more current, the greater the drop, in proportion to the wire resistance, which is a function of wire diameter).

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Ohhh so the PDP takes in current and outputs current. So why is it called a power distribution hub? I am a little confused on the second part.

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Because it distributes power. Power is voltage * current, so if you’re distributing current (and voltage), you’re also distributing power. The PDP is just like the big breaker box in your house. There’s a big main feed coming in from the power company, and lots of breakers that feed out to different sets of outlets in your house. Each outlet gets the same voltage but is limited to a certain amount of current by the breaker in the breaker box.

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Ohhh I see! So P = IV. The current for each motor could be different, but the V is the same for all. Therefore, the power for each motor depends on the current, which is determined by the breaker. Did I get it right?

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Close. The current is limited by the breaker, but the motor current actually depends on the mechanical load (e.g. speed and torque) on the motor output (essentially, what is the motor’s output power–mechanical power in this case rather than electrical).

So once you set a specific speed, it will draw that much current to match it?

Based on the amount of torque it has to apply to (try) to reach that speed, yes. This relationship is called the motor curve. Here’s an example (the actual points on the curve are different for each kind of motor, but the overall shape/relationships is essentially the same for all DC motors).

oh ok thanks!

Going back to this for a second:

I “play with” a number of different voltages at work. Up to 480VAC 3-phase in one or two cases. You don’t want to mess that one up… ZZAAAP!

I also “play with” a number of different amperages, up to 200A. I take great care around that one, too… Caught a fraction of that (at 48VDC, thankfully) across a finger once. Don’t want to do that again. Finding equipment that can handle 200A gets interesting, too, at least at the usual sources.

I will say that depending on the application, it’s not uncommon to have high voltage low current, OR low voltage “high” current. My company will sometimes use CIMs… on a 12VDC 50A power supply.
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For FRC use, 12V DC across a 120A main breaker is the maximum total (though note the breaker discussion, the main breaker can handle more for a short time).

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Don’t end up like Marv!

-sFMqj

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This has nothing to do with robots.

Just a weird side note of saying something is high voltage, like the 13,200 volt substation at work is considered medium voltage. It’s strange I’ve never found a hard cut off between medium and high voltage. Usually once its over 20,000 volts its called high voltage.

I have a 300,000 Volt Van de Graaff generator in my classroom. I had to convince my administration to let me order it, because they were convinced that such a high voltage would be dangerous.

The analogy that almost got them was, “Voltage is like height for gravity. Nobody is afraid of a speck of dust that’s a mile up, but everyone is cautious about a car two feet off the ground. This thing is more like the speck of dust than the car. The most dangerous things in my classroom, hands-down, are the 12V batteries we use for FRC. 300,000 V is perfectly safe.”

They persisted a little, so I did the math. The self-capacitance of a 30-cm diameter VDG is 4 x pi x e0 x r = 1.67x10^-11 F. At 300,000 V, it can hold a charge of Q = CV = 5 microcoulombs at an energy of 0.5CV^2 = 0.075 joules, or about the same as a paper clip one meter off the ground.

It’s mildly amazing to me that people do not understand that voltage isn’t energy, it’s energy per charge. That means that low voltages can be quite dangerous if there’s enough charge in play, and high voltages can be quite safe if the charge is very small.

Eventually they gave up and let me get the thing, and now we use the thing to do the stuff.

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Most people are surprised to learn that as little as 50 milliamps can put a person into respiratory arrest, potentially causing death. But, you need some voltage to actually flow that kind of current through the skin of the human body, which is fairly high resistance. But, at higher voltages, the resistance of the skin breaks down and currently can flow much more easily in the interior of the body where resistance is lower.

Electricity is one of those things that can be difficult to understand because most of the time you can’t see it. But, it’s important to learn about because it can hurt you if you aren’t careful with it. 12 volts DC is not likely to ever electrically injure anyone unless you short out a high current source (like a lead acid battery). Once you start getting up over about 90 volts, or in some cases maybe over 48 Volts, that’s where you want to take more precautions.

So, long story short, don’t stick your dinner utensils in an outlet, and don’t take a bath with your toaster, but FRC robots are generally fairly safe electrically. Although I will note, the battery used in FRC is capable of supplying several hundred amps. That’s enough current to weld with. So, don’t short it out. That would be if the red/positive terminal directly touched negative/black terminal through a path of low resistance (wire, metal).

I like the water analogy but I prefer using a person trying to push a block analogy. Using water just brings up flashbacks to fluid dynamics.

But Voltage is like the force of trying to slide something on the ground and the current is the velocity of the object. You can easily push something very fast with little force if it is light. But if it’s heavy it takes a lot of force to get even a little velocity.

“Power = effort x flow” is an extremely useful concept, and comes up across a bunch of physics fields.

P = Fv in mechanics
P = VI in electricity
P(ower) = P(ressure)Q in fluids

I have a hard time with the persistence of water analogies when talking about electricity, not because they’re wrong, but because most high school and even undergraduate students have minimal if any experience with water systems. For electrostatics I usually use height/topography (borrowing from Michael Faraday), and for circuits I use people flow.

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