Let's Make Robots!

Volts and Amps... For Dummies

Well Folks,

It seems that I might be turning into the lay-person lingo translator around here so I thought I would use my new-found position to start posting some basic concepts...

I have noticed a lot of posts about running motors, motor drivers and the like so I thought I would do a post about one simple thing, there may be more to come.

Today's lesson is volts, amps and logic signals.

Here we go...

Volts are water-pressure and amps are the diameter of the pipe.

Volts are guys walking down a hallway and amps are how much each has in his back-pack.

Volts are horsepower and amps are torque.

Volts are PSI and amps are CFM.

High voltage is a light-weight bullet going wicked fast.

High amps is a huge ship at 2 MPH (KPH) slamming into a dock and not stopping

Volts and Amps (Pratical Crap)

You can stuff a crap-load of volts through a tiny wire

Amps need big wires (Jumper-cables for your car, yo)

A lot of amps make things hot. 

Volts zap you, amps kill you. (Static electricity is wicked high volts, low amps)

There are lots of math things to calculate in terms of how volts relate to amps but forget that for now. As a basic idea, if you double the volts, you half the amps. For example: I have a tablesaw that I can wire for 110V or 220V. At 110V it sucks 15 amps, at 220V it sucks 7.5 amps.

Volts, Amps and Logic (For your robot brain)

Your robot brain talks to all the other electronic doo-dads using a logic-level signal. This signal is usally 5 volts but very, very low amps. A sonar sensor talks to your brain using a logic-level signal, for example. Seeing that you need some-sorta amperage to run really anything (a motor, light etc.) you will need a driver. A transistor is just a little switch that can talk to your "brain" (using a logic-level signal) and switch bigger things on. A darlington driver is just a bunch of transistors stuck into a neat little chip. A L293D motor driver is a bunch of transistors (stuck in a neat little chip) that are set-up in a way to run 2 motors forward and reverse. Both can take a signal from the brain and turn things on. In terms of servos, you will notice they have 3 wires. 2 wires are power (good power, with some amps -straight from the batteries) but the third is a logic-level signal to tell it what to do.

In a nutshell:

A darlington driver will run (usally) 8 things on or off (and fits on the picaxe 28 board)

A L293D Motordriver will run 2 motors forward AND reverse (Also on the 28 board)

Servos (at least the signal wire) can run directly off the brain (add a 330R resistor on the signal wire) -For more info see the "start here" post -Good stuff

LED's are super tiny, draw tiny amounts of power and you CAN run them directly off the brain.


O.K. That's all you get, and now you know... And knowing is half the battle!


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Here are my thoughts. At the risk of turning this (back) into an academic discourse.

> Volts are water-pressure
yep! (but read on)
> and amps are the diameter of the pipe.
disagree: ampère (unit for current) represent the speed of the water through the pipe (in mass/time). The diameter (cross section area really) is like the conductivity of the circuit. Bigger pipes = less resistance (better conduction).
but: you do need bigger pipes for bigger currents of course.

> Volts are guys walking down a hallway
hmm: down the the hill maybe, the higher the hill, the more volts (when electrical force compares to gravitational force)
> and amps are how much each has in his back-pack.
disagree: the backpack load is called "charge" in electronics, with an added bonus simplification: each electron (guy) has the exact same load or charge (measured in Coulomb btw). That's why we often forget about the charge per electron altogether. Thanks quantum guys!

> Volts are horsepower
disagree: electrical power is horse power. power always is power. always measured in Watt.
but: in DC electrics, power is calculated as a product of Volts and Amps
> and amps are torque.
wow: torque is a b*tch in its own right! let's see if I can tackle this.
disagree I think: torque is special way to describe mechanical force (in relation to leverage). I don't know of a good analogy for leverage. Let's forget about this one.

> Volts are PSI
right on! More specifically the difference in PSI (Pascal) between two compared places, e.g. the two leads on a battery. Or the water pressure at sea level and at 20000 leagues beneath. The dutch word for Voltage is "spanning" which translates to "pressure difference".
> and amps are CFM.
you yank: now you have me googling. ... Ah CF "per" M. (Almost a pint per second apparently.)
wikipedia agrees: but they maintain mass/time, not volume/time. So make that pounds (kilogram) per minute.

> High voltage is a light-weight bullet going wicked fast.
almost agree: high voltage is a bullet (regardless of mass) dropping from a wicked high skyscraper (minus the air drag). Sure it will go wicked fast. But the voltage is in the height, not in the speed.

> High amps is a huge ship at 2 MPH (KPH) slamming into a dock and not stopping
almost disagree: you are describing a wicked heavy electron. Which does not exist (did I say "thank you" to the quantum guys yet?). But then again: the ship could represent a shipload of electrons and it would work again.

Power would be a product of Volts and Amps remember? Imagine that ship dropping from a skyscraper! Would reach the same speed as a light weight bullet, but would kill a lot more pedestrians.

I like the falling-down-the-stairs analogy best.

Marbles that are lying still on a step of the stairs have an amount of potential energy each. The higher up the stairs, the more energy in each marble, the more potential to do damage. Once they start going down, they accelerate, gain in kinetic energy , fall down the steps, reach the bottom, slam dents in your hardwood floor, come to a stop. Energy gone. You'd better haul them upstairs, if you want to see them do that again. (I'm guessing you don't.) You could drain the energy from the marbles before they do damage. Just put up some kind of barrier along the way. A toy xylophone on the bottom step for example. Marbles hitting the keys on the instrument will produce music (work). The more energy per marble, the louder the music (power).

Now compare electrons in a battery pack at the "high" connector. Lying still until unleashed. Lots of energy in each electron. The higher the "potential difference" or "electrical pressure", the more energy each. But each electron has the exact same charge, as each marble has the exact same mass. When unleashed without anything in their path to slam into (hardwood, LED, whatever), they will speed down the "bottom" connector of the battery (a short circuit). Nothing to put that energy in. Somethin's gotta give (Rik's Law of Thermodynamics). But when they do meet some resistance, they will lose (some or most or all) energy there. A resistor in the circuit will suck some of that energy out and it will probably heat up (work). The more energy per electron, the more heat (power). But if the Resistance is too much, the electrons will not be unleashed to begin with. Or not as vigorously anyway.

And that is where the analogy falls flat.

Oh, and don't forget to haul them electrons back up the battery, if you wish them to do more damage.


Yeah, the analogy I've always liked best is aqueducts, like the Romans used to use -- they built channels for water to run through, but they were elevated, 20 or 30 feet high. The water is the electricity, and the height of the aqueduct is the voltage. If you want to get some work out of that water, you can drop it down to the ground. As rik said, if it just falls straight down unimpeded, it'll hit the ground pretty hard, but not do anything useful. If you put a waterwheel in its way, it'll turn the waterwheel and give you power, and in the process it will slow down and fall gently to the ground.

If you had a lot of power and you needed to get it to a water wheel at a remote location, you could build a big fat water channel 30 feet wide and just a few feet off the ground, or you could build an aqueduct higher up, and that way you could send less water, because at the other end they could drop the water off of the aqueduct and get more power from each drop of water. That's why power lines are at such a high voltage (110,000 volts or so, I believe) -- it's like building an aqueduct several miles high. That way you can send just a little bit of water and they can still get lots of power out of it at the other end because it has such a long way to fall.


Here's something that annoys me when I read about electrical stuff: people using the word "amperage".  I suppose it's logical to assume that Volts -> Voltage, so the same thing works for Amps, but it doesn't.  The proper word is "current", and it's measured in Amps.  Be sure to use the right terminology!  OK, that's enough of a rant, I'll be quiet now...

I think I just should have stopped at "Volts zap you, Amps kill you"  :)

P.s. You guys are Punks... and I love you.

back at yer! 8-o

BTW: that part totally hit the nail on the head. (Forgot to mention that)