Current

Note from Akuli:

To follow along, you need:

The LED-and-resistor setup from lesson 2: power supply, wire that fits inside power supply plug, alligator clip cables, 1kΩ resistor, LED
A multimeter that can measure currents

TODO: picture

What is current?

Akuli so, let's talk about what is current

Akuli to reiterate, voltage is electricity wanting to go from place A to place B, but not necessarily going

Akuli current is electrons actually moving somewhere

adder Current is, how much charge did get through past the resistance?

Akuli you don't necessarily need resistance to have current

Akuli it's just how much charge (that is, electrons) is moving

adder Sure.

adder OK.

Akuli a valid unit for current is electrons/second

Akuli the problem with that unit is that it is really small

Akuli like insanely small

Akuli so, you would need huge numbers to describe anything meaningful

Akuli ok?

adder So we count it as Coloumbs?

Akuli a coulomb is a big pile of electrons

Akuli a very big pile

Akuli so yes, we use coulombs/second

adder OK.

Akuli but in electronics, you don't usually say coulombs/second, you say amperes, which means the same thing

adder OK.

Akuli so, an ampere (abbreviated: A) is just a large number of electrons per second

Akuli ok?

adder OK.

Akuli like we discussed with voltage, this is inverted for historical reasons

Akuli so a positive amount of amperes from A to B actually means that electrons are traveling from B to A

Akuli this will probably become clear when we go actually measure the circuit again

adder OK.

Examples of currents

Akuli but first, i want to give you some kind of idea about what to expect

Akuli 1 ampere is a lot of current

Akuli your entire device may use 1 ampere if it's doing a lot

Akuli for example, a phone charger typically outputs just a few amps

adder So, usually we work in mA?

Akuli yeah

Akuli today i bought one for charging my phone in my car, and i think it said 2.2A on it

Akuli so if you're looking at an entire device, you may see a few amps

Akuli if you're looking at an individual component, expect mA or uA

adder OK.

Akuli (u = micro, one thousandth of a milli)

Akuli let's get even more concrete :)

Akuli so phone charger, 2.2 A

Akuli ok?

adder OK.

Akuli car engine starting: hundreds of amps for a short duration

Akuli you really need a lot :D

Akuli ok?

adder OK.

Akuli LED indicator light: somewhere between 1mA and 10mA, depending on type of LED and how bright you want the light to be

Akuli ok?

adder OK.

Akuli (those are not hard limits)

Power supply specifications

Akuli hmm, do i have more simple examples

Akuli let's actually find out what your power supply promises to deliver

Akuli unplug it from the wall and inspect it, there's a label that says what it's designed to output

adder Oh no.

adder Barely tucked it in.

adder Is this necessary?

Akuli not right now, but you will eventually want to know that when you run into it

Akuli i can send a pic of mine

adder Sure.

Lesson Image — Akuli sent a picture.

Akuli PRI (primary) is basically input and SEC (secondary) is basically output

adder OK.

Akuli so this one promises to deliever 300mA

Akuli this power supply is useful for a small device that doesn't need a lot of power

Akuli but 300mA wouldn't be enough to charge a laptop

Akuli laptop chargers usually say something like 3A or a bit more

adder I see.

Akuli voltage has to match too, of course

Akuli you generally want a power supply that has the same voltage and enough amps (more is fine, it just won't provide as much as it could)

Akuli ok?

adder The same as what?

Akuli the same as the device expects

adder OK.

Akuli for example, i can use this power supply with most guitar pedals, because guitar pedals are designed to take 9V power

Akuli (ignoring the fact that this power supply puts out way more than 9V if you don't have any load connected...)

Why does Akuli's "9V" power supply output more than 9 volts with no load?

adder How so?

Akuli why it outputs more than 9V with no load?

adder Yeah.

Akuli basically, the power supply is simple and dumb

Akuli the correct word is: unregulated

Akuli most power supplies adjust their output cleverly to hit a target voltage

adder I see.

Akuli this one does not, there's literally 6 parts inside the thing and no logic of any kind

adder :)

Akuli instead of "most", i should have said "most new"

Akuli this type of power supply used to be more common

Akuli digital devices don't like these, they like to assume the voltage is quite accurately what it should be

adder I see.

Akuli unregulated power supplies are also less efficient due to the simplistic design

Akuli so basically, for a number of reasons, you don't find these as much as you used to

Akuli ok?

adder OK.

Akuli now, we actually talked about current there

Akuli it's pretty much impossible to talk about electricity without talking about voltage and current

Akuli when i say "load", i really mean that some current is taken from the power supply

Akuli "no load" means current is zero

adder OK.

Akuli so even though current is today's topic, we have already discussed it before when we e.g. talked about measuring batteries

Akuli ok?

adder OK.

Measuring current

Note from Akuli:

At this point, adder has the LED-and-resistor setup from lesson 2.

Akuli enough talk, let's measure current :)

Akuli the first caveat i'll add: current is NOT between two points like voltage

Akuli current is THROUGH a thing, not between two things

Akuli so when you measure current, you connect the multimeter so that the thing you're measuring goes through the multimeter

Akuli it's more involved than just touching two points to read a voltage

Akuli ok?

adder OK.

Akuli let's do exactly that

Akuli disconnect the circuit at some point (doesn't matter where really)

adder Inside of PS is probably the easiest.

Akuli :DDDDDDDDD

adder That's not what you meant?

Akuli yeah, any point of the circuit you put together with alligator clips and such :)

Akuli don't mess with the 230V AC input side :D

adder OK, disconnected.

Akuli now, put the meter in current mode

adder A? mA? uA?

Akuli select A if unsure

Akuli you don't want to surprise the meter with too much current

adder OK.

Akuli in many meters, you would also need to change where the red probe is connected, in yours i think you don't need to

adder Says V Ohm mA uA

adder Where the red probe is right now.

Akuli ah, so it's all the same in yours

Akuli unusual, but i remembered it being like that :)

adder OK.

Akuli so, reconnect the circuit but so that the current travels through the meter

Akuli that is, black probe and red probe to the two things that are no longer connected to each other

adder Does the probe ordering matter?

Akuli try it :)

Note from Akuli:

At this point, adder has disconnected the red alligator clip from the LED's longer leg, and inserted the meter there.

adder gets weird current readings because the multimeter probe is plugged to the wrong connector in the multimeter

adder Just the sign flips, but I'm also measuring a different figure.

adder 0.22A vs ~0.3A

adder One of those is negative.

adder Also, led doesn't glow.

Akuli something weird is going on

Akuli what point did you disconnect?

adder Long leg.

adder Of LED.

Akuli so everything else is still the same, led(short leg) -> resistor -> alligator --> power supply negative?

adder Yeah, looks so to me.

Akuli meter should go between power supply positive and LED long leg

adder Yeah.

adder What is weird?

Akuli LED should glow, and 0.22A is way too much

Akuli it should be more like 0.006A

adder Well, I don't know. :)

Akuli hmm, walk me through the whole thing part by part, starting from the middle of the plug (power supply positive)

adder Wire from the inside to the gator -> gator -> LED long -> LED short -> resistor -> gator -> outside of PS.

adder But I reverted this as it wasn't working.

adder Otherwise there was a meter like you said in between.

adder LED now glows.

Akuli let's try again...

Akuli disconnect alligator from LED long leg, LED should turns off

adder OK.

Akuli connect that same alligator thing instead to one of the meter probes

Akuli then, touch the other probe to LED long leg

Akuli what happens?

adder 0.178A

Akuli and no LED glowing?

adder Nope.

Akuli your meter is doing something weird here

Akuli at least, that's my guess right now

Akuli try the mA setting of the meter

adder It's now showing 0.012A when I don't touch anything, just the one probe.

Akuli ahhhhh

Akuli i think you're touching something somewhere that you shouldn't

adder 5.6mA

adder LED works.

Akuli that's the reading :D

adder Touching it pretty much the same.

Akuli what exactly do you mean touching?

adder Like I said, this "port" on multimeter isn't designated "A".

adder Only "mA" and "uA".

adder There is a separate port that says 10A max.

Akuli ahhh yeah that's the one you should use with the "A" mode

Akuli that's why the meter was confused

adder >.>

Akuli you have a different meter than i remember?

adder Same meter we bought.

Akuli then i'm not remembering it properly :)

Akuli but yeah, now you're getting the correct reading

adder Getting 0.006 on different port.

Akuli 0.006A = 6mA

adder Yeah.

Akuli that's just the accuracy you're getting with that :)

Akuli the "10A max" one is great if you don't know how much current you're going to have

adder OK.

Akuli you're unlikely to get 10A out of a power supply anyway, so meter is probably happy with it

Akuli if you ever exceed the current, there's a fuse that you can replace and it will likely fix it

adder I see.

Akuli so there we go, 5.6mA :)

Akuli try swapping the probes

Akuli (now that we are getting good readings...)

adder -0.005

Akuli and in mA mode?

adder But it takes a while to bump from 0.003

adder No, in A mode.

Akuli the A mode is really intended for bigger currents

Akuli so it's probably all sorts of inaccurate at such "small" currents as a few mA

adder 5.66

Akuli should have minus sign

adder Yeah.

Akuli which way around are the probes now?

Akuli which probe points towards power supply positive?

adder Red probe touching long leg.

Akuli yeah

Red probe	Black probe	Measured current
red alligator clip cable (power supply positive)	longer leg (plus leg) of LED	5.6mA
longer leg (plus leg) of LED	red alligator clip cable (power supply positive)	-5.6mA

Direction of current

Akuli so current comes from power supply positive, goes in through the black probe, out through the red probe, into LED and resistor and back to power supply

Akuli this is what the meter calls negative current

Akuli positive is in through the red, out through the black

Akuli ok?

adder OK.

Akuli as with voltage, the actual movement of electrons is the opposite

adder Gonna take a while while I clear out this in my head.

Akuli :)

adder It's super confusing right now.

adder Got + -, red black, etc.

Akuli black is always -

Akuli red is always +

Akuli meter shows current as going from +(red) probe to -(black) probe

Akuli power supply wants current to flow from its + to -

Akuli does this maybe clarify the conventions?

adder Nope, sorry. :)

Akuli alright :)

Akuli then it'll just take some time getting used to it all :)

adder Yeah.

Akuli but basically, the multimeter in this mode is just an electron counter

Akuli it counts how many electrons are going from black wire to red wire

Akuli while disturbing the circuit as little as possible as it does so

Akuli ok?

adder OK.

LED current for standard LEDs

Akuli so, now you have an idea of what 6mA through an LED looks like

Akuli this depends on the LED

Akuli some LEDs are brighter than others at the same current

Akuli ok?

adder OK.

Akuli standard LEDs similar to the ones you have are usually designed to handle about 20mA without issues

Akuli and for an indicator light you usually need 10mA or less

adder OK.

LED current from datasheet

Akuli for different types of LEDs, you can usually download a datasheet from the website where you buy the LEDs

Akuli and the datasheet says how much it can handle

adder OK.

Akuli here's an example of a datasheet of an LED i used in a project:

Akuli shared a PDF datasheet. Click here to open the datasheet.

Akuli this is a 3-leg LED that can emit red or green light based on which legs you connect

adder I see.

Akuli i used it to show not only whether the device is on, but whether it's in a mode i called "go brrr"

adder Looks pretty Greek right now.

Akuli :)

Akuli let me explain how to find out the important stuff from here :)

Akuli for this 3-leg LED, you probably want to first know what each legs does

Akuli there's basically 2 LEDs inside it, but not 4 legs like you may expect

Akuli so you want to find out how that is

Akuli ok?

adder OK.

Akuli in the picture on the first page, there's a diagram on the bottom right

Akuli each --->|--- symbol is an LED

Akuli (that's actually the symbol of a diode)

adder OK.

Akuli it looks like an arrow, and that arrow is the direction of current needed to make the LED show light

Akuli why are they labeled SYG and SUR, i don't know

Akuli i'd expect RED and GREEN or similar

adder :)

Akuli but anyway, one of those is the red, the other is the green

Akuli so to power on the LED, you would feed in plus from either outside leg (depending on which color you want), and minus from middle leg

Akuli then current would flow in through either outside leg, and out the middle leg, just like it wants

Akuli you also need a resistor

Akuli ok?

adder OK/

adder OK.

Akuli so, how much current?

Akuli that's the only other detail you will likely need to dig out of the datasheet

Akuli scroll down to second page

Akuli "Absolute Maximum Ratings at TA = 25 C"

Akuli absolute maximum means, do not exceed this under any circumstance or the device may break

adder I see.

Akuli usually you want to stay a little bit away from absolute maximums

adder I see.

Akuli and what's the first thing the table says? forward current :)

Akuli forward current, as in current in the direction needed to light the LED

adder OK.

Akuli reverse current would be current in the opposite direction

Akuli so absolute maximum is 25mA

Akuli in my project, i wired the LED so that it gets about 10mA if the power supply is exactly 9V

Akuli i designed the project to work with "9V" supplies, including those that output more than 9V

adder I see.

Akuli so even if the power supply outputs more voltage, the LED will be fine

Akuli and 10mA is plenty to see it

adder Is this something tweaked by a resistor?

Akuli yes!

Akuli let's talk about that :)

Akuli or do you want to keep a short break?

Akuli this might be a good time for a break

adder Nope, we're good.

Akuli alright :)

Note from Akuli:

See the next lesson (resistance) for tweaking a resistor to get the desired LED current.

The important equation of currents

Akuli i'll mention one more fact about currents that is kinda important here

Akuli the important equation for voltages i presented was: V(A -> B) + V(B -> C) = V(A -> C)

Note from Akuli:

I explained the "important equation of voltages" in the first lesson.

Akuli there's a similarly important thing for currents

Akuli it goes like this: sum(currents in) = sum(currents out)

Akuli ok?

Akuli (i'll explain what this says shortly)

adder OK.

Akuli this basically says that electrons don't appear or disappear

Akuli this is usually presented for intersections of wires, but it actually applies much more broadly than that

adder OK.

Akuli almost any component you can think of satisfies this, if you consider the currents through all of its legs

Akuli resistors, LEDs, capacitors, IC chips (e.g. the 555 timer), a whole computer, anything really

adder OK.

Akuli the only exception i know is devices that literally shoot electrons

Akuli like the CRT tube of an old tv or computer monitor

adder OK.

Akuli so if you're not messing around with anything like that, this equation holds for any component :)

Akuli that's why, in the case of 2-leg components, we talk about just one current

adder OK.

Akuli whatever goes in through one leg must come out through the other leg

Akuli the same current

adder OK.

adder This is what law?

Akuli good question

Akuli for intersections of wires, this is kirchhoff's current law

adder OK.

Akuli in my mind, the more general version that applies to anything other than electron shooting devices and such, is just common sense

Akuli if you understand what current is, it should seem obvious

adder OK.