Akuli so, if a device takes in 1000W of electrical power, it cannot output 2000W of power in any form, electrical or otherwise
Akuli i mean, it could do that for a moment if it stores some power inside it
Akuli but not continuously
adder OK.
Akuli so that's how the physics law about energy translates to power
Akuli you can accumulate energy and release a powerful burst of it, but continuously, input power >= output power
Akuli ok?
adder OK.
Current of 1.5kW heater
Akuli in most circuits, most of the power becomes heat
Akuli after all, what do you do, make electrons wiggle around between atoms of your resistors :)
adder OK.
Akuli so, let's calculate how many amps of 5V power would be needed for that heater, if it ran on 5V
Akuli the formula is: power = voltage * current
Akuli 1.5kW = 5V * current
Akuli ok?
adder OK.
Akuli the current is.... 300 amps!
adder :>
adder Meaning if we want a reasonable current, we need to increase the push.
Akuli yes!
Akuli let's try a 230V power supply (that is, your wall; glossing over AC vs DC details for now)
Akuli 1.5kW = 230V * current
Akuli ok?
adder OK.
Akuli the current is 6.521739130 A
Akuli so this thing by itself won't trip your 10A breaker, but two of these would
Akuli ok?
adder I see.
Akuli we can learn a couple things from here
Akuli first, heating big things takes up a huge amount of power
Akuli this thing is for heating a room
Akuli that's wildly different from a little LED and resistor setup
Akuli ok?
adder OK.
Why high voltages are used
Akuli second, for big power it's convenient to use a big voltage
Akuli let's say you're using a long extension chord, and it has 0.1 ohms of resistance
adder Is this why energy is transferred along power lines at high voltage?
Akuli yes!
Akuli let's look at the long extension chord example
Akuli i could actually measure an extension chord
Akuli hmm, too small for my meter to read it
Akuli so we'll have to imagine a super crappy/long extension chord
Akuli ok?
adder OK.
Akuli let's say you put 6A through a line with 0.1Ω resistance
Akuli the wire itself eats 0.6 volts
Akuli if your whole supply is 5V, that's horrible news
Akuli if your whole supply is 230V, you really don't care that much
Akuli ok?
adder OK.
Akuli also, if your supply was 5V, you would need 300 amps, not 6 amps, so this situation would be way worse...
Akuli ok?
adder OK.
Akuli so, wire thickness and material and such matter much less at higher voltages
Akuli this is of course a tradeoff, high voltages are more dangerous and they tend to jump from one cable to another cable next to it
Akuli but that is a worthy tradeoff if you need a lot of power
Akuli ok?
adder OK.
Phone charger explained
Akuli so, let's look at a phone charger with power in mind
Akuli output power needed: 5V * 2.2A = 11W
Akuli ok?
adder OK.
Akuli input power needed: 230V * ??? = 12W
Akuli i bumped 11W to 12W, because inevitably some energy will become heat
adder Yeah.
Akuli phone chargers do heat up, though not by that much
Akuli solving that gives 52.17 mA
Note from Akuli:
The current consumed by a 2.2A phone charger can be a few times larger than this calculation would suggest, perhaps 0.1A or even 0.15A. This has to do with the 230V input being AC instead of DC. Look up "power factor" if you want to learn more about this. I won't go into more detail here, because we will mostly work with DC.
Akuli so, the phone charger really doesn't need that much current, if it's fed from a high voltage and it can do a good job converting
Akuli ok?
adder OK.
Akuli now, how does the conversion work?
adder :)
Akuli the trick is to convert the energy to something else than electrical energy
Akuli in phone charger, magnetic energy
Akuli physics people know that changing the magnetism of a chunk of iron requires energy
adder Why is magnetic more convenient?
adder Why can't we just amp this up?
Akuli well, let's look at other options
adder Why the intermediate step?
Akuli you could try a resistor
Akuli but a resistor can't really create more amps
Akuli it's a 2-leg component, the amps you feed in come out the other side unchanged
adder DIODE
Akuli as far as heating is concerned, a diode behaves just like a resistor (we'll get to this in a moment)
adder We talking about phone chargers?
Akuli yes
Akuli in general, the components you are familiar with aren't good at making more amps
Akuli but magnetism physics is
adder OK.
Akuli typically you use either a transformer (older, unregulated power supplies) or an inductor (newer, switching-regulated power supplies)
Akuli but either way, it's magnetism
adder OK.
Akuli this isn't the only option
Akuli you could, for example, use a 230V motor to spin a 5V generator
Akuli but motors and generators use... magnetism
Akuli so you would have a louder, less efficient, overall crappier way to get the same result
Akuli ok?
adder OK.
Akuli you could shine light at a sunpanel
Akuli but those are quite inefficient
Akuli so much of the energy would be wasted as heat
Akuli ok?
adder OK.
Akuli so a transformer or an inductor is really the way to go here
Akuli if you do things right, you can reach pretty nice efficiencies, 90% is not at all unreasonable
Akuli ok?
adder OK, doesn't matter for now, we have a black box that amps up from small current to adequate current.
Akuli yes! :)
Akuli with that attitude, you can do a lot of electronics
Power consumed by components
Akuli so, let's talk about what happens to energy after it comes out of the power supply and it's sprinkled over our components
Akuli ok?
adder OK.
Akuli the way this works varies a lot component to component
Akuli but i'd say there's two kinds: components that generate heat, and a few special cases
Akuli a capacitor would fall under those special cases, a perfectly working capacitor produces no heat at all (we'll talk about that later)
adder OK.
Akuli the same goes for a perfectly working inductor or transformer (as we just discussed...)
Akuli but most other things (LEDs, diodes, resistors, transistors) just make heat in various ways
Akuli ok?
adder OK.
Akuli for simplicity, i'll focus on two-leg components
Akuli a typical two-leg component (e.g. resistor, diode) makes heat like this: power = voltage * current
Akuli that is, the same formula that goes for power supplies
adder OK.
Akuli so 5V into a 1kΩ resistor consumes 5mA of current, and the power would be 5V * 5mA = 25mW
Akuli ok?
adder OK.
Akuli this is one thing you may run into if you try to set up a resistor for an LED but your supply voltage is high
Akuli for example, let's say you have a 24V power supply, and you want a simple "power on" indicator LED at 10mA
Akuli the most common type of resistors (like the ones you have) are made to handle 250mW
Akuli so you're kinda close to the limit here
Akuli it will be fine, but you can see that this could be a problem
Overheating components
adder What happens on >250mW?
Akuli good question
Akuli for components that heat up like resistors, the story is roughly the same
Akuli if you apply too much power for a short duration, nothing happens
Akuli however, things may break for other reasons; check datasheet if it's not just a simple resistor
Akuli ok?
adder OK.
Akuli if you apply too much power continuously, it may start smoking, and if you have way too much power, it may catch on fire
Akuli i've gotten resistors to smoke in the past, but never fire
adder :>
Akuli surprisingly enough, my resistors still measured ok after a moment of smoking, even though they were dark colored
adder lol
Akuli people who repair electronics often find burnt resistors (and other components), they rarely catch on fire but often get dark etc
adder I see.
Akuli it's often so that one component fails, and then another component gets more voltage than it should, hence more power
adder Cascading failures.
Akuli :)
How to avoid overheating
Akuli so, what if you need more than 250mW?
Akuli you can buy bigger resistors
Akuli there's e.g. 0.5W, 1W, 2W, 10W resistors
Akuli they are physically bigger, because they need more area to get the heat out
adder OK.
Akuli you can attach a heat sink (mostly a thing for transistors and such, also power resistors)
Akuli a heat sink is a chunk of metal (usually aluminum) whose job is to distribute the heat around
Akuli so instead of one spot that reaches a high temperature, everything will be slightly warmer
adder I see.
Akuli trying to look for some reasonable picture of a relatively big audio amplifier
Akuli sent a picture. The picture is the thumbnail of this YouTube video.
Akuli on the left and right, you see big heat sinks
Akuli you also see some transistors attached onto them
adder Yeah, I know what's a heat sink. Had it on my pi's.
Akuli yes :)
Akuli obviously there's other ways, like water cooling, but those are rarely used outside computers
adder OK. :)
Power of resistor: convenient equations
Akuli i'll finish with a couple convenience things
Akuli so the basic formula for power is power=voltage*current, and that works for more than resistors even though people usually present it as being only for resistors
Akuli basically any component that isn't clever about temperature management, or somehow special like capacitor, behaves that way
Akuli ok?
adder OK.
Akuli for resistors specifically, here are some convenient formulas:
Akuli P = V^2 / R
Akuli P = R I^2
Akuli where V is voltage, R is resistance, I is current
Akuli ok?
adder OK.
Akuli the first one is just P = VI, but replace I with V/R as you know
Akuli the second one is also P = VI, but replace V with RI
Akuli ok?
adder OK.
Akuli this is useful if you don't want to work out the voltage or current
Example: Maximum current of Akuli's 0.1Ω 0.5W current measuring resistors
Akuli for example, i have some 0.1Ω 0.5W resistors
Akuli i bought them for measuring currents
Akuli that is, i pass some current through the resistor and i measure its voltage to get an idea of how big the current is
Akuli ok?
adder OK.
Akuli what's the maximum current that can be measured this way?
Akuli (hint: pick a formula that doesn't involve the voltage, since you don't really care)
adder P/R?
Akuli almost
adder sqrt(P/R)
Akuli yeah :)
Akuli so, what's the current? :)
adder 5A?
Akuli almost
Akuli sqrt(5) A
adder lol
Akuli so, 2.23 amps
Akuli plenty for most things i want to measure
Akuli ok?
adder Hate that I needed to fire up a Python shell to compute 0.5/0.1.
Akuli :D
Example: Maximum voltage of standard 1kΩ 0.25W resistor
Akuli let's do another example
Akuli how much voltage can your 1kΩ resistors handle?
Akuli (hint: pick a formula without current)
adder Are we lacking info? P?
Akuli your resistors are 0.25 watt
adder Ah.
Akuli that's the standard small resistors
adder 0.25W * 1000
adder 250V?
Akuli 250 watts is a lot
Akuli 6 of those and you have the heater i linked :DDD
Akuli ehhh sorry
Akuli i asked about voltage, nevermind
adder :>
Akuli the answer is sqrt(250) volts
adder Oh fuck me
adder whoops
Akuli :D
Akuli so, 15.8 volts
adder I see.
Akuli can't use those for that LED indicator light on 24V power supply
adder :)
Akuli must pick higher resistance to get less current
Combining multiple resistors to handle more power
Akuli or, here's a trick that is sometimes useful: combine multiple different resistors, either in series or in parallel
Akuli we haven't talked about this yet, but three 1Ω resistors connected to each other correctly is same as one 1/3 Ω resistor
Akuli ok?
adder Why would chaining up resistors decrease the total resistance?
Akuli depends on how you connect them
Akuli if you do: ----resistor1---resistor2---resistor3----
Akuli then resistances are added
Akuli and you would get 3Ω
adder Aaah I see.
Akuli if you do:
Akuli ,---resistor1---.
Akuli | |
Akuli ---o---resistor2---o----
Akuli | |
Akuli `---resistor3---'
Akuli then you get 1/3 ohms
adder Right.
Akuli i used this exact setup in my stereo amplifier build
Akuli in a traditional audio amplifier, you have two output transistors for each speaker
adder Cool.
Akuli one output transistor increases the speaker's voltage ("pull up"), another to decrease it ("pull down")
Akuli inserting a few 0.33Ω resistors makes it easier to tweak those transistors to basically behave nicely when the amplifier is idle
Akuli the point is, i needed 0.33Ω resistors, and all the output going to speakers would go through them
adder I see.
Akuli so by chaining some 1Ω resistors i had, i "made" 0.75W 1/3 Ω resistors
adder :)
Akuli so that sort of tricks can be used to work around power issues, if you don't want to buy the right thing
Akuli i think that's all you need to know about power :)
adder OK. :)
Note from Akuli:
The first kind of chaining I showed is called series, and the second is called parallel.
For resistors in series, you can add the resistances as I explained. But we didn't discuss how to combine the resistances of multiple parallel resistors with different resistances (the only example was three 1Ω resistors). There is a formula for that, and you will find it easily if you look for it online.
