Our first electronics project is also one of the most useful tools for electronics projects - a bench power supply. While you're probably reading this in a room with dozens of power supplies, (Phone, laptop, whatever) using them will come with a ton of problems.
Generally speaking, power supplies are designed to give no more power than the thing they were sold with could ever use, and only at the voltage (or voltages) that gizmo was designed for.
A lot of them are cheaply made, too. Sometimes important safety features are left out because they never expected them to be used for any other purpose. If your project needs more power than they're meant to give, the results are impossible to predict. They could be catastrophic.
I like making things and I don't like house fires, so let's build a safe, controllable power supply. As a bonus, you'll learn the fundamentals of electronics, stuff that you'll take with you to every following project.
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Let's have a look at what we're aiming for:
Only the finest illustrations!
This is our demonstration Bench PSU, an imaginary model I just made up.
It has Four displays, two knobs, a button, three sockets, three indicator LEDs, power and reset controls. Don't get overwhelmed! Let's break them down bit by bit.
VOLTAGE
Voltage (Symbol = V) is a measure of electrical pressure. That is quite literal, it describes how hard the electricity will be forced through whatever it is you've connected it to. The more voltage is forced through, then the more power will be able to come along for the ride.
That means that controlling the voltage is EXTREMELY important. If we don't supply enough voltage to whatever we want to run, it just won't get enough power. It'll act strange or just not work.
Likewise, if we supply too much voltage, we can have results anywhere between "my model train is going too fast" to "oh, it's on fire now".
The display shows us exactly what the voltage output is that very moment, and the knob adjusts it smoothly from zero to maximum. There is no keypad nor enter key because it can be useful to see how a circuit behaves when voltage is brought up smoothly, rather than in steps, or all at once.
CURRENT LIMIT
This section is current control. Current is measured in Amperes, usually shortened to Amps (Symbol = A)
If Voltage is a measure of electrical pressure, then Current might be thought of as how wide the pipe it's coming down is. Current is very different from Voltage in that it's consumption is not forced, but instead up to the circuit that's consuming it.
For example, say we have a lamp which is designed for 12 Volts and 0.5 (one half) Amperes. If we connected it up to a power supply of 12 Volts and 5 Amperes, it would happily light up without a single problem. It would take the 0.5A that it needs, and ignore the remaining 4.5 Amps the supply could give.
That makes it sound like it's not important to control current, and a lot of the time, that's true. However, it's possible for a damaged or wrongly made circuit to short out, or otherwise pull far too much current than it should do. If that happens, it could become a fire hazard, or maybe make a sudden loud bang with nasty ass smoke pouring out, so we better control it just in case.
Unlike with Voltage, we don't set an exact amount of Current. Instead, we set an allowed maximum. The display shows this, and the knob adjusts it much like with the voltage control.
If the allowed current is exceeded, the power supply will detect this and immediately cut the supply. A buzzer will sound, the green LED will go dark, and the red LED will light up. The supply will stay cut until either the PSU is turned off and back on again, or the RESET button is pushed.
It goes without saying, but you should probably disconnect whatever tripped the cutoff circuit before resetting the PSU!
Outputs
The Red socket is our power output, at whatever voltage you set. Above that is the Current Draw indicator, which shows you the actual amount of current your project is using.
(If you want a straight power number, you can multiply Current draw * Volts. The answer is your power in Watts, Symbol = W)
The Green socket is GND (Ground, Common or Earth), the return path back from what you're powering, and so has Zero Volts. It exists to complete the circuit, a mandatory electricity thing, without which nothing would work.
The Black socket is electricity's Dark Gary. It does the equal and opposite of the Red socket, providing negative voltage. That is to say, if Red pushes electricity "towards" ground, then Black sucks electricity "away" from ground, to itself. Why would we want this?
Well, it's a surprise tool that'll help us later.
NEXT UP: Bench Power Supply Part 2: We have to talk about Mains Electricity now unfortunately
If you'd like to see this series continue, and you can afford it,
please put a buck in the tip jar!
I earn only £800 a month, spend more than half of that on rent alone, and so supplies and tools are a significant expense. Your support would be greatly appreciated! Who knows, if this really takes off I might buy a freezer.
