Components and Connectors

Every component has one thing it does and one thing it refuses to do. Learn the refusal and the datasheet becomes readable.

How to read any component#

There are thousands of parts and about eight behaviours. You are not learning a catalogue, you are learning a small number of behaviours and how to recognise which one you are holding.

For every component, ask four questions in this order:

1. What does it do to current? Resist it, store it, block it in one direction, amplify it, switch it.
2. What is its limit? Every part has a rating it will not survive: voltage, current, power, temperature. Almost every failure is a limit exceeded.
3. How does it fail? Open or short. This matters enormously for fault-finding, and different parts have strong tendencies.
4. What does it do that you did not ask for? Every real component is a mixture. A resistor has inductance. A capacitor has resistance. A wire has both. Class 7 and Class 9 are largely about these unwanted extras.

Resistors#

A resistor opposes current and turns the difference into heat. That is all it does, and it does it extremely reliably.

Reading them. Four bands: two digits, a multiplier, a tolerance. Five bands: three digits, a multiplier, a tolerance. The tolerance band is usually gold or silver and slightly separated, which is how you know which end to start from. Surface mount parts are marked with numbers instead: 472 is 47 followed by two zeros, so 4.7 kΩ.

The rating that catches people is power, not resistance. A quarter-watt resistor asked to dissipate half a watt will work, get very hot, drift in value, discolour the board, and eventually fail. Use P = I² × R before you choose the physical size, every time.

How they fail: almost always open circuit, often after a period of drifting high. A resistor that has gone open is easy to find. A resistor that has drifted from 100 Ω to 140 Ω because it ran hot for two years is the fault that takes an afternoon.

Capacitors#

A capacitor stores charge in an electric field. Its useful behaviour is that it opposes a change in voltage: it resists being made to change quickly, and it takes time to charge and discharge.

That time is the RC time constant, τ = R × C, and it is one of the most useful numbers in this course. In one time constant the capacitor gets 63 per cent of the way to its target. In five, it is there for all practical purposes.

You will use capacitors for four things, and it is worth naming them because a capacitor chosen for the wrong job is a common design fault:

The one that bites theatre people: electrolytic capacitors are polarised and will vent, loudly, if you reverse them. They also dry out with age and heat, which is why a piece of equipment that worked when it was put in the store in 2014 does not work now. Failed electrolytics are the single most common fault in old show equipment, and they are visible: a domed top or crusted residue.

And the one that hurts: a large capacitor in a mains power supply holds a dangerous charge after the equipment is unplugged. Sometimes for minutes. Treat every large capacitor in mains equipment as live until you have measured it, and discharge it properly rather than by shorting it with a screwdriver.

Diodes and LEDs#

A diode passes current in one direction and blocks it in the other. It is the simplest useful non-linear part and it appears everywhere.

The key number is the forward voltage drop: about 0.7 V for a standard silicon diode, about 0.3 V for a Schottky, and 1.8 to 3.4 V for an LED depending on colour. That drop is nearly independent of current, which is what makes the next section true.

LEDs, and why they need a resistor#

An LED is a diode that emits light, and it has the diode's steep curve. Below its forward voltage it passes almost nothing. Above it, current rises almost vertically. There is no comfortable middle where it self-regulates.

So an LED connected straight across a supply does not settle at a sensible current. It draws as much as the supply will give, heats up, and a hotter LED conducts more at the same voltage, which heats it further. That is thermal runaway, and it takes seconds.

The fix is a series resistor that sets the current:

R = (V_supply − V_forward) ÷ I_desired

For a red LED at 20 mA on a 5 V rail: (5 − 2.0) ÷ 0.02 = 150 Ω. Then check the power: 0.02² × 150 = 0.06 W, so a quarter-watt part is comfortable.

The LED resistor tool does this and shows the working, but do the arithmetic by hand at least twenty times this term, because it comes up in Class 6, Class 11 and the capstone.

Where this matters professionally: every LED fixture in your rig contains a constant-current driver doing exactly this job in a more sophisticated way. When somebody tells you an LED fixture "dims badly on a dimmer", the reason is in this section: a phase-controlled dimmer varies the voltage, and voltage is not what an LED responds to sensibly.

Wire it before you wire it#

A pushbutton, a pull-down, and a counter. Press it once and read three. Then take the pull-down out and watch what an input does with nothing holding it, which is not zero and is not one.

Inductors, and the kick#

An inductor stores energy in a magnetic field, and it opposes a change in current — the exact mirror of a capacitor.

You will meet inductance mostly as a nuisance and once as a hazard. Any coil of wire is an inductor: a relay coil, a solenoid, a motor winding, a transformer, a contactor.

The hazard is what happens when you interrupt the current. The magnetic field collapses and the inductor generates whatever voltage it needs to try to keep the current flowing. Switching a 12 V relay coil can produce a spike of hundreds of volts across the switching device. It will destroy a transistor instantly and it will erode a mechanical contact over time.

The cure is a flyback diode across the coil, reverse biased, giving the collapsing current a loop to circulate in. It costs a few cents and its absence is one of the most common faults in home-built show electronics. Class 5 and Class 6 are built around this.

Semiconductors, in one page#

You will meet these properly in Class 5 and Class 11. For now, recognise them.

• Transistor (BJT). A small base current controls a much larger collector current. Three legs. Used as a switch or an amplifier. Cheap, forgiving, needs a base resistor.
• MOSFET. A voltage on the gate controls current between drain and source. Three legs, looks similar, behaves differently: it takes almost no steady current to hold it on, but it is static sensitive and it needs its gate driven properly. The workhorse for switching LED strips, solenoids and motors.
• Voltage regulator. Three legs, takes a messy input voltage and produces a clean fixed output. A linear one burns off the difference as heat, which is why a 12 V to 5 V linear regulator running a metre of LED strip needs a heatsink or a rethink.
• Optocoupler. An LED and a light sensor in one package, with no electrical connection between them. This is how a 5 V microcontroller safely tells a 230 V circuit what to do, and it is the most important part in this list for a technical director.
• Integrated circuit. Anything from a logic gate to a microcontroller. Read the number on top and search it; the datasheet's first page is written for exactly this moment.

Connectors: why the industry chose what it chose#

Connectors are where theatre electronics actually fails, so they get an hour.

Every connector in a venue is a set of answers to the same five questions: how much current, is it signal or power, does it need to lock, does it need to be sexless or gendered, and what must it be impossible to plug into by mistake.

The two rules#

One: a connector must make it impossible to do the wrong thing. This is why DMX is on 5-pin XLR and not 3-pin, even though only three pins are used. Somebody decided that plugging a DMX line into a microphone input should be physically impossible, and they were right. When you see a rig using 3-pin XLR for DMX, that safeguard has been thrown away to save money, and the failure it causes will be intermittent and hard to find.

Two: the connector is not the weak point, the strain relief is. A cable almost never fails in the middle. It fails where it enters the connector, because that is where the flexing is concentrated. Class 4 is about that sentence, at a bench, with an iron.

Transformers#

Two coils sharing a magnetic core. Alternating current in one induces a voltage in the other, in the ratio of their turns, and nothing conducts between them.

V_secondary ÷ V_primary = N_secondary ÷ N_primary

A 230 V to 12 V transformer has about nineteen times as many turns on the primary. Power is conserved less the losses, so the current ratio is inverted: twelve volts at five amps out is about 230 volts at 0.28 amps in.

Three reasons they matter to you, and only one of them is about changing voltage:

Isolation. There is no conductive path between primary and secondary. This is the strongest form of isolation available and it is why an isolating transformer appears on the safety card.

They only work on AC. A transformer fed DC is a short circuit with a time constant, and it will draw current until something gives. This is not a subtlety, it is the second most common way people destroy transformers.

Inrush. Energising a transformer at the wrong point in the mains cycle saturates the core and draws a very large momentary current, which is why a rack of transformer-based equipment trips a breaker at switch-on and runs happily afterwards. That is Class 5.

Toroidal transformers are quieter and more efficient and have worse inrush. Switch-mode supplies have replaced them nearly everywhere, which is why almost everything in a modern rig has the switch-mode signature you will find on a scope in Class 8.

Fuses, breakers and the things that pretend to be them#

A protective device is characterised by two numbers and a curve: the current it will carry indefinitely, the current at which it opens quickly, and how long it takes at everything in between.

The trap is the slow-blow. A device with an inrush needs a fuse that tolerates it, and fitting a fast-blow "because it is the same rating" gives you equipment that blows a fuse every time it is switched on. Fitting a fast-blow where a slow-blow belongs is the more common direction; fitting a larger fuse to stop the nuisance is the dangerous one.

And the PTC is not a fuse. It goes high resistance while the fault persists and recovers when it cools, which means a fault behind a PTC sits there cycling rather than announcing itself. It protects the board and it hides the problem.

At the bench#

Two hours.

Block A — Identify fifty parts (40 min)#

Fifty components in trays. For each: name it, read its value from its markings, measure it, and say how it fails. Log all four.

Some of them are marked in ways you have not seen. That is deliberate: the skill is reading an unfamiliar marking system by working out what it must mean, not recalling one you were taught.

Block B — The LED that dies (30 min)#

Calculate the correct series resistor for a red LED on 5 V at 20 mA. Build it. Measure the actual current and the voltage across each part, and confirm they sum to the supply.

Then, on the sacrificial rig, with eye protection, connect an LED with no resistor and watch the current on the bench supply's display. Record how long it takes and what the final current was. You get one LED for this and you will remember it.

Block C — Capacitor time constants (30 min)#

With a 100 kΩ resistor and a 100 µF capacitor, predict the time constant, then charge the capacitor and time it with a stopwatch to 63 per cent of the supply voltage. Repeat with 10 kΩ.

Then discharge a 4700 µF capacitor through a 100 Ω resistor and measure how long it takes to become safe to touch. Write that number down. It is the reason for the rule about capacitors in mains equipment.

Block D — Connector inspection (20 min)#

A tray of connectors, half of them faulty. Find the faults by eye and by meter: bent pins, cold joints, strands escaping the barrel, missing strain relief, a shield connected at both ends when it should not be, a cracked shell. Photograph each fault and write one sentence on how it would present during a show.

Numbers from this class#

Self test with model answers

Answer it yourself first, out loud or on paper, then open the answer. Reading the answer without attempting the question teaches you almost nothing.

1. 1Why does an LED need a series resistor rather than just the right voltage?
2. 2Calculate the series resistor and its power for a blue LED at 20 mA on a 12 V rail.
3. 3What is the flyback problem, and what does it cost to solve?
4. 4Why is DMX on 5-pin XLR when it only uses three pins?
5. 5A board has a 100 nF ceramic capacitor next to every chip and a 470 µF electrolytic at the supply entry. Why both?

Note. These are model answers, not the only correct ones. If yours differs and you can defend it with the arithmetic and a measurement, that is worth more than matching the wording.