Power Supply, Switch, Load: Reading a Simple Circuit

A schematic is not a picture of the inside of a device. It is a map of where current can go. Once you can trace a path on paper you can find the one place the path is broken.

Every circuit has three parts

1. A source: the battery, the power supply output, the wall outlet. It provides the push.
2. A control: the switch, relay, or transistor that decides whether the path is complete.
3. A load: a lamp, a heater, a DC motor in a pump, a speaker, or a circuit board that does the work and uses the power.

Add the wires that join them and you have a circuit. If current is not flowing, exactly one of those four things is at fault, and the meter tells you which.

Switches are the control you meet most, and they are described two ways at once. By feel: a toggle switch flips, a rocker switch tips one end in, a pushbutton switch springs back when you let go, and a membrane keypad is a printed version of the same contact. By what the contacts do: a single-pole switch breaks one conductor while a double-pole switch breaks two at once, which is how a mains switch opens hot and neutral together, and a single-throw switch has one on position while a double-throw switch moves its common contact between two outputs.

Symbols to recognize on sight

cell             one long line and one short line; the long line is positive
battery          several cells stacked, long and short alternating
resistor         a zigzag, or a plain rectangle on newer drawings
lamp             a circle with an X or a loop inside
switch           a line lifted away from a contact
fuse             a loop, or an S, between two terminals
circuit breaker  a switch with a small hook or an arc through it
capacitor        two parallel plates; a curved plate means polarized, so it goes in one way
inductor         a series of loops or humps, like a coil of wire
transformer      two coils facing each other, often with lines between them for the core
diode            a triangle pointing into a bar; current flows the way the triangle points
LED              the same triangle and bar with two small arrows coming off it
ground           three horizontal lines, each shorter than the one above
chassis ground   a small hatched bracket or rake

Crossing versus joining

Two lines that cross with no dot are two wires passing each other with no connection. A dot at the junction means they are joined. Older drawings used a little hop over the crossing wire to say the same thing. Misreading a dot is how a beginner sees a short that is not there.

Following the power path

This is the single most useful skill in the unit. A device is dead; trace the path in order and measure at each step.

1. The outlet and the cord. Is there voltage at the receptacle, and is the cord intact at both ends?
2. The plug and the inlet. Many devices have a detachable cord that works loose.
3. The fuse or breaker. Measure across it: a good fuse has almost no voltage across it, an open one has all of it.
4. The switch. Across a closed switch you should read almost nothing; the full supply voltage across it means it is open.
5. The power supply. Does its output rail read the voltage printed beside it?
6. The load. If the supply is good and the load does nothing, the fault is in the load or in the wiring to it.

You walk the path until the voltage disappears. The part between the point where it is present and the point where it is gone is the part that is broken.

Block diagrams

A block diagram drops the symbols and draws each section as a labeled box with arrows for the signal flow: mains in, power supply, controller, display, motor drive, motor. It will not tell you which component failed, but it tells you which section to put the probes in, and it is the first page you should look at in a service manual for a device you have never opened. Signals are drawn flowing left to right and power top to bottom, so read it that way.