Common circuit devices: some basics
Resistors: ‘resisting’ the flow
By Ohm’s law (E = IR), a potential difference of 1 V will force a current of 1 A through
a resistance of 1 .
Resistors in series add resistance. Two resistors in parallel present an ‘equivalent
1 1 r xr
resistance’ Rt such that Rt    1 2
r1 r2 r1  r2
Resistors are often used in series with another circuit element to ‘limit’ the amount of
current flowing through that device. For example, you usually place a resistor in series
with an LED (see below).
Other common applications:
Voltage dividers
Loads
Capacitors: storing charge
A 1 F capacitor connected to a 1 V supply stores 6.28 x 1018 electrons. Common
capacitors have values in the microfarad range.
Capacitors in series add capacitance. Capacitors in parallel present an equivalent
1
1
1
 ... 
capacitance according to Ct  
C1 C2
Cn
Electrolytic capacitors have polarity. Observe the markings on circuit diagrams and on
the devices themselves.
Warning: capacitors can retain their charge for a long time. Use caution when handling:
always assume that a capacitor might be charged!
The voltage on a capacitor increases exponentially, according to a time constant. This is
determined by the product of the capacitance and the resistance in series with the
capacitor.
Applications:
filtering
removing dc
RC circuits: integrator, differentiator, timer
Diodes and rectifiers: one way traffic!
current
These are semiconducting devices that allow current to
pass in one direction only. It is important to note that a
diode does not begin conducting until the forward voltage
reaches a threshold value (for silicon diodes, 0.6 V; for
germanium diodes 0.3 V). The diode therefore drops the
applied voltage by the magnitude of this threshold.
Diodes can be damaged if a voltage is applied in the nonconducting ‘reverse’ direction that exceeds a reverse
threshold (Vr) or if the current in the forward conducting
direction exceeds a breakdown value (If).
voltage
Diodes are used in many ways:
LED’s (light emitting diodes) as pilot lamps or indicators (note an LED does not
‘turn on’ if the applied voltage is less than its forward voltage (usually about 1.7
V). LED’s cannot handle much current and therefore need a limiting series
resistor.
Inexpensive voltage droppers,
Voltage regulators,
Protection from reverse polarity,
Protection from transient ‘voltage spikes’,
Waveform ‘clippers’
and most importantly as rectifiers.
Diodes also can be combined into primitive logic gates.