Voltage: The amount of work required to move electrons along a conductor.
Voltage (V) is measured as a potential difference between two poles, and can be considered analogous to pressure or force.
Current: The movement of electrons along a conductor.
Current is measured in amperes (amps, A), and is analogous to the flow of a fluid through a pipe. The greater the movement, the greater the current.
Resistance: The opposition of a conductor or device to current flow.
Measured in Ohms (Ω), resistivity is the nature of a substance to resist current flow. Note that resistance does not reduce the energy (or work) being exerted to move the electrons: conservation of energy requires that energy is not “destroyed.” The energy that would be moving electrons—but which cannot, because of resistance—is emitted in some other way, most commonly as heat or light. (This is, of course, how a light bulb works: the filament conductor resists current flow, and as a result emits light and heat.)
Ohm’s Law defines the mathematical relationship among voltage, current (I, measured in amps), and resistance (R, measured in Ohms):
V = I x R
Current is directly proportional to voltage, and indirectly proportional to resistance; in other words, if voltage increases and resistance stays the same, then amperage increases. If voltage stays the same and resistance increases, then amperage will drop.
Direct Current (DC) and Alternating Current (AC)
A direct current (DC) circuit draws current from a battery source, and the current flow is in one-direction. DC power can vary in both amperage and voltage. In an alternating current (AC) circuit, current flows in both directions, oscillating from one to the other very quickly. (The speed of the alternations is measured in hertz, or frequency of oscillations.) AC power has the advantage of being able to be “stepped up” or “stepped down”—voltage can be increased or decreased using a transformer, allowing current to be delivered over much longer distances.
Peak Voltage: the maximum voltage reached by the signal.
RMS (Root Mean Square) Voltage (current): the maximum effective voltage (current) of the signal.
Because AC voltage varies from peak voltage, through zero, to peak voltage (in the other direction), the voltage is only at peak for a portion of the time the current flows. The root mean square voltage is the equivalent DC voltage of the circuit. Generally speaking, the relationship between RMS and peak voltage can be expressed as:
RMS = 0.7 x Peak
Peak = 1.4 x RMS
(These equations also apply to current.) Unless otherwise stated, listed voltages can be assumed to be RMS; a 6V AC power supply will supply 6V RMS, or 8.6V peak.
Electrical Circuits
Series Circuits
In a series circuit, current can only flow along a single path, through any loads or devices connected in the circuit. Each load or device adds resistance to the circuit; as a result, the total resistance of the circuit is cumulative.
Figure 1: Three lamps in series
In the example above, three lamps, each with a resistance of 2Ω, are wired in series in 24V DC circuit. In order to calculate the amperage draw on the circuit, we need to first calculate the resistance. Because in series circuits resistance is cumulative, we can simply add the resistances of individual components together:
2 + 2 + 2 = 6 Ohms
Once we know the total resistance, calculating the total amperage draw is a simple matter of applying Ohm’s Law:
V = I x R
I = V/R
I = 24/6 = 4 amps
Of course, if we knew the amperage draw of the lamps but not the resistance, we can perform a similar calculation, adding the amperages together and applying Ohm’s Law once again.
Parallel Circuits
In a parallel circuit, all of the positive terminals are connected together and all of the negative terminals are connected together, providing multiple paths for electron flow.
Figure 2: Three lamps in parallel
Because of this, resistance must be calculated differently in a parallel circuit:
Calculating amperage draw using Ohm’s Law, we can see that a parallel circuit draws more amperage than a series circuit:
V = I x R
I = V/R
A = 24/0.667 = 35.98 Ohms
Series Parallel Circuits
Series parallel circuits get an order of magnitude more complicated as they contain parallel branches which have devices in series within them. However, by breaking these circuits down branch by branch, resistances in these circuits can be calculated relatively easily.
Figure 3: Six lamps in a series parallel circuit
If we number each parallel branch from the top, branch one has two lamps, each with 2 ohms of resistance; branch two has one lamp with 2 ohms resistance; and branch three has three lamps, each with 2 ohms of resistance. We know that in series circuits, resistance is cumulative, so we can calculate the total resistance of each branch by adding the resistances of each device on each branch together:
For branch one:
2 + 2 = 4 Ohms
For branch three:
2 + 2 + 2 = 6 Ohms
Once we’ve calculated the resistances of each branch, we can use the formula for calculating total resistance in a parallel circuit from above:
Understanding series and parallel circuits and Ohm’s Law can help us not only understand existing circuits, but also to manipulate devices in a circuit to more efficiently use voltage. Compare the three-lamp series and parallel circuits above. As we saw, the total amperage draw in the parallel circuit was much greater than that in the series circuit. If we had six 2 ohm lamps in the circuit, the total amperage draw would increase in the parallel circuit, but decrease in the series circuit:
For the series circuit, the amperage draw would decrease to 2 amps:
For the parallel circuit:, the amperage draw would increase to 72 amps:
Consequently, careful construction of series parallel circuits can aid in keeping our amperage costs down.
Normally Open and Normally Closed
Switches (toggles, pushbuttons, keyed switches, etc.) come in two general flavors: normally open, and normally closed. A normally open switch (or contact) is one which requires action to allow current to flow. In other words, a normally open switch must be engaged to close the circuit. A normally closed switch is just the opposite; a normally closed switch requires action to break, or open, the circuit.
Figure 4: Block diagrams for normally open and normally closed switch
Basic Electricity
by Rich Dionne
Sep 7, 2012
Basic electricity terms
Voltage: The amount of work required to move electrons along a conductor.
Voltage (V) is measured as a potential difference between two poles, and can be considered analogous to pressure or force.
Current: The movement of electrons along a conductor.
Current is measured in amperes (amps, A), and is analogous to the flow of a fluid through a pipe. The greater the movement, the greater the current.
Resistance: The opposition of a conductor or device to current flow.
Measured in Ohms (Ω), resistivity is the nature of a substance to resist current flow. Note that resistance does not reduce the energy (or work) being exerted to move the electrons: conservation of energy requires that energy is not “destroyed.” The energy that would be moving electrons—but which cannot, because of resistance—is emitted in some other way, most commonly as heat or light. (This is, of course, how a light bulb works: the filament conductor resists current flow, and as a result emits light and heat.)
Ohm’s Law defines the mathematical relationship among voltage, current (I, measured in amps), and resistance (R, measured in Ohms):
V = I x R
Current is directly proportional to voltage, and indirectly proportional to resistance; in other words, if voltage increases and resistance stays the same, then amperage increases. If voltage stays the same and resistance increases, then amperage will drop.
Direct Current (DC) and Alternating Current (AC)
A direct current (DC) circuit draws current from a battery source, and the current flow is in one-direction. DC power can vary in both amperage and voltage. In an alternating current (AC) circuit, current flows in both directions, oscillating from one to the other very quickly. (The speed of the alternations is measured in hertz, or frequency of oscillations.) AC power has the advantage of being able to be “stepped up” or “stepped down”—voltage can be increased or decreased using a transformer, allowing current to be delivered over much longer distances.
Peak Voltage: the maximum voltage reached by the signal.
RMS (Root Mean Square) Voltage (current): the maximum effective voltage (current) of the signal.
Because AC voltage varies from peak voltage, through zero, to peak voltage (in the other direction), the voltage is only at peak for a portion of the time the current flows. The root mean square voltage is the equivalent DC voltage of the circuit. Generally speaking, the relationship between RMS and peak voltage can be expressed as:
RMS = 0.7 x Peak
Peak = 1.4 x RMS
(These equations also apply to current.) Unless otherwise stated, listed voltages can be assumed to be RMS; a 6V AC power supply will supply 6V RMS, or 8.6V peak.
Electrical Circuits
Series Circuits
In a series circuit, current can only flow along a single path, through any loads or devices connected in the circuit. Each load or device adds resistance to the circuit; as a result, the total resistance of the circuit is cumulative.
Figure 1: Three lamps in series
In the example above, three lamps, each with a resistance of 2Ω, are wired in series in 24V DC circuit. In order to calculate the amperage draw on the circuit, we need to first calculate the resistance. Because in series circuits resistance is cumulative, we can simply add the resistances of individual components together:
2 + 2 + 2 = 6 Ohms
Once we know the total resistance, calculating the total amperage draw is a simple matter of applying Ohm’s Law:
V = I x R
I = V/R
I = 24/6 = 4 amps
Of course, if we knew the amperage draw of the lamps but not the resistance, we can perform a similar calculation, adding the amperages together and applying Ohm’s Law once again.
Parallel Circuits
In a parallel circuit, all of the positive terminals are connected together and all of the negative terminals are connected together, providing multiple paths for electron flow.
Figure 2: Three lamps in parallel
Because of this, resistance must be calculated differently in a parallel circuit:
Calculating amperage draw using Ohm’s Law, we can see that a parallel circuit draws more amperage than a series circuit:
V = I x R
I = V/R
A = 24/0.667 = 35.98 Ohms
Series Parallel Circuits
Series parallel circuits get an order of magnitude more complicated as they contain parallel branches which have devices in series within them. However, by breaking these circuits down branch by branch, resistances in these circuits can be calculated relatively easily.
Figure 3: Six lamps in a series parallel circuit
If we number each parallel branch from the top, branch one has two lamps, each with 2 ohms of resistance; branch two has one lamp with 2 ohms resistance; and branch three has three lamps, each with 2 ohms of resistance. We know that in series circuits, resistance is cumulative, so we can calculate the total resistance of each branch by adding the resistances of each device on each branch together:
For branch one:
2 + 2 = 4 Ohms
For branch three:
2 + 2 + 2 = 6 Ohms
Once we’ve calculated the resistances of each branch, we can use the formula for calculating total resistance in a parallel circuit from above:
Understanding series and parallel circuits and Ohm’s Law can help us not only understand existing circuits, but also to manipulate devices in a circuit to more efficiently use voltage. Compare the three-lamp series and parallel circuits above. As we saw, the total amperage draw in the parallel circuit was much greater than that in the series circuit. If we had six 2 ohm lamps in the circuit, the total amperage draw would increase in the parallel circuit, but decrease in the series circuit:
For the series circuit, the amperage draw would decrease to 2 amps:
For the parallel circuit:, the amperage draw would increase to 72 amps:
Consequently, careful construction of series parallel circuits can aid in keeping our amperage costs down.
Normally Open and Normally Closed
Switches (toggles, pushbuttons, keyed switches, etc.) come in two general flavors: normally open, and normally closed. A normally open switch (or contact) is one which requires action to allow current to flow. In other words, a normally open switch must be engaged to close the circuit. A normally closed switch is just the opposite; a normally closed switch requires action to break, or open, the circuit.
Figure 4: Block diagrams for normally open and normally closed switch