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DC voltage: what is it? (Circuit symbol and cable color codes)

August 1, 2021 from Electrical4U < ; svg viewBox = "0 0 951 1024" class = "icon twit" & gt; < / svg> < svg id = "face" viewBox = "0 0 585 1024" class = "icon" & gt; < / svg> < svg id = "lkn" viewBox = "0 0 512 512" class = "icon" & gt; < / svg>

Table of Contents

What is DC voltage?

DC voltage means "direct current voltage". Although it sounds confusing, the term "DC" has been used more broadly to refer to a system that has constant polarity. DC voltage is therefore a voltage that produces or would produce a direct current.

In contrast, an alternating voltage is a voltage that produces or would produce an alternating current.

DC in this context refers more generally to quantities that do not regularly change polarity or have zero (or actually zero) frequency. AC refers to quantities that regularly change polarity at a frequency above zero.

Voltage is the difference in electric potential per unit charge between two points in an electric field. Electrical energy is generated by the movement and existence of charged particles called electrons.

The movement of electrons creates a potential energy difference between two points. We call this potential difference voltage.

There are two types of electrical energy; AC and DC. As mentioned, the voltage acquired from the DC source is known as the DC voltage.

DC voltage has a constant value.

And it's called VDC. The frequency of the DC voltage is zero (or almost zero). Therefore, DC voltage systems will not change polarity during operation.

DC voltage symbol

The Unicode character -U+2393 "⎓" is used for DC applications. Sometimes it is also symbolized by a straight line.

Several DC sources are available in a wiring diagram to obtain DC voltage. A battery is the most commonly used source for DC voltage. The figure below shows a battery symbol.

Battery Symbol

Another commonly used circuit symbol for a DC voltage source is shown below.

DC Voltage Source Symbol

The ideal DC voltage source has zero internal resistance.

But a real DC source will always have some internal resistance.

In an ideal voltage source, the voltage drop across the source is zero. But in the case of a practical voltage source in the real world, some voltage drop occurs. This voltage drop increases as the current increases.

The V-I characteristics of the ideal and effective DC voltage source are shown in the figure below.

VI Characteristics of Ideal and Efficient DC Voltage Source

Color Code of DC Voltage Leads

Systems of wiring they are color coded to identify individual wires. This type of system is particularly useful for identifying cables during maintenance activities.

The term "direct current" is often used in place of "DC voltage" when discussing wire color codes (note: DC power is not interchangeable with the term DC voltage. But "DC power" is used more broadly to indicate that the circuit itself is DC, not AC).

IEC DC Power Circuit Cable Color Codes

The IEC (International Electrotechnical Commission) provides a standard for the wiring code.

The US national electrical code differs from the IEC.

IEC standards for DC voltage cables are shown in the table below.

IEC Function Standard

LabelColor Protective EarthPEGreen - Yellow Bare Two-Wire DC System Positive L+ Brown Negative L-Gray Two-Wire Grounded DC System Positive Circuit (Grounded Negative) L+ Brown Negative circuit (negative grounded) MBlue Positive circuit (grounded) MBlue Negative circuit (positive grounded) L -Gray Three-wire grounded DC system Positive L + Brown Mid-wireMBlueNegativeL-Gray IEC Standards for DC Voltage Cables

United States DC Power Circuit Cable Color Codes

The US National Electrical Code has discouraged the use of ungrounded systems for safety reasons. Therefore, there are no wire color codes for ungrounded DC systems under the United States National Electrical Code.

The recommended wire color codes for grounded systems under the United States National Electrical Code are those shown in the following table.

Feature US National Electrical Recommended Code Label color Protective earth PGBare, green or green-yellow Ungrounded two-wire DC system Positive L + No recommendation (Red) Negative L-No recommendation (Black) System DC Two-wire grounded ground Positive circuit (with negative ground) L+ Red Negative circuit (with negative ground) N white Positive circuit (with positive ground) N white Negative circuit (with positive ground) L-Black DC system with three-wire groundingPositiveL+ RedMid-wireNwhiteNegativeL-BlackRecommended US National Electrical Code

Difference Between DC and AC Voltage Voltage

The main difference between DC voltage and AC voltage is shown in the table below.

Direct voltageVoltage CAD Definition The direct voltage induces a direct current of constant value. force that derives from current span>

In some scenarios, it is necessary to reduce the DC voltage.

For example, suppose a battery or cell is used to provide DC voltage.

You have a 9V battery, but the circuit only requires 3V. You will need to reduce the supply voltage in order to connect the battery to the circuit.

The DC voltage can be reduced using diodes and resistors.

How to reduce DC voltage with diodes

A diode only conducts when it is forward biased. When connecting a diode, there is a certain voltage drop across it.

The voltage drop in small silicon diodes is about 0.6-0 .7 V, germanium diode 0.25- 0.3 V, Schottky diode 0.


We can further increase the voltage drop if we use diodes in series with the load or circuit. This has been illustrated in the following circuit:

DC Voltage Reduction with Diodes

In our example, we need a 6V drop between the 3V load and the 9V battery. If we are using silicon diodes, we need to add 4 or 5 diodes in series as shown in the figure above.

The anode of the first diode is connected to the positive pole of the battery.


And the anode of the second diode is connected to the cathode of the first diode. If you need to create an even voltage drop, you can join multiple diodes in the same way.

How to reduce DC voltage with resistors

Resistors can be used to create a voltage divider circuit. Using this circuit we can achieve any desired voltage drop.

You can get half the input voltage by connecting two resistors of the same value in series.

But if you need a specific voltage drop, you can use the voltage drop formula below:

DC voltage reduction with resistors

From the above formula you can get the voltage across resistor R2. This is the voltage you need. But from this formula alone, you cannot find the resistance of R2.

For this you must assume the resistance of a resistor, input and output voltage. Insert these values ​​into the following equation to find the resistance of R2.

For example, you have a 5V battery input. And the circuit requires a 3V output. Let's say you have a 10Ω resistor. Now find the resistance of the second resistor R2.

Example of DC voltage reduction with

Comment increasing dc voltage

Increasing voltage means increasing the voltage level.

You can step up the DC voltage by using a DC-DC power converter called a boost converter.

Upconverter is a class of switch-mode power supplies (SMPS) that contains at least two switches. semiconductors (diode/transistor) and an energy storage element (inductor/capacitor).

Filters are used to reduce voltage ripple. Usually filters are made of capacitors or a combination of capacitors and inductors. And it is connected to the load side or the power side of the converter.

In the operation of the boost converter, the power remains the same. When the voltage increases, the current decreases, which makes the power constant.

In AC power systems, a transformer is used to step up and down the voltage level. This relies on alternating current (AC) to create a variable flux and induce current from the primary side to the secondary side of the transformer via electromagnetic induction.

But why in DC power systems we have direct current (DC), which is constant, we cannot generate this changing flux.

So a transformer will not work in DC power systems because there is no change in the magnetic field to induce the voltage.

The boost converter used to increase DC voltage and the buck converter used to reduce DC voltage.

How is DC voltage represented on multimeters?

Multimeter is a measuring device used to measure voltage, current and resistance. Multimeters vary in size and features.

To measure voltage, set the knob to the given voltage range. And connect the probes to the voltage terminals of the multimeter.

We can measure AC and DC voltage. To measure DC voltage, set the multimeter to the DC mark.

Different voltage ranges provided in some DMMs, such as 2V, 20V, 200V, etc.

If you need to measure the voltage below 20V, set the knob to 20V.

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