Table of Contents
| Stan Zurek, DC, Encyclopedia Magnetica,
|reviewed by Jeff Jones, 2021-04-03|
Commonly, DC current circuit often simply means constant voltage circuit (specifically such that is not AC or alternating).
In the narrowest meaning it refers to a value which is constant over time, but depending on type of application and context the exact meaning can be quite different.
In practice most electric circuits or devices that are rated to operate from a “DC” source do so at a condition of constant voltage rather than constant current. For example, a battery-powered device will operate with a constant voltage as provided by the battery with the current varying accordingly to the load requirements, this may be from zero current when the device is switched off, some small current in a “standby” mode, to a nominal current of 100% of rated load when the device is switched on.
Therefore, most “direct current” circuits are actually “direct voltage” circuits, typically referred to as V DC, VDC, Vdc etc. Similarly for current: I DC, IDC, Idc, and so on.
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Strictly “constant” voltage or current is used only in specialised applications such as measurements, where the stability is critical for operation, such as DC insulation resistance testing3) or radar power supply applications where the required stability can be at the level of 0.01 ppm (135 μV in 15 kV).4)
Voltage sourced from a battery (of any type) is not strictly constant, and decreases up to 25% (or more) during the discharge process. Measuring changes in this voltage provides information about the amount of the energy remaining in the battery.5)6)
In a wider sense, the DC component of a waveform is the static offset or bias (DC offset) contained in a given waveform. Such static offset can characterise the operating point of a class A transistor amplifier, or the mathematical mean of the signal (voltage or current), which itself can contain any other variability.
For certain waveforms, DC component may be measured by a multimeter or oscilloscope, by selecting the “DC mode” or “DC coupling”. Conversely, the AC component can be measured by using the AC mode or AC coupling. However, the exact meaning also varies, depending on device or application - for example, in a typical oscilloscope “DC coupling” typically means that AC+DC signals are measured (no filtering), whereas “AC coupling” removes (filters out) the DC component.7)
Nevertheless, there are devices which require DC current (with voltage adjusted or varying as necessary for the given load conditions), examples being low-resistance ohm-meters8) and LED lighting drivers.9)
DC voltage can be generated by a switched-mode power supply. However, the filtering is never perfect and the voltage contains some disturbances called “ripple”, typically related to the switching frequency.10)
In some applications the filtering is carried out by passive means such as output capacitor or inductor, but to achieve better stability also linear regulation is used, by means of a transistor circuit with a suitable feedback.11)
In other applications, highly pulsating current is required for correct operation, as in all switch-mode power supplies. Many of them are designed to be DC-to-DC converters, meaning that they operate with constant input and output voltage (DC voltage), but the current depends on the load (as in a battery circuit).
For example, in a flyback transformer, the internal current flowing through the primary winding is typically a unipolar pulsating ramp. Such fluctuations of current are typically filtered out with a local capacitor, so that the pulses do not have to be supplied by the external circuit (and similar for the output voltage).
Off-line power supplies of higher power (>75 W) are required to have power factor close to unity. Power factor correction can be achieved with a boost converter (a type of switch-mode power supply).12)
In both cases the output voltage is pulsating and needs filtering, for example with a capacitor, which acts as an energy storage for the time where the AC voltage is near zero. This produces an output DC voltage with some pulsation or “ripple”, as dictated by the load impedance and the size of the filter capacitor.
The current flowing into the capacitor is highly pulsating, with the charging occurring only around the peaks of the rectified voltage. The output current is dictated by the load, and can have shape similar to the output voltage.
For half-wave rectification just one diode is needed, and then only every other pulse is present (as compared to the full-wave rectification). To supply the same load, proportionally larger peaks of input current are required and larger ripple can be expected, or a larger capacitor needs to be used.
In three-phase circuits there is some overlapping between the waveforms (due to 120° phase shift) and even without filtering the rectified voltage does not fall to zero. Three-phase full-wave rectification requires 6 diodes and is equivalent to 6 half-sine pulses overlapping each other, with 60° phase shift. For high-power DC applications special transformer are built which have multiple windings, with effective phase shift of down to 6° (60 effective pulses).13)
In a constant-voltage circuit, the output current depends on the load impedance. Therefore, the current can change from zero to full load, and also to overload. It is also possible that the current will become negative.
For example, from the viewpoint of a rechargeable battery, the current is positive when the battery supplies the load, but it needs to become negative in order to charge the battery. Therefore, even though the current changes polarity, the circuit is still considered to be classified as “DC”. This is because the voltage across the battery remains with the same polarity, so it is a “DC voltage” circuit.
DC as oppose to AC
The name “DC” is used frequently to denote that the the supply voltage is unipolar (e.g. typical battery or DC power supply), rather than “AC” or alternating (e.g. AC mains supply provided by national grid at 50 Hz or 60 Hz).
For example, soft magnetic materials exhibit losses under alternating magnetisation. The resulting B-H loop can be decomposed into dynamic losses (eddy currents, and excess loss) and static due to hysteresis loss. Under very slow magnetisation the magnetising current is changing but at a sufficiently slow rate so that the dynamic losses become negligible, and only the “static” hysteresis loss is remaining.
Therefore, for the data shown in the illustration, a frequency of 0.1 Hz can be considered quasi-static (quasi-DC) even through the magnetising current is sinusoidal and alternating.14)
DC component or DC offset
DC component (DC offset) is the the average of the waveform over some interval of time, suitable for a given analysis.
Pulse-with modulation is used to control the amount of energy stored temporarily and delivered to the output. The pulsations arise because of the switching character of the power supply. Current increases when the given inductor is connected to the primary voltage source, the energy is stored in the magnetic field of an inductor, and then the inductor is disconnected from the primary side, and connected to the secondary side, for example by means of freewheeling diode. The energy is then depleted from the inductor and delivered to the output, during which the current reduces.
The DC offset in each waveform is clearly visible. Only the ripples are responsible for dynamic effects such as core loss, skin effect and proximity effect, but the resistive losses are caused by all components of the current, including the DC component.