What are the sources of power loss in an ammeter?
The measured current and the internal resistance of the ammeter are the sources of power loss in the device. The ammeter circuit offers low resistance so that a small voltage drop occurs in the circuit.
How to measure the current in a circuit?
To measure the current, generally the ammeter is connected in series with the circuit. This device is mainly used for measuring a small amount of current, and the current is measured in the milliampere or microampere range.
Which type of ammeter is used to measure AC supply?
In the moving iron ammeter, the coil freely moves between the poles of a permanent magnet. This ammeter can measure both the alternating and direct current. The Electro-dynamometer ammeter also works on both AC and DC supply and is highly accurate. Rectifier type ammeter is used for measuring AC supply.
Why does the ammeter circuit offer a low resistance?
The ammeter circuit offers low resistance so that a small voltage drop occurs in the circuit. An ammeter is a thermally sensitive device that is affected by internal as well as external environmental conditions. Temperature contributes to the reading of the device.
The Potential Problems with Harmonic Current
In general, a harmonic current is a sinusoidal current with a frequency that is an integer multiple of the fundamental frequency of the mains or usually 50 or 60 Hz.
Relationship Between Harmonic Current and Power Factor
The presence of harmonic current also causes a decrease in the power factor (the ratio of active power to apparent power). Since only the fundamental frequency produces active power, the volt-amp product of the harmonics contributes to reactive power, which increases the apparent power without changing the active power.
What Causes Harmonic Current
To mitigate the effects of harmonic current, it is important to understand what causes it. In short, harmonics result from non-linear loads (i.e., loads whose current vs. voltage characteristics do not form a straight line) that create distortion, of which rectifiers on the front end of power supplies are a good example.
Regulation of and Limits for Harmonic Current
Harmonic current limitations are regulated by IEC 61000-3-2/EN 61000-3-2, which applies to equipment with an input current of up to 16 A per phase. The goal of this standard is to maintain high power quality and reliability of the mains to protect the distribution network and connected devices and it has serious implications for power supplies.
Conclusion
Harmonic currents of switched-mode power supplies can seriously impact the power quality of the mains, negatively affect other devices operating on the same lines, and lead to such issues as overheating, saturated transformers, and the need for larger cables.
Introduction
As engineers we are often looking at ‘what if’ scenarios to ensure our designs work under as many application conditions as possible. For conditions under which our designs do not work properly, we at least do not want either our product to become damaged or our designs to damage something else. With power supplies one of the concerns often is “wha...
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The Ideal Power Supply
Current Limiting in Power Supplies
As was mentioned above, all power supplies will have some form of current limiting and most of the time it will be controlled and non-destructive. This discussion will focus on limiting the output current from a power supply to protect the power supply in the event the load demands too much current and would otherwise damage the power supply. Input current limiting is often implement…
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Fuse Current Limiting
Perhaps the simplest form of power supply output current limiting is to place a fuse in series with the output terminal of the power supply (Figure 3). This method would be effective but is not often employed in power supplies because it is relatively easy to draw excessive load current (i.e., shorting the output terminals or plug of the power supply) and accidentally blow the fuse. In add…
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Constant Current Limiting
A common method that has been implemented to limit the output current from power supplies is to monitor the output current and reduce the output voltage when the current limit is reached while maintaining the maximum output current (Figure 4). In this implementation, the output voltage during current limiting is dependent upon the impedance presented by the load during th…
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Fold-Back Current Limiting
To address the component stress issues present in constant current limiting and mentioned above, some power supplies are designed with fold-back current limiting. This implementation can be confusing to the user. In a fold-back current limited power supply the output voltage and output current are both reduced after maximum output current is detected (Figure 5). A standar…
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Hiccup Mode Current Limiting
Perhaps the most common implementation for current limiting now employed in power supplies is known as hiccup mode. This mode of over-current protection can be thought of as an active version of fuse protection mentioned earlier in this discussion. With hiccup mode current limiting protection, the output voltage of the power supply is shut down when an over-current situation i…
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Conclusion
The Potential Problems with Harmonic Current
In general, a harmonic current is a sinusoidal current with a frequency that is an integer multiple of the fundamental frequency of the mains or usually 50 or 60 Hz. Additionally, only the fundamental frequency's current creates active power, measured in watts, which can also be referred to as true or real power. On the other hand, harmonic current...
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Relationship Between Harmonic Current and Power Factor
The presence of harmonic current also causes a decrease in the power factor(the ratio of active power to apparent power). Since only the fundamental frequency produces active power, the volt-amp product of the harmonics contributes to reactive power, which increases the apparent power without changing the active power. To better understand this, a review of power triangles is in or…
See more on allaboutcircuits.com
What Causes Harmonic Current
To mitigate the effects of harmonic current, it is important to understand what causes it. In short, harmonics result from non-linear loads (i.e., loads whose current vs. voltage characteristics do not form a straight line) that create distortion, of which rectifiers on the front end of power supplies are a good example. Because the ac input voltage is a sinusoidal waveform a linear sys…
See more on allaboutcircuits.com
Regulation of and Limits For Harmonic Current
Harmonic current limitations are regulated by IEC 61000-3-2/EN 61000-3-2, which applies to equipment with an input current of up to 16 A per phase. The goal of this standard is to maintain high power quality and reliability of the mains to protect the distribution network and connected devices and it has serious implications for power supplies. In this standard, there are applicatio…
See more on allaboutcircuits.com
Mitigating Harmonic Current
In order to comply with the IEC 61000-3-2/EN 61000-3-2 standard, harmonic distortion must be mitigated. For power supplies, such mitigation is best achieved using either passive or active power factor correction, the circuits of each are shown down below in Figure 5. Passive power factor correction involves adding fixed line frequency filtering at the input of the power supply a…
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Conclusion
Harmonic currents of switched-mode power supplies can seriously impact the power quality of the mains, negatively affect other devices operating on the same lines, and lead to such issues as overheating, saturated transformers, and the need for larger cables. Compliance with IEC 61000-3-2 is necessary to maintain high power quality and protect other mains connected devices. At CU…
See more on allaboutcircuits.com