Hot answers tagged

44

It's possible, and I've seen it done (the gear in question being a wideband amplifier for an EMC probe set), but for isolated low voltage only. The main drawback is the possibility of shorting the plug supplying power as swipes past the outer (ring) conductor. While it might be acceptable for a specialty power input with limited voltage and current, it ...


41

Nuclear power plants in particular have extremely critical needs for power even when they are not generating, as decay heat requires circulating cooling water through not only the reactor long after shutdown, but also possibly some of the recently spent fuel stored in pools of water outside the reactor. As a result they are not only able to draw from the ...


34

Your linked description shows that the tweeter is intended to be used with a woofer for music reproduction. The description doesn't cover use of the tweeter as a specialist ultrasonic transducer on its own. The power rating indicates that the tweeter is intended to be used with a 350 W average/700 W peak audio amplifier driving both tweeter and woofer. For ...


32

Higher frequencies are much more affected by the inductance of the power lines. 400 Hz is fine on an aircraft, but over long distances the power factor would be extremely poor. 60 Hz was an educated guess (as I understand), but it has turned out to be about right.


31

It depends on the circumstances. Without knowing anything else, we don't know whether power is proportional to \$V\$, \$V^2\$, or neither. If a variable voltage source is connected to a resistor, then the power is proportional to \$V^2\$. If a variable voltage source is connected to a constant-current load (something which admits the same amount of current ...


30

When a power station is up and running the power it generates can be used to power its internal operation. However, many power stations do not have the ability to bring themselves back into service after a major outage. (referred to as a "Black Start"). It may not be worth the capital expenditure to provide adequate backup power (that would only be ...


29

What @Frog says about losses is true, however, that's not the real reason for utility frequency to be around 50-60Hz. HVDC systems have essentially no reactive losses, yet they did not really become widespread. The choice of utility frequency is largely historical, and frequencies outside the 25-100Hz range were simply prohibitive around 1900 from the ...


28

Trolleys with overhead wires often disconnect momentarily and the high inductance creates an arc of the same current that allows some continuity but with some drop of voltage or power. Whereas AC might cutout in < 10ms. But the real reason is DC traction motors have greater efficient torque needed to start. You might see some DC motor Torque capability ...


28

This is an ideal diode circuit. It permits current to flow when the left hand side is at a higher voltage than the right, but blocks current flow otherwise, like a regular diode, but it has minimal forward voltage drop. The circuit is described in this power electronics tips article


27

Yes there is, it is called an electronic load.


24

From what I know, transmitting current over long(er) distances is more efficient using AC. That's true if you can step up the voltage. In the case of the trolleybus the wires are energised at the "end-user" voltage, 600 V. Note that the overhead will be broken up into isolated sections and these will be fed from the nearest trolleybus substation. ...


23

If you want to convert the voltage to 5 volts, you should not use a resistance-voltage divider. That way you will indeed create 5 volts, but as soon as you apply a load the voltage will drop. Instead, you have two options in general to regulate voltage. The first option using a linear regulator, and the second option is using a switch-mode power supply. ...


22

In June 2004, the Economist, A financial / economy magazine should be read with a lot of caution when it comes to technical claims made therein – the people writing, interviewing, editing such a magazine aren't engineers! Just the same applies the other way around: I wouldn't trust IEEE Spectrum to be the best source for discussion of economy. in reference ...


21

Modern PC parts still need those rails, for example, SSDs usually use 5V for main operation. No, NVMe SSDs as attached by M.2 get a 1.5 V and a 3.3 V rail, We use 5V in USB devices to charge up cellphones and power personal beverage coolers. So the need is not going away (any time soon) just because Intel has published a new standard. USB is a very good ...


21

It appears that the problem lies with the power ratings of loudspeaker components being the power rating of the system and not that of the individual component. For example, in the 1970s, Philips were specifying the ratings of individual speaker components. In later years, it may have been felt that specifying the system power would make it easy to select ...


19

The current marked on a motor should be the continuous, full-load operating current. The power marked on a motor should be the full-load mechanical power delivered by the shaft. A 3 Hp motor should deliver about 2240 watts. 2240/4600 is 0.487. That tells you that the motor's full-load power factor multiplied by its full load efficiency is 0.487. For a rough ...


19

What happens when the tip slides across the other contacts? TRS connectors are not ideal for power delivery because incorrect shorts can happen as the connector is mated.


19

Resistors are all the same size no matter the resistance. It's convenient for the manufacturer to test how much power a certain-sized resistor can take before it burns up. And it's the same for all resistors of that size. Also the power rating is the same regardless of the resistor's exact resistance (within the tolerance). Cables with different current ...


18

It has been used for power in the past, but not so much any more, because it has proven to be a bad idea to use one connector for multiple purposes. Imagine the damage when accidentally plugging 12V supply into audio output or microphone input. That is also the reason why you don't want to use USB connectors for anything else than USB or HDMI connectors for ...


18

An unloaded power transformer is a pure inductance. As you increase the load, the power factor improves. So, take a Variac, plug an incandescent light into it, and you should be able to dial up any power factor you like.


15

History As other answers have observed, protecting both the power supply end and equipment end during connection and disconnection requires active electronics. The use of quarter-inch jacks originates in a time before silicon devices had been invented, so this simply was not possible. By the time electronics existed and could be put into something portable ...


15

The frequency of the power grid is a great compromise. Make the frequency higher and you get smaller (read: cheaper) transformers and (somewhat) smaller generators and motors. But, you get higher hysteresis loses in the transformers' cores and higher radiative loses in long power lines as well. The above consideration about 100 years ago ended up with the ...


15

At least in The Netherlands (also referred to as "here" where I live), it is for historical reasons. Trolley busses date back to 1882, the one in The Netherlands in Arnhem (still operational) was started in 1949. The speed controls were easier for DC then, in the absence of electronics we know today, and the available series-connected DC motors ...


13

Cables are rated for power too, but somewhat indirectly. They have a stated thermal limit as well as a maximum working voltage (insulation strength). Nevertheless it's usually more convenient to think of their current handling capability since their primary purpose is to do exactly that: move current. You would arrive at the cable's current rating from the ...


13

Study the circuit board and the components on it. Trace along the suspected positive and negative power tracks. Look for components that have identifiable positive and negative terminals. Electrolytic capacitors have their negative terminals clearly marked. If you can find data sheets for any integrated circuits on the PCB, see which are the power and ...


13

The 'efficiency' they are referring to is Conversion Ratio as I found in the wikipedia article about Rectifiers - Conversion ratio (also called "rectification ratio", and confusingly, "efficiency") η is defined as the ratio of DC output power to the input power from the AC supply. If we go by this convention, assuming transformer and ...


13

The ability of speaker wire to carry the current involved is rarely much of an issue except a few quite unusual loudspeakers. The most notable in this regard was probably the Apogee Scintilla, with a rated impedance of one (1) ohm. That extremely low impedance translated to low voltage and quite high current. But, those are still remembered (thirty years ...


13

It's a 1-way low impedance current switch implementation. The circuit basically behaves like a diode, without the downside of having a voltage drop. (It still has but much lower). It's to avoid power going back to the USB port when powered from an external source. When powered from the USB, current flow through Q2A and will block Q2B, the Q1 gate will then ...


12

If the signal is represented as a voltage \$v(t)\$ or a current \$i(t)\$ and it is connected to a (1 ohm) resistor, the power dissipated in the resistor is proportional to \$v^2(t)\$ or \$i^2(t)\$. Apart from that, defining power as a positive, increasing function of the signal amplitude has useful mathematical properties.


12

Cables are rated for power - it's just that it's not a useful way to think about it. The upper rating on cable size will be determined by how hot it gets before its insulation is damaged - and this will depend directly both on conductor size and insulation characteristics. Cables aren't intended to drop voltage, they are intended to carry current with an ...


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