Hot answers tagged

91

There's no such thing as "lossless" anything in electronics, and there's not a single IC that's designed to do what you want. But here are some different supply ideas. Since you didn't specify current consumption or efficiency, let's look at three different approaches: Non-isolating Zener supply 5% efficiency or less Plug-in timers that are ...


53

The jargon for this type of connector is "switched jack". Not only is it common for barrel jacks to have switches, but also for some other types of jacks, like phone jacks of all types. When you insert the plug, the connection between pins 3 and 2 breaks. This is useful for cutting a battery out of the circuit when a DC adapter is plugged in. Imagine you ...


49

You are looking for a slip ring. This is a device designed to do exactly what you describe - transmit power and/or signals to a rotating object. Generally, they work by having a rotating circular contact on one side and a spring-loaded pin which pushes against it on the other. They are not super reliable long term or in harsh conditions, but should be ...


49

TL;DR; Your friend is telling you how to destroy a battery, potentially violently. Use a proper voltage regulator circuit instead. Batteries are not designed to be used a voltage regulators. While it is true that in an ideal world your proposed circuit would work (after all, an ideal battery drops its rated voltage at any current), in practice things are a ...


48

ALL current will heat the voice coil of a speaker. But AC current is useful to reproduce sounds (which is what a speaker is made for). On the other hand, DC current will produce the equivalent amount of heating as an equivalent AC current, but it will produce nothing but a fixed offset (versus moving the cone in and out to produce sound). And while you ...


47

An alternative to slip rings is the rotary transformer. This consists of two cup-shaped cores that face each other, with the windings inside the cups. If you drive the primary at a high frequency (we used 25 kHz), the cups can be ferrite and the whole thing can be quite compact (we did about 100W in a unit that was about 1" thick overall and about 3" in ...


43

Conceptual answer: Capacitors are essentially two plates that are mounted next to each other, with a gap between them so that the plates don't touch. That's why it's drawn as --| |-- on a diagram. Direct current can't jump the gap between plates, because it would take a massive amount of voltage to force the electron to jump the gap between plates. The ...


40

Consider an antenna that converts all the electrical energy from a power oscillator to an electromagnetic radio transmission. Let's say 1 watt is fed to the antenna and 1 watt is the emitted radio wave power. That power is flung out in most directions; just like a lightbulb emits light power in (virtually) all directions. The lightbulb isn't just a similar ...


38

You should not be so hard on your professor. Much of the confusion newcomers to EE struggle with is that we talk about theoretical IDEAL circuits as part of the teaching process. In ideal circuits things often act rather contrary to your intuitive and experimental notions of how things actually work. Things like short circuits, transformers, diodes, and ...


37

It's nothing to do with synchronization. It has to do with ensuring safety of utility workers. The inverter should be quick to disconnect in the case of a grid failure (seconds) wait a period of time (in this case 5 minutes) after the grid is restored before beginning to supply power out to the grid. See, for example this exchange (the "standard" in ...


34

Let me show you exactly why this is a bad idea to use a battery in this manner. The following simulation shows an example with a 100mA load. As you can see this forces 100mA of current to flow in the battery as well. If you use an alkaline battery this will be causing a charging action which cause an eventual heating of the battery and it may explode or at ...


33

The voice coil on a speaker is effectively a big inductor. It happens to also generate sound, but the loops of wire in a magnetic field make it act like an inductor. Inductors change impedance with respect to frequency. This is because any change in current through the system must build up the magnetic field in the coils. The faster you oscillate the ...


31

A "better" way is to send 12 V (or more) over the coax and have local regulation to 5V at each RaPi outlet. You can use cheap buck regulators available on Ebay (a few GBP or dollars) to take the 12 V DC and efficiently convert to 5 V locally. With 12 V being sent down the wire and with local switching buck regulators, the overall current down the coax is ...


30

Very clever, but that's not how it works. By your reasoning you should not only be able to make the frequency infinite, but also 4 Hz, or 100 Hz, or \$\sqrt{2}\$ Hz, all at the same time, with the same signal. And that's why you can't do that: a repeating signal can have only 1 fundamental frequency, which is 1/period. It would be the same as taking 2 ...


30

The components that make up DC adaptors (inductors, transistors, capacitors, diodes, ect) are all rated for a certain current and/or power dissipation. Components that can handle 1000A vs. components that can handle 5A are orders of magnitude apart in cost, size, and availability. For an example let's look at an inductor that could be used in a 1000A ...


30

The voltage for the Hall–Héroult process is inconveniently low (and the current too high) for efficient parallel operation so they use a whole bunch of cells in series. From this source ("Studies on the Hall-Heroult Aluminum Electrowinning Process"): The optimum current density is around 1 A cm-2 with a total cell current of 150-300 kA and a cell ...


29

Transformers are AC only. Running DC through a transformer basically gives you a heater. Critically, transformers work through the fact that a change in magnetic field induces a voltage in a wire. The critical portion is that the change is required. In an ignition coil, the change is created by simply connecting and disconnecting the ignition coil from ...


29

Direct current is more difficult to interrupt because it is continuous. Alternating current alternates polarity so it crosses zero on its own which helps to extinguish the arc produced across the breaker contacts when they open. The contacts doesn't get this help if it's DC so the contacts have to be hardier. Interrupting DC with an AC relay/breaker could ...


28

A resistor divider will do what you want, but at this voltage there are some issues you can normally ignore: The top resistor has to be able to handle 1 kV. Those are harder to get than "ordinary" resistors, and are often not linear with voltage at the high end. Power dissipation. Even what would normally be a "large" resistor, like 1 MΩ, ...


27

The combination of 2 things can ruin your day/life: current and duration of that current. The following diagram, issued by the IEC as quoted in the body of the parent page, shows the danger zones: AC-1 zone: Imperceptible AC-2 zone: Perceptible AC-3 zone :Reversible effects: muscular contraction AC-4 zone: Possibility of irreversible ...


27

I would be surprised if a cheap DC toy motor contained a rectifier diode since it's designed to be a DC motor, not a DC generator A cheap DC motor of the type that has a permanent magnet stator uses brushes and a rotor commutator to continually reverse the current into the rotor coil thus the effect is like feeding AC into the coil: - If you didn't do ...


25

Although the question has provided limited details, this answer presents a somewhat different hypothesis from the standard assumption that there's an inductive coil hidden in there somewhere. The charger in question possibly uses a Piezoelectric Transformer instead of the magnetic (inductive) transformers usually seen for isolation. Does the charger looks ...


25

If you look at the charge profile of a lithium battery you'll see that at certain points it changes from constant current to constant voltage charging: - This means that some form of "in series" charge control mechanism needs to be present to act initially as a constant current source then change to a constant voltage source. This charge control circuit ...


25

You could use a couple of resistors to half the voltage seen by the voltmeter. You would then just need to double the reading. This would protect the voltmeter from damage. simulate this circuit – Schematic created using CircuitLab


25

Consider this: You have your 1kV power supply, with an impressively insulated wire going out to a resistor. Coming back, you have a return wire with light insulation because, hey, it's grounded! Now the ground wire breaks at the power supply. Your "safe" ground wire is now at 1kV. Is that light insulation sufficient?


24

The loop of conductor that carries the 10 kA current has non-zero inductance. That means a large amount of energy is stored in that loop as \$\frac{1}{2}LI^2\$. If there's a break in the circuit, the inductance will raise the voltage at the break in order to keep the current flowing, while there is still stored energy is available to drive it. This will be ...


24

even touching both ends of an AAA battery with a metal wire produces sparkles and heat To analyze this circuit, you have to consider both the internal resistance of the battery and the actual resistance of the wire. Since a real wire has non-zero resistance, some power will indeed be delivered to the wire and turned into heat. But also, since a real ...


24

You certainly can use Square D "QO" panels/breakers on DC And the reason you can do that is Underwriter's Laboratories (UL) listed the breakers for DC service, and the labeling and instructions (which are approved by UL) state that explicitly. This was backed by testing in UL's lab. This is a condition of using them in mains power (NEC 110.2). Naturally,...


24

Your original digital signal is bent out of shape because it has been high-pass filtered by the tape recorder. Let's have a look at Korg Volca service manual and dig in the schematics, here is the input circuit for Sync: Presence of R158 hints at trouble as this is not some simple digital input circuit. On the contrary Q9 is in linear mode, biased by the ...


23

The inverter has a software delay. This is intentional. It waits 5 minutes to make sure it is connected to a stable grid. An inverter could theoretically connect and go full power in seconds. But it doesn't. For example, if after a power failure, all inverters immediately went online a started outputting full power, the network would be overwhelmed and will ...


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