73 votes
Accepted

Why do CPUs need so much current?

CPUs are not 'simple' by any stretch of the imagination. Because they have a few billion transistors, each one of which will have some small leakage at idle and has to charge and discharge gate and ...
alex.forencich's user avatar
56 votes

Why is there a small cut in this ampere meter shunt?

To be accurate, a current shunt must have a specific resistance value. The method used here to get that resistance just right is to build the shunt with a too-low resistance, then trim it by cutting ...
Kevin Reid's user avatar
  • 7,524
53 votes

Why can't I power everything with a 9 V DC battery?

Archimedes said, “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” This is the nature of the thought exercise you’re asking. And the answer is, yes, given a ...
hacktastical's user avatar
  • 52.7k
45 votes
Accepted

Measuring current with a multimeter - What did I do wrong?

You placed an ammeter in parallel to a voltage supply, which created a short circuit through the meter and most likely blew the 10A fuse in the ammeter socket. What you want to do is put the meter ...
Chris M.'s user avatar
  • 1,330
45 votes
Accepted

12 V car battery short circuit with a wrench that has resistance of 0.5 ohms, how much current?

A wrench does not have a resistance of 0.5 ohms, it's way lower. Your basic multimeter cannot measure resistances to better than an ohm or so, the resistance of leads, and the unreliability of ...
Neil_UK's user avatar
  • 165k
37 votes
Accepted

Why can't I power everything with a 9 V DC battery?

Batteries are chemical energy stores. The size of the battery determines the amount of electrolyte that can be accomodated and that determines the amount of energy that can be stored. The battery can ...
Transistor's user avatar
  • 175k
29 votes

Using a 2 A, 48 VDC rated switch in a 8 A, 12 VDC circuit

tl; dr: nope. The switch will fail. I think you’re misinterpreting how switch ratings are derived. While the two proposals would appear to handle similar amounts of power (96W), switches are specified ...
hacktastical's user avatar
  • 52.7k
28 votes
Accepted

Why to disconnect the negative/ground terminal, not positive?

There is no real inherent distinction between breaking one side of a loop or the other side- it's all in series so breaking the negative or the positive side of the supply keeps electrons from flowing....
Spehro Pefhany's user avatar
27 votes
Accepted

What is negative current?

understand that voltage is relative to ground, I prefer to disagree. A voltage is against a reference point. Often that reference point is ground but not always. Taken the above into account your ...
Oldfart's user avatar
  • 14.3k
22 votes

What can I put a kilowatt into? (Or: help me save my hairdryers)

To dissipate \$ 1\mathrm{kW} \$ at \$ 14\mathrm{V} \$ you need a resistor with a resistance of \$ R = \frac{\mathrm{V}^2}{\mathrm{W}} = \frac{\left(14\mathrm{V}\right)^2}{1000\mathrm{W}} = 0.196\...
user4574's user avatar
  • 12.1k
21 votes

Amps / current doesnt add up to the expected figure - Ohm's law failure on my part!

2.9 ohms (measured with power disconnected through circuit) As the lamp heats, its resistance rises sharply. Ohms law is never wrong except when misapplied.
Andy aka's user avatar
  • 451k
21 votes
Accepted

What happens if a motor draws more amps than a battery can provide?

I'll answer your first scenario only, because I think it's more interesting. Let's dissect: the battery I'm using can only provide 20 amps But what that really means? What would happen if you try to ...
anrieff's user avatar
  • 5,391
20 votes
Accepted

Getting the energy consumption in joules over a period when having current and voltage

Convert \$\text{m}A\$ to \$A\$. Multiply \$A\$ and \$V\$ for each time period to get \$W\$. Multiply \$ W\$ by the length of the time period (\$0.1\text{ s}\$) to get \$J\$. Add up all the \$J\$. ...
hobbs's user avatar
  • 7,320
20 votes

How are switch ratings determined?

Switch ratings are related to the designed breaking capacity: it's much more difficult to break a circuit while current is flowing than it is to simply not impede flowing current while the circuit's ...
Mister Mystère's user avatar
19 votes

What causes a fuse to blow, the current or the power?

So if a fuse is rated for 12V DC and 20 Amps, this would be equal to 240 Watts. If a different voltage is supplied, will this change the Amps at which the fuse will break? Does the fuse technically '...
Andy aka's user avatar
  • 451k
17 votes

Why do CPUs need so much current?

According to Wikipedia, top CPUs released in 2011 had some 0.5 to 2.5 billions of transistors. Assuming a CPU with 1 billion of transistors consumes 64A of current, the average current is only 64nA ...
Dmitry Grigoryev's user avatar
17 votes

Is there any advantage to have a dedicated hole to measure milliamperes on a multimeter?

Meters are often left connected to voltage sources for purposes of monitoring them. Further, they often use the same knob for both mode selection and power on/off. If a meter uses the same knob to ...
supercat's user avatar
  • 46.6k
16 votes
Accepted

How many amps can a car battery supply?

First, it highly depends on the battery. Some cars have much beefier batteries, measured in Amp Hours. We arn't even talking about Electric Vehicle battery banks which are massive. Then it depends on ...
Passerby's user avatar
  • 72.9k
16 votes

5V LDO blowing up only when testing current draw

I think people are distracted by the PMOS, but the real reason is a voltage spike when the 24V is connected, due to inductance of the wiring that is even higher when the multimeter is connected, ...
Jim Paris's user avatar
  • 2,685
16 votes
Accepted

In below picture, is ammeter AC or DC? Is there direction for connection with load in series? If so what if I connect in wrong direction?

It's a moving iron meter hence this symbol: - Taken from here Moving iron meters are used for DC AND AC so, the meter needle will only move in a clockwise direction irrespective of current polarity. ...
Andy aka's user avatar
  • 451k
16 votes

Using a 2 A, 48 VDC rated switch in a 8 A, 12 VDC circuit

No, you can't use that switch in that circuit; both current and voltage ratings need to be satisfied separately. The voltage rating has to do with isolation and contact arcing, the current rating has ...
ocrdu's user avatar
  • 9,120
14 votes
Accepted

What will happen when the current is too high for a switch?

Switch damage due to excess current goes one of 3 ways: 1) If the damage occurs as the switch is opening (which is the common failure condition when dealing with motors and solenoids) the switch ...
WhatRoughBeast's user avatar
14 votes
Accepted

Standard Volt and Ampere

This all brings me to wonder, how much of what we use is calibrated to someone else's meter. And how many of those meters, are calibrated to yet someone else's.. and on and on. Seems like one big ...
Enric Blanco's user avatar
  • 5,771
14 votes

Why is there a small cut in this ampere meter shunt?

That is how the shunt is calibrated for accuracy as it reduces the area to pass the current.
Solar Mike's user avatar
  • 6,529
14 votes
Accepted

What is the ampacity of 14 gauge silver wire?

In general the current rating of a wire is limited by the wire's ability to dissipate heat to its surroundings without getting too hot. That in turn depends on the maximum safe wire temperature and ...
Peter Green's user avatar
  • 22.1k
14 votes

In below picture, is ammeter AC or DC? Is there direction for connection with load in series? If so what if I connect in wrong direction?

There are three clues: The scale isn't linear so that implies that it is a moving iron meter. The ≂ symbol indicates that it can handle DC and AC. (The '-' indicates DC and the '~' AC. The ...
Transistor's user avatar
  • 175k
14 votes
Accepted

What is the relationship between the length of a wire, its AWG, and its amperage rating?

A wire that will carry 15A without overheating, will carry 15A whether it is 1 meter long, or 1 km long. What will change is the resistance, and resistance is directly related to voltage drop. If you ...
Math Keeps Me Busy's user avatar
13 votes

5V LDO blowing up only when testing current draw

I agree with VillageTech in incriminating the PMOS. It has a capacitive load in the source and an inductive load in the drain (your mutlimeter wires), add a bit of parasitic inductive/capacitive ...
bobflux's user avatar
  • 74.8k

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