Understanding Amps: How Can They Be Determined Without Knowing the Circuit?

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In summary, the conversation discusses how electrical devices display the number of amps they use and how this can be determined without knowing the circuit. The speaker explains that designers know the components in the circuit and the voltage source, which has low resistance. They also mention that power ratings for devices like computer motherboards refer to the maximum power that can be consumed with all options installed. Adding more resistance in parallel will decrease the total resistance and increase the amperage, while plugging more resistance in series can blow a fuse.
  • #1
Wishbone
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I often see on electrical devices, they say how many amps the device uses. But I wonder how can that be? if you do not know the voltage and the resistance (im guessing they do know the resistance) how can they tell you how many amps it pulls? Also doesn't it matter what components are in the circut before the device? What if the circut divides into 100 parallel resistors right before the device, won't the amperage be significantly less? And what if these 100 resistors are in series, won't the voltage then be considerably less, causing a lowering in amperage?

The question I am asking, is how can they make a determination of the amps used without knowing the circuit.
 
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  • #2
Wishbone said:
The question I am asking, is how can they make a determination of the amps used without knowing the circuit.

"They" designed the circuit so they do know what's inside.
 
  • #3
And the voltage source for the type of circuits you are talking about is known, and has low source impedance. I'm assuming the devices you are asking about are ones that plug into the AC Mains power outlets in the walls, etc. All power consuming devices are plugged in parallel into this power source, so one device doesn't affect another by being in series with it. And the source resistance (or impedance) of the AC Mains is very low, so its voltage level does not droop much when a maximum load is placed on it.
 
  • #4
Well I an see what you mean for things like TVs or Vacuum cleaners. However, for something like computer hardware, you cannot determine the amperage for a motherboard, or RAM, unless you know all other components of the computer and power supply, no? Or for a more general example, you simply cannot make any determination about the amperage of a device when you move it from one circuit to another, correct?
 
  • #5
Wishbone said:
Well I an see what you mean for things like TVs or Vacuum cleaners. However, for something like computer hardware, you cannot determine the amperage for a motherboard, or RAM, unless you know all other components of the computer and power supply, no? Or for a more general example, you simply cannot make any determination about the amperage of a device when you move it from one circuit to another, correct?

Ah, that's a better question. When the designers put a power rating on a configurable device like a computer motherboard with empty memory sockets, they are listing the maximum power that can be consumed by the device, when fully populated with options. If you open up your PC (with the power off) and look at the sticker on the power supply, it will list the maximum output currents that are available from the supply for its different output voltage wire harnesses. The input power label on the outside of the PC and in the PC's Owner's Manual correspond to the maximum total output power of the PC's power supply, plus some fraction due to the lost heat because the power supply is not 100% efficient in converting the input AC Mains power to its output supply rails.
 
  • #6
Ah excellent explanation, my final question is: why would adding greater resistance (more RAM chips, video cards, or other stuff that draws power) increase the amperage, and thus bringing it to the maximum levels? How come pugging more resistance (ie. vacuum cleaner, tv) into a outlet can blow a fuse? I figure the more resistance you add to a circuit with a constant voltage source, the lower the current will be.
 
  • #7
Because the added resistances are in parallel, thus REDUCING the total resistance as seen by the power supply.
 
  • #8
I understand now, thanks.
 

1. What is electricity?

Electricity is a form of energy that results from the flow of electrons through a conductive material. It can produce light, heat, and other forms of energy.

2. What is the difference between AC and DC electricity?

AC (alternating current) electricity flows in both directions, while DC (direct current) electricity only flows in one direction. AC electricity is used in most household appliances, while DC electricity is commonly used in batteries and electronic devices.

3. How does electricity flow through a circuit?

Electricity flows through a circuit due to the difference in potential energy between the positive and negative terminals of a battery or power source. The electrons flow from the negative terminal, through the circuit, and back to the positive terminal.

4. What is resistance and how does it affect electricity?

Resistance is the measure of how much a material or component resists the flow of electricity. It is measured in ohms and can affect the amount of current flowing through a circuit. Higher resistance means less current can flow, while lower resistance allows more current to flow.

5. How is electricity generated?

Electricity can be generated in various ways, including through chemical reactions in batteries, through electromagnetic induction in generators, or through solar panels. Power plants also use different methods, such as burning fossil fuels or harnessing the power of water, to generate electricity.

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