Current Ratings

[windozeuser]

I hope someone could clear something up for me. If you have say a power supply rated at a max of 2A output current. Then you connected a device that draws 1A, will the 2A manifest through the circuit? Why NOT?

If it only draws 1A what happens to the spare 1A


This is like an LED, which will draw huge amounts of current and burn itself out. if you don't connect a resister in series with it. What I don't get is why does it draw that much current is it internal resistance is low?

What is Impedence?

 

[Joel Rainville]

I hope someone could clear something up for me. If you have say a power supply rated at a max of 2A output current. Then you connected a device that draws 1A, will the 2A manifest through the circuit? Why NOT?

Turn on the water. Does the ocean end up in the sink as soon as you turn the tap on?

If it only draws 1A what happens to the spare 1A

It stays in the ocean Seriously, there's no "spare 1A". The circuit draws a certain current, and that's it. Your power supply is rated at 2A because it *can* output up to 2A. Past that, a fuse should blow somewhere, to avoid frying other components in the PS and your circuit (which should also be protected by a fuse BTW)

This is like an LED, which will draw huge amounts of current and burn itself out. if you don't connect a resister in series with it. What I don't get is why does it draw that much current is it internal resistance is low?

Exactly. No or low resistance is like a short.

What is Impedence?

https://en.wikipedia.org/wiki/Impedance

 

[zachtheterrible]

You can think of your entire circuit as a load, or one big resistor across your power supply, although its hard to calculate exactly what the resistance of the entire circuit is.

When a resistor is placed across a power supply, it only lets a certain amount of current flow through it (ohms law). The lower the resistance the higher the current and vica versa.

This is like an LED, which will draw huge amounts of current and burn itself out. if you don't connect a resister in series with it. What I don't get is why does it draw that much current is it internal resistance is low?

If a small LED is placed across a 1.5v battery, there is no need for a resistor. If its placed across a higher power source like 4v it will draw too much current and wil need a resistor. Again, you can think of the LED as a resistor.

The water analogy will help you understand this very well. Think of voltage as water pressure, the water as current, and a resistor as the diameter of a pipe that the water flows through. If the pipe is large, as much water as possible will flow through the pipe.

 

[ljcox]

Joel & Zach's analogies are very good.

If a power supply is rated at 2A, that is the maximum current that can be safely drawn from it. If you try to draw more, then components will start to heat up and if enough excess current is drawn (it won't happen at 2.01A) then either a component will overheat and fail or, if there is a self regulating feature in the system, it will limit the current at a safe level.

Len

 

[Someone Electro]

Its the Ohms law that is doing this.

Resistance is mesured is a unit caled Ohm.The higher the resistance the lower the curent is going to be.

Voltage / Resistance = Curent

So if conect an 100Ohm over a power suply that is giving 10 V the curent will be: 10V / 100 Ohm = 0,1A or 100 mA (miliamps {1A = 1000 mA})

So the resistance is whats limiting the curent.

The 2A is the power suplys rated max.Its the bigest curent it can suply.If the load draws more then probobly an fuse will blow to protect it.The cheap "wall wart" power suplys.These mostly have an termal fuse on the winding so wen it gets too hot (Becose the load is drawing too much curent) the fuse shuts down the suply or it cod catch fire becose it wod keep geting hoter and hoter.These termal fuses dont work instantly at overload.(An normal fuse wod triger instantly)So you cod draw well more than the suply its rated for.But for only a minute or so.

Heating is the main problm.Since wen curent flows it creates heat.

So the more curent you draw the more curent is going trough the components in the suply.And more curent means more heat.And at some point there is so much heat being produced that the componnets over heat.This os someware betwen 80 and 200 C° (Depends on what componnet it is)

 

[1st Mil-tech]

A battery rated at two amps will put out more than two amps. Its rated output is made to help designers get the most life out of it without losing its efficiency. You can get more than two amps out of a D cell but watch out it may explode or overheat. that is why they put ratings on batteries.

Your question asked what happens to the excess current.

It stays in the battery. :wink:
The battery only puts out the amount of current the circuit was designed to draw. If you put a LED across the battery without a current limiting resistor the current will continue to increase as the heat will cause thermal runaway. POOF :cry: :shock:

The resistor will limit the current, as it is the highest resistance in the series circuit. Most LEDs are designed to run at less than 5 milliamps of current. (There are exceptions, such as automotive and stop-sign type applications) Also If they are pulsed they can handle more current and therefore more brightness. But then there has to be sufficient time to physically cool down.

If you have a 12 volt battery and want to use a 1 milliamp LED. use ohms law.

R = E/I or 12v / .001a = 12000 ohms or 12kohms

12v/12ohms = 1amp = 1000 milliamps
12v/120ohms = .1amps = 100 milliamps
12v/1200 ohms = .01amps = 10 milliamps
12v/12000 ohms = .001amps = 1 milliamp

Hope this helps your understanding

Second question
What is Impedance?
Impedance is also a form of resistance, but is used to help explain circuits that are used with varying frequency (alternating current) and has much more complex math involved, rather than Direct current.

 

[windozeuser]

Ah Thank you all
I understand it much better now.