basic doubts about Voltage & Current

[mdfrahim1]

I went to a electronics shop to buy a voltage supply capable of delivering from 0 to 30V (i think you guys call it PSU). The shopkeeper asked me please tell me the correct ratings of Voltage and Current.
Here is when I started to get confused. All my study till now looks to contradict this.

Current flows depending on the completion of a circuit. So, a voltage supply of +5V will give a different current everytime I will change the load (or resistance). So how is it possible for us to know the current untill the circuit is complete ? How can we even know the range..again teh range varies with the load.

 

[ericgibbs]

I went to a electronics shop to buy a voltage supply capable of delivering from 0 to 30V (i think you guys call it PSU). The shopkeeper asked me please tell me the correct ratings of Voltage and Current.
Here is when I started to get confused. All my study till now looks to contradict this.

Current flows depending on the completion of a circuit. So, a voltage supply of +5V will give a different current everytime I will change the load (or resistance). So how is it possible for us to know the current untill the circuit is complete ? How can we even know the range..again teh range varies with the load.

hi,
shopkeeper asked me please tell me the correct ratings of Voltage and Current.

He is asking, whats the maximum current you would expect to draw from the PSU [power supply].. NOT what your circuit is going to draw at any instant.
The higher the current capabilty of the PSU the higher the cost [usually]

If you are just driving a PIC and a few LED's, a +5V at 0.5Amp would do the job.

For general work, if you want a fully adjustable 0 to +30V, I would suggest about 1 to 2Amps.

If you have a specific application that requires a fixed voltage say +5v at 1Amp, it will be far cheaper than a fully variable PSU

 

[shaneshane1]

I went to a electronics shop to buy a voltage supply capable of delivering from 0 to 30V (i think you guys call it PSU). The shopkeeper asked me please tell me the correct ratings of Voltage and Current.
Here is when I started to get confused. All my study till now looks to contradict this.

Current flows depending on the completion of a circuit. So, a voltage supply of +5V will give a different current everytime I will change the load (or resistance). So how is it possible for us to know the current untill the circuit is complete ? How can we even know the range..again teh range varies with the load.

it all depends on what the load is, and at what what current the load should draw, a voltage of 5V can draw all sorts of current as a max, eg: if you look at the data sheet for your application, say a LED it might say, 2.1 volts and 30mA max,

so you would do the following

5V - 2.1V = 2.9 then 2.9V/0.030 = 96ohms resistor

so your LED is at 2.1 volts and 30mA current from a 5V supply

i hope this helps!!!

 

[dknguyen]

You can estimate the current draw of the circuit through calculations you know...it's not too hard to be really conservative when estimating maximum current.

If this is for a general bench power supply that is to be used with many circuits, you probably have a feel for the power level the circuits will be. Usually it's under a few amps for electronics unless it's a larger power circuit (with motors or things like that).

 

[Leftyretro]

I went to a electronics shop to buy a voltage supply capable of delivering from 0 to 30V (i think you guys call it PSU). The shopkeeper asked me please tell me the correct ratings of Voltage and Current.
Here is when I started to get confused. All my study till now looks to contradict this.

Current flows depending on the completion of a circuit. So, a voltage supply of +5V will give a different current everytime I will change the load (or resistance). So how is it possible for us to know the current untill the circuit is complete ? How can we even know the range..again teh range varies with the load.

Any specific PSU unit will have a maximum current and voltage rating. It's up to you to decide how much power you may require in the future and size the PSU you obtain for that maximum power draw.

Lefty

 

[ecerfoglio]

I went to a electronics shop to buy a voltage supply capable of delivering from 0 to 30V (i think you guys call it PSU). The shopkeeper asked me please tell me the correct ratings of Voltage and Current.
Here is when I started to get confused. All my study till now looks to contradict this.

Current flows depending on the completion of a circuit. So, a voltage supply of +5V will give a different current everytime I will change the load (or resistance). So how is it possible for us to know the current untill the circuit is complete ? How can we even know the range..again teh range varies with the load.

The current rating is, as ericgibs said, "the maximum current you would expect to draw from the PSU [power supply].. NOT what your circuit is going to draw at any instant."

You may think that it is as, say, a "one ton" truck. It can carry a load of up to one ton (1000 Kg), but it may also carry only 200 Kg or even go empty.

(Sorry about the metric examples, feel free to "translate" them into pounds if you like )

 

[audioguru]

(Sorry about the metric examples, feel free to "translate" them into pounds if you like )

"Blokes" on TV talk with a severe English accent about how many stones they weigh.

Then they talk about pounds as if it was money.
What are they talking about?
Cheery-O, mates.

 

[mdfrahim1]

Since I am new I don't have much idea about how much current will my circuit draw. Probably after some experience I will be more comfortable.
But can I assume that for electronics circuits 1A of current is good enough.

And my aim was to buy a PSU which can be used across different loads. I wa just trying to experiment so really don't know the actual loads.

 

[shaneshane1]

But can I assume that for electronics circuits 1A of current is good enough.QUOTE]


I would say yes, look at it this way, if your using heaps of IC'S in your circuits,you can pretty much rule them out and dont even count them as current,until you apply something to the input/output of course,

and your LED'S that you will most likely be using often only draw around 30mA each, and there are 1000mA to make up 1Amp (thats a lot of LED'S)

you should only need more than 1Amp if you are going to draw heaps of current eg:motors.

1Amp will be fine for you.

 

[mdfrahim1]

Thanks a lot Shane.

Just for my info. While considering circuits and calculating currents shall i replace IC's by a short circuit or a open circuit ?

What is the internal resistance of an LED ?

 

[shaneshane1]

Thanks a lot Shane.

Just for my info. While considering circuits and calculating currents shall i replace IC's by a short circuit or a open circuit ?

What is the internal resistance of an LED ?

What

i think your taking on to much infomation about to many things at once?

why do you need to know the internal resistance of an LED for?

ill give you quick example how a LED works

A LED has a maximum mA it can draw before it will burn out, if you were to put that LED connected to say, a car battery, it would burn out straight away or even explode in your face because the LED will try and draw more current than it can handle, so you need to connect a resistor suited to that LED, the best way for you to work out all that info is to mabye go to your local electronics shop and buy there parts catalogue, it will have all the info you need, eg: if you wanted to buy a LED and test to see how current and voltage and resistors work together

inside the catalogue you pick a LED you want to use , and it might say

red
30mA max
1.8V

so that is telling you that its red and you need to supply 30mA at 1.8Volts

so now you need to find a resistor that will supply your LED with 30mA

so you get the power supply you will be using eg: 12V and you minus the voltage drop across your LED and thats 1.8Volts

now you have 10.2V so to find the correct resistor you simply divide the mA it will use which is 30mA by the 10.2Volts, and your resistor will be 340ohms resistor(340R) the the equation goes like this

12V - 1.8 = 10.2V then 10.2V / 0.030mA = 340R

so i hope that this example helps you understand a little more about how current & volts work.

 

[mdfrahim1]

Absolute fantastic information ........Thanks a lot.

But can i think that internal resistance of LED is 1.8V/30mA.... I know its not i think this is its impedance ?

 

[shaneshane1]

im not sure!!!

why are you so concerned about internal resistance, how is that going to benefit you?, iv never needed to find out the internal resistance of an LED?

 

[ericgibbs]

Absolute fantastic information ........Thanks a lot.

But can i think that internal resistance of LED is 1.8V/30mA.... I know its not i think this is its impedance ?

hi,
The LED is a semiconductor, you dont think of it in terms of resistance or impedance.

As Shane points out,, the specification sheet for a LED will state its forward operating voltage and its maximum working current.

In your example Vfwd = 1.8V and 30mA.

 

[mdfrahim1]

OK,I will not consider the internal resistance.

The reason I was bothered abt. was that I was thinking just the reverse of what you guys think.

You guys think rating of a LED first , check the resistance and determine the resistance. I was thinking of a resistance first and then calculating the current.

Anyhow, i think lets leave this. I understood what I wanted.

Thanks a lot guys.

 

[crashsite]

(Sorry about the metric examples, feel free to "translate" them into pounds if you like )

Now, I'm really confused. When I "translate" to pounds I get some shillings left over and I'm not quite sure if I should save them or spend them....

 

[crashsite]

OK,I will not consider the internal resistance.

Actually, you were not completely out of line to consider the LED's internal resistance (or that of any diode). Sometimes it can be a circuit consideration and is fairly easy to calculate but, damned difficult to measure.

Every first-year electronics student is dismayed when he/she breadboards their first one-transistor amplifier and discovers how lopsided the output sine wave, at the collector, is compared to the nice even one applied to the base of the transistor from the signal generator. The distortion is because of the changing resistance of the transistor's BE diode junction changing the amplifier's gain as the signal voltage (and thus the base current) varies.

So, while it's not common to actually measure or calculate the resistance, it must be acknowledged and dealt with.