Hello everyone! I want to ask something! Its weird question but its important for me! I know very well resistance, the Ohm's Law, resistors etc.. So why I ask this question if I know very well some basics? The reason is that resistors doesn't show in the color code, how much voltage or current they can handle until their break down, (of course this value is different for each value of the resistors, I.e. the breakdown voltage isn't the same for the resistor 330 ohm and another one 1K) So how to find how much current or voltage the resistor can handle depending of its impedance value until it will breakdown?

 

I already asked about this question long time ago. refer to the photo.
mostly used 1/4w resistors can handle about 250v but be careful with high voltage. and current can derive from its 'power rating'
if you apply 10v on 1/4w resistor than it can only handle 1/4*1/10 A

 

While components may break down owing to high voltage flash over, this is not the limiting factor for resistors.
Resistors are damaged primarily by heat, or more precisely, by lack of proper heat dissipation.
A resistor is therefore rated by its wattage which is determined by physical size and construction.
In practice, resistors should be underrated to half its rated value. For example, a resistor rated at 1W would be driven to lower than ½W.

Physical mounting, PCB construction and air flow all play an important role in helping to keep a resistor cool.

How does one determine the wattage dissipated by the resistor in circuit?

There are three variations of the same power calculation:

Power = I x V
Power = I x I x R
Power = V x V / R

Hence if you know the reduced power limit and the resistance value you can calculate the maximum voltage allowed:

Voltage = square root of ( resistance x power)

For example, assume the resistance is 200Ω and the wattage is 1W.
Derate the wattage to ½W

Maximum voltage = square root (½ x 200) = square root of (100) = 10V

Here is a photo of typical resistor sizes and wattage:

Here is a graphic to assist you in determining the voltage or current limit for a given resistance and wattage.

Here is another example on how to use the chart.

Suppose you intend to use a 1kΩ ¼W resistor.
The 1k line intersects the ¼W line at about V = 15V and I = 15mA

Hence the voltage across the resistor should not exceed 15V.
Moreover, at 10 to 15V you ought to move up to the next higher wattage rating, i.e. ½W for reliable operation.

As another example, in a 5V circuit, you can use ¼W resistors as low as 200Ω.

 

I know the 3 formulas given above! But i didnt bit understood what you mean! But let's give an exercise! Let's say I have a resistor 1K and I apply 12V and 300mA! How to understand how many voltage/current it can handle (let's say it will be overheat) until its "death" point! I ask that so as to be clear with that! Let's say I want to light a bulb 2,5 volts and I want to add a resistance! I should to know the right value of the resistance so as to succeed the right brightness! I will not put what resistor I want! I need the resistor that can handle the range of the voltage/current so as to succeed the right brightness! For example I will not put 1 Ohm because the resistor will be broken and bulb will dead, and not a 1 Mohm because the bulb will not lights! So how to understand what resistor with particular voltage/current/power handling range will be right?

 

I know the 3 formulas given above! But i didnt bit understood what you mean! But let's give an exercise! Let's say I have a resistor 1K and I apply 12V and 300mA! How to understand how many voltage/current it can handle (let's say it will be overheat) until its "death" point! I ask that so as to be clear with that! Let's say I want to light a bulb 2,5 volts and I want to add a resistance! I should to know the right value of the resistance so as to succeed the right brightness! I will not put what resistor I want! I need the resistor that can handle the range of the voltage/current so as to succeed the right brightness! For example I will not put 1Ohm because the resistor will be broken and bulb will dead, and not a 1Mohm because the bulb will not lights! So how to understand what resistor with particular voltage/current/power handling range will be right?

You do not understand resistors and Ohm's law.

If you apply 12V across a 1kΩ resistor, the resistor will conduct 12mA, not 300mA!

 

You do not understand resistors and Ohm's law.

If you apply 12V across a 1kΩ resistor, the resistor will conduct 12mA, not 300mA!

I don't said how much current will pass through the resistor! I said I supply 12 volts and 300mA! I have supplier with range 3,4.5,6,7.5,9,12 volt and 300mA

 

Start by learning ohms law. If you apply 12 volts across a 1K resistor the current through it will be 12 mA If you pass acurrent of 300 mA through a 1K resistor then the voltage across it will be 300 volts. A resistor that has 300 mA flowing through it with 12 volts across it will be a 40 ohm resistor.

Les.

 

I know the 3 formulas given above! But i didnt bit understood what you mean! But let's give an exercise! Let's say I have a resistor 1K and I apply 12V and 300mA! How to understand how many voltage/current it can handle (let's say it will be overheat) until its "death" point! I ask that so as to be clear with that! Let's say I want to light a bulb 2,5 volts and I want to add a resistance! I should to know the right value of the resistance so as to succeed the right brightness! I will not put what resistor I want! I need the resistor that can handle the range of the voltage/current so as to succeed the right brightness! For example I will not put 1 Ohm because the resistor will be broken and bulb will dead, and not a 1 Mohm because the bulb will not lights! So how to understand what resistor with particular voltage/current/power handling range will be right?

For your question, a light bulb will be rated by voltage and wattage.
From this you can determine the current and resistance.
Put a resistor of similar value in series with the light bulb in order to reduce the brightness. To determine the exact value to use you have to do this by trial and error. The exact solution is complex. This is because the resistance of the light bulb is not constant. The resistance changes with current and applied voltage. Secondly, the perception of brightness is non-linear.

 

I don't said how much current will pass through the resistor! I said I supply 12 volts and 300mA! I have supplier with range 3,4.5,6,7.5,9,12 volt and 300mA

Again, you do not understand Ohm's Law.
The power supply may be rated at 300mA. That does not mean that the circuit will have a current of 300mA.

 

Every resister has its 'power rating' look close MrChips's posting above. Only this value needed for choicing right resistor.
Other value like voltage, current will followed by your circuit design. Your power souce voltage, bulb resistance, your chosen resistor value is the key factor. and calulate current.
if current*voltage across your chosen resistor=wattage. this wattage should not exceed its 'power rating' understand?
Hope you understand. If not you are not understand Ohm's law clearly. cheers.

 

I know that, thank you! I know very well Ohms Law! I know how to calculate with Ohms law! But my question is different! Sorry I was not specified! I have 1K resistor and light bulb 2.5 volts. I supply this circuit with 12V/300mA! I want to know (sorry I didn't specify this) if I will increase the voltage to 20V it will breakdown? If no how much it can handle until its death? For example I have a resistor 6K that can handle from 1,5V to 15V (Warning!!! This isn't the real value of its handling I know! I just make an example!) So I make this circuit with the bulb 2,5V and trying to increase the voltage! I know that it can handle only until 15V, until its breakdown! So I have to apply 3V to 14 volts knowing that the resistor can handle the voltage but it will be heated very much so I put a cooling system and I have not problem! That why I need a voltage/current range so as to know what resistor value I will put! If my circuit have a supply 40V, I should to put a resistor that can handle a range of voltage/current about 20-50V or similar without breakdown, with result my circuit will be safe! I will not put for example a 6K that can handle as I said in example 1,5 to 15 volts! I will put a resistor with range 20V to 50V! That I want to know! If the question I did is still not specific or it have less informations let me know about that! But if question is unspecified and it haven't accurate solution, leave it! Don't worry! I will make some experiments for this theme so as to be clear! I just wanted to ask if there is some formulas that explain this theme!

 

As I said, there is no formula for this.

Start with a 1000Ω 1W resistor and see if you like the brightness.
Gradually reduce the value of the resistance and see the effect. The resistor will get hot. If you burn your finger on the resistor then you need a higher wattage resistor.

 

Again, you do not understand Ohm's Law.
The power supply may be rated at 300mA. That does not mean that the circuit will have a current of 300mA.

I know that ;-)! The current that will pass through the resistor will be 12mA as you said right! I know that 300mA will not be in all circuit! The current depending of the component position on the circuit supply each component with current way less than 300mA! The source that I have, supply 12V/300mA! The current then will be decreased depending the position of each component! Some components will take 50mA of this currents, other 100mA, other 25mA, and other 25mA! The total current will be always 300mA never lower never higher as the 1st Kirchoff law!

 

Ohm's law is not calculating matters. It has profound back ground. Just understanding Voltage,Current is also no so easy.
Many ppl just calculate mecanically but not clearly understand what that means.

Sorry, but you said you supply 12v/300mA. That's a fault sentence. 300mA is a current. Current have meaning in the CURCUIT. Not stand alone. I hope you think more about this. I am not Offensive just for your understanding. cheers.

 

As I said, there is no formula for this.

Start with a 1000Ω 1W resistor and see if you like the brightness.
Gradually reduce the value of the resistance and see the effect. The resistor will get hot. If you burn your finger on the resistor then you need a higher wattage resistor.

Now you understood what I mean! But I'm don't worry if there is not any law for this! I will try to make my own table of each resistor handling after some experiments and based on this table I will be clrear ;-)! So thank you very much. I need 3 things for that! 1) A multimeter with temperature calculation, 2) all the values of the resistors and 3) some basic experiments! If I will make it right this table of the values something that will take to me some long time, but I'm don't worry, I will use this table and maybe will post it! The table that have all the basic informations about resistors, including their voltage/current range values ;-)! This is very important for me how much voltage/current can handle a resistor knowing its breakdown voltage! Isn't that good?

 

I know that ;-)! The current that will pass through the resistor will be 12mA as you said right! I know that 300mA will not be in all circuit! The current depending of the component position on the circuit supply each component with current way less than 300mA! The source that I have, supply 12V/300mA! The current then will be decreased depending the position of each component! Some components will take 50mA of this currents, other 100mA, other 25mA, and other 25mA! The total current will be always 300mA never lower never higher as the 1st Kirchoff law!

NO!!!

Total current is NOT always 300mA,

Please learn to apply Ohm's Law.

Current = Voltage / Resistance

 

OTL

 

NO!!!

Total current is NOT always 300mA,

Please learn to apply Ohm's Law.

Current = Voltage / Resistance

I know and this thing! The total current is not 300mA because of thermal losses of the conductors and all the thermal losses of each component! The total energy cannot be equal with the to total energy produced by the source! I actually referring if there was not any losses and all components work perfect and lossless! I know all the basic things! If there was not losses the total current could be exactly the same with the total current of the source!

1) Total Is = Total Ic (without losses)
where Is = current of the source and Ic = the current of the circuit
2) Total Is ≠ Total Ic (With losses)
The thing that you try to explain me is the second formula (Total Is ≠ Total Ic) I know that and thanks for that! But I speak always with the 1st formula where Total Is = Total Ic, namely the source supply 12V/300mA to the circuit and according to this formula the total current will be ALWAYS 300mA (without loses!!!!) But because we have losses the more smaller is the circuit the more it is close to the current of the source but never equal!!!


PS
Ohms law have 3 formulas:
V=R*I
I=V/R
R=V/I
I know to use ohms law!

 

I know and this thing! The total current is not 300mA because of thermal losses of the conductors and all the thermal losses of each component! The total energy cannot be equal with the to total energy produced by the source! I actually referring if there was not any losses! I know all the basic things!

Ohms law have 3 formulas:
V=R*I
I=V/R
R=V/I

No No No!!!

Stop saying you know the basic things because it is obvious that you don't.

Go back and use Ohm's Law properly.

Calculate the current through a resistor by dividing the voltage ACROSS the resistor by the resistance of the resistor.

Do this every time and don't make assumptions.

(And don't repeat any mumbo jumbo about thermal loss and total energy because you are revealing your ignorance - I'm trying really hard to be nice.)

 

OK you are right! Thanks for the tip! And sorry about that I was rude! Its my fault!