question about voltage drop

in a electronic book i saw this paragraph

" if the source cannot supply 10w. its out put voltage drops with the excessive current load.Then the current is reduced to the amount corresponding to the amount
of power the source can produce."

here the question that i have is how do you know maximum power that can be supplied by a source ?

and how can a source drop its out put voltage by it self ?
can a battery do that ?

A cheap wall-wart power supply might have an output of 9VDC when its load is 200mA but its voltage might be 18V when its load is only 10mA.

A little watch battery cell is 1.5V when its load current is 10uA but its voltage is almost zero when its load current is 20mA.

Because they have an internal resistance that forms a voltage divider with the resistance of the load.

A cheap wall-wart power supply might have an output of 9VDC when its load is 200mA but its voltage might be 18V when its load is only 10mA.

A little watch battery cell is 1.5V when its load current is 10uA but its voltage is almost zero when its load current is 20mA.

Because they have an internal resistance that forms a voltage divider with the resistance of the load.

A cheap wall-wart power supply might have an output of 9VDC when its load is 200mA but its voltage might be 18V when its load is only 10mA.

A little watch battery cell is 1.5V when its load current is 10uA but its voltage is almost zero when its load current is 20mA.

Because they have an internal resistance that forms a voltage divider with the resistance of the load.

A cheap wall-wart power supply might have an output of 9VDC when its load is 200mA but its voltage might be 18V when its load is only 10mA.

A little watch battery cell is 1.5V when its load current is 10uA but its voltage is almost zero when its load current is 20mA.

Because they have an internal resistance that forms a voltage divider with the resistance of the load.

If you don't know the capacity of your source, you can do the following experiment.
Put to your source a small load I=0.01A and register the voltage.
Then increase the load to I=0.02A and register the voltage
and so on
continue until the voltage drops certain quantity that can be 5 or 10% or any other given specification.
usually for a certain range of loads the voltage almost mantains constant when it gets out of the range specified that is your maximum load. P=IV
that is your maximum Power.

ok thankz

can you guys tell me normally when when a power rating is given by a resister you can find the voltage when that amount of power is dissipated.so the resister should be able to handle this amount of voltage right.but why does many have a working voltage less than that ?

Just use the ordinary power calculations for the power in a resistor. They use the voltage across the resistor called V, or the current in the resistor called I. The resistance of the resistor is called R.

Power= V x I.
Power= V squared divided by R.
power= I squared times R.

A resistor is made from materials that survive very high temperatures. So a resistor at its max allowed power is extremely hot and will burn you or melt a plastic capacitor or wire insulation that is nearby.

For a low power, the most usually used resistors are the charcoal and metallic film. And heat affects the value of the resistor more the charcoal one, so you must consider not reach the maximum power and temperature.
Example for a 1/2 W Resistor you must consider 0.8*1/2= 0.4 watts as a new maximum operating power.
For a metalic film resistor 70°C is the maximum temperature without afecting the value of the resistor. You can take care not to exced that, if so the resistor value will decrease. So 60°C is a good value not to exced.

I'll hope that this help you.

yea true using P=v^2/R we can get p. but in a certain electronic book it was stated that

"
the maximum working rating off a resistor is the maximum allowable voltage that the resistor can safely
withstand with out internal acing . the higher the wattage rating of the resistor the higher the maximum working voltage rating."

which means you can find out the maximum working voltage using P=v^2/R

but again the book says " with very large resistance values the maximum working voltage rating may actually be exceeded before the power rating"

why is this is happening .this is confusing if this is so why cant the resister indicate it power rating to be the value using its actual working voltage.

You are confusing a resistor's max allowed power dissipation rating which must be calculated for each resistor in a circuit, with a resistor's max allowed voltage rating so it does not arc. Of course a low value resistor will over-heat if a voltage that is not even high is applied to it.

so there is no connection between voltage rating and the power dissipation rating i thought that you can find I using w=(I^2)*R
and from I we can find V using V=IR or W=VI. i thought this V is the max allowed voltage rating. isnt this so ?

Look at a datasheet for resistors.
I think (I haven't looked for about 44 years) the max allowed voltage for a 1/4W carbon-film resistor is 200V or 250V.
I think the max allowed voltage for a 1/2W carbon film resistor is 300V or 350V.

Then calculate its power dissipation:
1) A 220 ohm 1/4W resistor dissipates 1/4W when its voltage is only 7.4V.
2) A 56 ohm 1/4W resistor dissipates 1/4W when its voltage is only 3.75V.
3) A 180k resistor dissipates only 0.22W when it has 200V across it.

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thankz audio gury

i have a small question

1) A 220 ohm 1/4W resistor dissipates 1/4W when its voltage is only 7.4V.

what happens if 7.4V is exceeded?

as you have stated max allowed voltage for a 1/4W carbon-film resistor is 200V or 250V. so if you use a voltage as much as 200V the power dissipated wouldnt that be more than 1/4 W ????

this is whats confusing me any wayz sorry i feel like im going in circles and asking da same question over and over sorry about that

nirVaan said:

i have a small question

1) A 220 ohm 1/4W resistor dissipates 1/4W when its voltage is only 7.4V.

what happens if 7.4V is exceeded?

Click to expand...

DUH!
What do you think happens to something that gets hotter than its max allowed temperature??? It might smoke. It might melt. It might catch on fire. It might melt its solder and fall off the circuit board.

as you have stated max allowed voltage for a 1/4W carbon-film resistor is 200V or 250V. so if you use a voltage as much as 200V the power dissipated wouldnt that be more than 1/4 W ????

Click to expand...

DUH! again.
The max allowed voltage is when the resistor arcs. The max allowed power dissipation is the max allowed heat. They are completely different.

ahhhh i think i get it now resister arching is not the same as resister over heating. but that just make me have more questions in my head lol.

if you cant connect a given resister to a voltage that is more that 4.7 V (if you do it will burn) why would the resistor manufactor even mention about max allowed voltage of 200 V.i mean surely you cant even dream about connecting this resister to voltage of 200V.

i hope its clear what im trying to say.i mean if you cant connect the resister to a voltage source of more than 4.7V. or it will burn shouldn't this be the max voltage ???. giving a max voltage of 200 v implies that you can connect this resister to any voltage below 200V and expect normal operations. but hey lets say you connect it to a 150V but if you do It might smoke. It might melt. It might catch on fire. It might melt its solder and fall off the circuit board

Added after 1 minutes:

i hope my question is clear

Added after 6 minutes:

the only thing that i can think of is at normal room temperature you cant exceed more than 4.7 V .but if you sum how manage to have few dozen air conditions air coolers and heat sinks around it and keep it temperature down and avoid it from over heating you can manage to connect it to a voltage source of 200 V. is this correct or am i utterly confused.

People don't use a fan to cool an overheated resistor. They simply use an bigger resistor instead that has a higher max allowed power dissipation rating.
Or re-design the circuit so that a low value resistor is not used with a high current in it when it wastes power as heat.

1/4W resistors are small so their max allowed power dissipation is 1/4W. Their max allowed voltage is 200V or 250V if the power dissipation rating of 1/4W is not exceeded.
People make vacuum tube amplifiers that use hundreds of volts so special high voltage resistors are used. Of course the values are high enough so that the power rating of the resistor is not exceeded.

200V across a 100k resistor is a power dissipation of only 0.4V so a 1/2W resistor will be pretty warm but will be fine.
200V across a 180k resistor is less than 1/4W so a 1/4W resistor is fine.

Your example of the max voltage of 4.7V on a resistor calculates to be a 44 ohms 1/2W resistor.
If you want more voltage across a 44 ohms resistor then use a 1W resistor. The max voltage is 6.63V for 1W of power dissipation. Or use a 2W resistor for a max voltage of 9.4V. or use a 5W resistor for a max voltage of 14.8V. Or use a 10W resistor for a max voltage of 21V. Or use a 25W resistor for a higher max voltage.

hmmmmm

i still cant undestand this fully
simply put in a resister you have a power dissipation rating right.

using W=V^2/R you can find the V right

if you connect the resistance to a higher voltage than this given V the resister will over heat right

so we should not connect the resister to a higher voltage than the given V right

if so why is there a another much more bigger voltage value given as max voltage

The power dissipation of a resistor is simply calculated with its resistance value and the max amount of voltage across it or the max amount of current through it.
Then use a resistor size (power dissipation rating) that is big enough so it doesn't over-heat.

The max allowed high voltage rating is the voltage that causes arcing (the electrons jump over the insulation).