Dont undersant capacitor's basic principe

[alphadog]

If i have 1F capacitor, and load it with 1V, then i connected a 1ohm resistor in parallel with the capacitor.
On the one hand, I = V / R = q / t , therefore a 1A current will flow for 1sec through the resistor.

On the other hand, the voltage on the resistor will be exponentially, and therefore the current will decrease exponentially from 1A to 0A.

What is wrong with my first assumption?

 

[Willbe]

If i have 1F capacitor, and load it with 1V, then i connected a 1ohm resistor in parallel with the capacitor.
On the one hand, I = V / R = q / t , therefore a 1A current will flow for 1sec through the resistor.

On the other hand, the voltage on the resistor will be exponentially, and therefore the current will decrease exponentially from 1A to 0A.

What is wrong with my first assumption?

At a constant current of 1A, your voltage will decrease 1V in 1 second. A resistor doesn't do constant current.

 

[crutschow]

I = V/R when the voltage is constant which, of course, it isn't here since the cap is discharging. The I equals 1A only at the first instant the resistor is connected to the cap.

Similarly I =q/t assumes all variables are constant. But obviously I is not not constant since the cap voltage is dropping as the cap discharges. Thus t will be greater than 1 second. To accurately describe the time under these circumstances you need to use calculus and the differential forms of the equations.

 

[alphadog]

Thanks i understand now.

Can you explain this for me please?
"The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly."

I dont understand why the capcitor can provide an amount of electrical charge faster than a battery
I cant beleive that i'm a third year student in electronics and dont understand it

 

[ericgibbs]

Thanks i understand now.

Can you explain this for me please?
"The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly."

I dont understand why the capcitor can provide an amount of electrical charge faster than a battery
I cant beleive that i'm a third year student in electronics and dont understand it

hi,
In a camera flash circuit, the battery charges a high voltage cap using a voltage step up circuit, driven by the battery.

The cap make take 2 or 3 secs to charge the cap, the high voltage cap is then discharged to fire the flash.

 

[alphadog]

hi,
In a camera flash circuit, the battery charges a high voltage cap using a voltage step up circuit, driven by the battery.

The cap make take 2 or 3 secs to charge the cap, the high voltage cap is then discharged to fire the flash.

Thanks,

But why not using the battery to operate the flash, directly?
The flash only cares about the current that flows through it and for how long in order for it to work well, right?
So can you give me an example with numbers for how the capcitor will do a better job than the battery?

 

[Nigel Goodwin]

Thanks,

But why not using the battery to operate the flash, directly?

Because it's far too low a voltage.

The flash only cares about the current that flows through it and for how long in order for it to work well, right?

No, the flash requires a high voltage - 350V or so - it won't work off a low voltage.

 

[alphadog]

Because it's far too low a voltage.



No, the flash requires a high voltage - 350V or so - it won't work off a low voltage.

But the capacitor's voltage cant be higher than the battery's voltage, so how come the capacitor provide the flash with voltage that is higher than the battery's voltage?

 

[ericgibbs]

But the capacitor's voltage cant be higher than the battery's voltage, so how come the capacitor provide the flash with voltage that is higher than the battery's voltage?

I dont understand why the capcitor can provide an amount of electrical charge faster than a battery
I cant beleive that i'm a third year student in electronics and dont understand it

I dont wish to sound unkind, but how on earth did you get to the 3rd year not knowing the basics...

 

[alphadog]

I dont wish to sound unkind, but on earth did you get to the 3rd year not knowing the basics...

Well I even asked a student that learns in a university which is ranked within the first 50 universities in the world and he couldnt answer me that.

So its not just me.

 

[alphadog]

But anyway, i'd love to understand it.

 

[ericgibbs]

But anyway, i'd love to understand it.

Basically:

Q = C*V

where Q = charge, C= capacity, t = time and Q = I * t

so I*t =C*V

Can you work it out from that.?

EDIT:
Capacitors

 

[Nigel Goodwin]

If it will help makes things clearer?:

Batteries - store energy in the form of a chemical reaction.

Capacitors - store energy in the form of an electro-static field.

 

[alphadog]

If it will help makes things clearer?:

Batteries - store energy in the form of a chemical reaction.

Capacitors - store energy in the form of an electro-static field.

How does it explain what you said that flash needs higher voltage than the battery's voltage, which the capacitor can provide? (But the capacitor's voltage cant be higher than that of the battery).

 

[alphadog]

Basically:

Q = C*V

where Q = charge, C= capacity, t = time and Q = I * t

so I*t =C*V

Can you work it out from that.?

EDIT:
Capacitors

It still doesnt explain (at leat for me) how the capacitor makes a better job in operating the flash than a battery.

 

[ericgibbs]

It still doesnt explain (at leat for me) how the capacitor makes a better job in operating the flash than a battery.

hi,

For explanation, let the Vbty = 10V, and the Cap = 10uF and the Cap charged voltage be 350Vdc

From I * t = C * V

Say, the 't' time to charge the cap is 1sec and the discharge time is 0.01sec.

Icharg= (C * V) /t

Let the eff=100% for the step up ratio of 350/10 = 35 times, assuming 350Vdc on the cap

So, I = (10^-5 * 350) / 1 = 350µA [flowing into the cap, allowing for the step up], the charge C on the cap is 10^-5 * 350

Let cap discharge time of the flash be 0.01sec

So now,
Idis = (10^-5 * 350)/ 0.01 = 0.35A

IF it were possible to create a flash directly from the battery, the battery current would have to supply [35 * 0.35A] for 10mSec...
which for a small dry battery would be impossible.

Xenon flash tubes require a high voltage across the electrodes to generate a 'flash', which is triggered by a HV trig pulse.

Is this any clearer.?

Flashtube - Wikipedia, the free encyclopedia

 

[flat5]

HowStuffWorks "How Camera Flashes Work"

 

[alphadog]

hi,

For explanation, let the Vbty = 10V, and the Cap = 10uF and the Cap charged voltage be 350Vdc

From I * t = C * V

Say, the 't' time to charge the cap is 1sec and the discharge time is 0.01sec.

Icharg= (C * V) /t

Let the eff=100% for the step up ratio of 350/10 = 35 times, assuming 350Vdc on the cap

So, I = (10^-5 * 350) / 1 = 350µA [flowing into the cap, allowing for the step up], the charge C on the cap is 10^-5 * 350

Let cap discharge time of the flash be 0.01sec

So now,
Idis = (10^-5 * 350)/ 0.01 = 0.35A

IF it were possible to create a flash directly from the battery, the battery current would have to supply [35 * 0.35A] for 10mSec...
which for a small dry battery would be impossible.

Xenon flash tubes require a high voltage across the electrodes to generate a 'flash', which is triggered by a HV trig pulse.

Is this any clearer.?

Flashtube - Wikipedia, the free encyclopedia

Thank you very much!!

One thing is not reasonable in your explanation:
How can you load a capacitor with 350V, using a 10V battery?
Once the capacitor's voltage reaches 10V, there will be no potential difference between the battery and the capacitor, and no current will continue to charge the capacitor to 350V.

 

[ericgibbs]

Thank you very much!!

One thing is not reasonable in your explanation:
How can you load a capacitor with 350V, using a 10V battery?
Once the capacitor's voltage reaches 10V, there will be no potential difference between the battery and the capacitor, and no current will continue to charge the capacitor to 350V.

Come on alphadog, you have a posted link showing the step up transformer in the camera flash circuit......

In a camera flash circuit, the battery charges a high voltage cap using a voltage step up circuit, driven by the battery.

I cant beleive that i'm a third year student in electronics and dont understand it

 

[alphadog]

OK i read about this step-up voltage circuit (i didnt know about it before),
But still there's something wrong with your theory.

If the capacitor's charge is 10^-5 * 350, and the discharge time (5*RC) is 0.01sec, then it doesnt mean that the current that will flow through the flash is 10^-5 * 350 / 0.01, becaues the current is not constant, but exponantial, so how can you say that, and afterwards compare it to a battery which DOES produce constant current?

EDIT:
and what does that mean:
"Let the eff=100% for the step up ratio of 350/10 = 35 times, assuming 350Vdc on the cap
"