Hi All,

Should it be possible to build an oscillator circuit that consumes 1W continuous regardless if the cap is connected or not? The cap to be charged is 10F 3.0V.

Also, another scenario same as above but when the cap is connected the input power drops by 10%.

Thanks!

Joe

 

Your post makes little sense.
Is the oscillator to charge the capacitor?
What is the input power to the oscillator?

 

Your post makes little sense.
Is the oscillator to charge the capacitor?
What is the input power to the oscillator?

Yes it is to charge the cap, input power is about 980mW with no cap connected and 950mW with cap connected

 

Is the voltage power to the oscillator less than 3V?

 

Yes it is to charge the cap, input power is about 980mW with no cap connected and 950mW with cap connected

Where do you want the power to go when there is no cap.?

 

Hi All,

Should it be possible to build an oscillator circuit that consumes 1W continuous regardless if the cap is connected or not? The cap to be charged is 10F 3.0V.

Also, another scenario same as above but when the cap is connected the input power drops by 10%.

Thanks!

Joe

Curiouser and curiouser.

 

Is the voltage power to the oscillator less than 3V?

Yes VCC is 3 - 3.3VDC

 

If your power supply is 3 V to 3.3 V and you are charging the cap to 3 V, why do you need an oscillator circuit at all?

Why do you want it to consume a constant 1 W whether the cap is connected or not?

Why do you want the input power to go down by 10% when it is connected?

 

Thus far it seems to me that this thread does not make much sense at all. This seems like the situation of an answer seeking a question!
Post #1 seems to make the assumption that the purpose of an oscilator is to charge a capacitor. My experience is that charging a capacitor is seldom the purpose of an oscillator in a "real situation."

 

One more "detail" is that to charge any capacitor the applied voltage must be high enough to cause current to flow into the capacitor. Usually that voltage is a bit above the intended charge voltage.

 

Yes VCC is 3 - 3.3VDC

To charge to 3.0 when the supply is 3.0 to 3.3 is hard. It can be done but......

that consumes 1W continuous regardless if the cap is connected or not

input power is about 980mW

Is the supply source limited to 1W? (the circuit cannot pull more than 1W because there is no more power)
OR
Must the circuit do the limiting?

Must the circuit stop charging when the cap reaches 3.0V? or 2.9V? or something?

 

There is a whole lot missing here!!
To force charge into a capacitor requres a higher voltage. If the charging voltage is the intended voltage for the capacitor to charge to, the charge time will approach "very long" as the voltage difference becomes smaller.
CONSIDER that to push "charge" into a capacitor the charging voltage must be greater than the intended charged voltage, otherwise no current will flow. There is no alternative scheme.

 

Agreeing with MisterBill2. If you use an absolute 3V power source and use it to charge a capacitor, assuming a cap with zero ESR, as the voltage approaches 3V the current will drop near to absolute zero and the cap will charge to 2.9V, 2.99V, 2.999V • • • 2.999999999999999999V but it will virtually never reach 3V. And as Mister Bill stated, with the slowing of the charge rate, it will take years to reach near absolute 3V. Decades later it will still be less than absolute 3V. That's why everyone is saying you need to use a source that is higher than 3V to charge a cap in a reasonable amount of time. And the bigger the cap the higher the ESR, the slower the charge and the less likelyhood it will reach the targeted voltage unless the charging source is greater than 3V.

If a cap is rated for 3V at 10% then using a 3.3V source is advisable. However, it is not advisable to use anything higher because there's a risk of overcharging the cap and possibly damaging it.

 

Yes VCC is 3 - 3.3VDC

Measuring Watts is more complicated than measuring current. To do exactly what you want I we need to measure the current and (3 to 3.3VDC) then multiply. If you have a computer involved, then OK, but to do this in the analog world it is hard and complicated. But if you allow us to only measure the input current, (300mA) and not worry about the input voltage it becomes easier.

 

Agreeing with MisterBill2. If you use an absolute 3V power source and use it to charge a capacitor, assuming a cap with zero ESR, as the voltage approaches 3V the current will drop near to absolute zero and the cap will charge to 2.9V, 2.99V, 2.999V • • • 2.999999999999999999V but it will virtually never reach 3V. And as Mister Bill stated, with the slowing of the charge rate, it will take years to reach near absolute 3V. Decades later it will still be less than absolute 3V. That's why everyone is saying you need to use a source that is higher than 3V to charge a cap in a reasonable amount of time. And the bigger the cap the higher the ESR, the slower the charge and the less likelyhood it will reach the targeted voltage unless the charging source is greater than 3V.

If a cap is rated for 3V at 10% then using a 3.3V source is advisable. However, it is not advisable to use anything higher because there's a risk of overcharging the cap and possibly damaging it.

On paper, it will never reach 3 V, but in practice it will reach 3 V pretty quickly.

For simplicity, let's say that you have a 10 F capacitor and had to keep the charge current to no more than 300 mA. If you could charge it at constant current, then it would take 100 s.

Now put a 10 Ω resistor between the source and the cap to limit the current to no more than 300 mA. That gives you a 100 s time constant.

Now say that you have a volt meter that has 4-digits and is perfectly accurate. How long will it take to read 3.000 V? The actual voltage needs to get to 2.9995 V? 870 seconds, or less than 9 times as long as the constant current case. If you are willing to accept the value from a 3-digit meter when it reads 3.00 V, then that is reduced to 640 seconds, or a bit over six times as long as the constant current case. Neither of these is years or decades.

 

Yes, the theoretical resistive charging of a capacitor states that it will take an infinite time to reach exactly 3V, but in practice, it reaches an engineering useful value in a reasonably short time. For example in 5 time-constants, it reaches 99.3% of the source voltage.

 

And, after taking near infinite time to charge to 3.0000000000000000… V. How long does it stay there when you start discharging it? Or even if you just let it sit there unconnected?

If the cap is being used for energy storage, it reaches 99% of capacity at 2.985V.

 

This thread provides the reasoning for choosing a higher voltage to use for charging a capacitor. At least in the practical realm.

 

Is the thread starter AWOL? Perhaps he realized that his request was extremely ambiguous.

 

Is the thread starter AWOL? Perhaps he realized that his request was extremely ambiguous.

Far too soon to tell. Lot's of people abandon a thread once their question's been answered, even though it would be appreciated by most to say, "Thanks."

But it's only been a few days since they last posted and people have lives outside of AAC (well, that's an unverified claim that I've heard from time to time, anyway). Maybe they've gone camping.