Decoupling capacitor

I think the formula is
time to 60% of charge=CxR
Where C= capacitance in uf
R=resistance
T=time in seconds
Example 100ufx10M=1000seconds

 

Have you considered a battery? If you can get the right voltage and capacity, your problem will be solved. Otherwise, the use you propose for a capacitor is not decoupling. So you want to supply 300 ma for 0.5 sec? What is the nominal voltage and how low can it decrease and still be acceptable. When I know that, I can calculate the capacitance needed to sustain that voltage range for that length of time while supplying that amount of current.

Ratch

 

Let me get this correct. The system will operate at 12 volts until it switches to another voltage source. You do not want to go below 9 volts during the switchover. The time for the switchover will be 0.1 seconds, not 0.5 seconds. The current drain will still be 0.3 amps. Do I understand everything correctly?

Ratch

 

OK, when the voltage is down to 9 V, and the current is 0.3 amps, the alarm looks like a 30 ohm resistor to the capacitor. We want the RC time constant to be about 5 times 0.1 sec, so that makes the capacitor value 16,667 mf. You may want to double that value because electrolytics are notorious for drying out and losing their capacitance. If the cap is 16.667 mf and the resistance is 30 ohm, the cap will go from 12 volts to 9.82 volts in 0.1 sec. If you want to sustain the same current for 0.5 sec to a drop down to 9.82 volts, you will need a 83.333 mf cap. Those are fairly big units. Are you sure you don't want to use a battery?

Ratch

 

That is a question for your local parts supplier or a catalog, but not me, I would think the 12 volt system could keep the right kind of battery fully energized without over energizing it.

Ratch

 

Hi,

Assuming a constant current discharge of 300 mA, and C = Q/V = (I * t)/V
- for 0.1 seconds, C = (.3 A * .1 s) / (12 V - 9 V) = 10 mF
- for 0.5 seconds, C = (.3 A * .5 s) / (12 V - 9 V) = 50 mF

 

It doesn't work that way. A capacitance that small would never be able to sustain that much current for that length of time. The cap would de-energize in an instant.

Ratch

 

The usage determines what kind of cap it is. In your case, what are you bypassing? Caps that big are sure to be electrolytics.

Ratch

 

You need to learn about decoupling and bypassing.

Ratch

 

The capacitors which have been specified are colossal. Do you mean μf rather than mF?

 

Yes, microfarads. I never heard of anyone saying millifarads, although the prefix is correct.

Ratch