# Capacitor time formulas (for noobs)

• 03-30-2005, 12:12 AM
KyleYankan
Capacitor time formulas (for noobs)
Hi everyone. Let me make it VERY CLEAR that I am not planning on using a capacitor as a tank circuit. I'm only writing this because I'm reading my electronics book, and ran across this. I thought I would point this out for the n00bs.

Time = Farads * Ohms

That means, that how many ohms your computer provides, times the farads of the capacitor is how many seconds it will last.

Volts = Amps * Ohms

therefore Ohms = Volts / Amps
therefore Time = Farads * (Volts / Amps)

Volts is usually 12.

So, for my 120 watt computer, I can figure out the amps by dividing by the voltage.

120 watts / 12 volts = 10 amps. I can plug that in my formula no problem.

Time = Farads * (12 / 10)

However, I need to know either
a) how long the capacitor I have will last
or
b) how big a capacitor is needed to last x seconds.

I said, I need a capacitor that can last 10 seconds.

10 = Farads * (12/10)
8.3333 Farads are required for a "normal" car startup. Do you have any 8 farad capacitors laying around?
• 03-30-2005, 06:11 AM
MikeH
Kyle,

You may want to go re-read a bit. Take a good look at the definition of "last". The capacitor voltage decays at an exponential rate. You need to define "last" as the point in time at which the voltage drops below the acceptable operating point for whatever is connected to it. The standard "RC time constant" curve defines that time at which the voltage reaches 63.2 percent of it's final value. If you are discharging from +12V, the RC time constant is the time at which the voltage decays to 4.416 volts (12 * (1-63.2)). There are very few DC-DC converters/inverters that will continue to operate at 4.4 volts...;)

HERE's a pretty good explanation.

• 03-30-2005, 07:56 AM
Chris31
Another capacitor thread :lol:

Check it out here >>>

I get a feeling noobies comes to this forum with a spare 1F capacitor on their hand. :rolleyes:
• 03-30-2005, 09:02 AM
MatrixPC
• 03-30-2005, 02:48 PM
Chris31
Haha

4.7F for 1 seconds protection :lol:
• 03-30-2005, 03:07 PM
slartiblartfast

Impossible. Has anyone thought about making a poor man's UPS, by buffering the 12v current with a gel cell battery and some relays or something?

Be a lot cheaper than dropping \$400 on capacitors.
• 03-30-2005, 03:19 PM
jcdillin
yes, plenty have done that with gel cells
• 03-30-2005, 03:21 PM
Chris31
Quote:

Originally Posted by slartiblartfast
Has anyone thought about making a poor man's UPS, by buffering the 12v current with a gel cell battery and some relays or something?

Not that Im aware off, no. :rolleyes:
• 03-30-2005, 03:21 PM
Chris31
Quote:

Originally Posted by jcdillin
yes, plenty have done that with gel cells

Ah man you ruinned it :lol:
• 03-30-2005, 03:32 PM
slartiblartfast
probably seems like the way to go.