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### Author Topic: Lab power supply  (Read 4565 times)

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#### zamboniman60

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##### Lab power supply
« on: December 01, 2006, 05:43:14 PM »
I'm working on a simple transistor-regulated (series-pass) power supply. I don't expect to pull a lotta current off this sucker, maybe 500mA, but I'm using a 4A 12V transformer, and four 1F supercapacitors in series to make the base raw power supply. Does anyone know how much ripple I can expect?

• Supreme Robot
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##### Re: Lab power supply
« Reply #1 on: December 01, 2006, 10:37:50 PM »
the basic equation to calculate how long a capacitor can hold 50% of its charge (after about 50% it basically stops working):

time (sec) that cap can power something = C*V*.5 / I

so . . .

1/(1/1F + 1/1F + 1/1F + 1/1F)*12V*.5/.5 = 3 seconds, if I did that right . . .

I wouldnt expect any ripple at all . . . would your 500mA draw do anything at really high frequencies?

#### Militoy

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##### Re: Lab power supply
« Reply #2 on: December 05, 2006, 10:41:48 PM »

• Supreme Robot
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##### Re: Lab power supply
« Reply #3 on: December 16, 2006, 01:47:22 PM »
Ive been looking into supercapacitors some more . . . basically, they arent as neat as I thought they were . . .

Imagine a 1 ohm resistor in series with the cap. What happens if during a sudden peak, you try to draw 2A from that cap? 2A*1ohm = an internal voltage drop of 2V i.e. useless. Also how much heat? 2A*2A*1ohm = 4 watts of peak heat generated inside the cap = boom.

(exploding capacitors make a lot of smoke and shoot stuff in the air, and is why I always wear safety goggles the first time I turn on a new circuit)

Check the resistance of those caps in the datasheet . . . will probably be around 30 ohms . . .

#### Militoy

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