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### Author Topic: Lab power supply  (Read 3203 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 »
If you are asking about the ripple on your raw supply to the front end of the regulator, you should get around 10% ripple (more or less), from a full-wave bridge with a capacitor equal to at least the "critical" capacitance. With a 60 Hz input, that's around 1000uF per amp of load. Any good linear regulator circuit should easily cancel out that much ripple. You can actually end up with less than 0.1% p-p ripple on the output, with a 723-based linear regulator, and proper compensation. IMHO, 250,000uF of capacitance (4 1F caps in series) won't buy you much more than a single 1000uF electrolytic cap will at the current you're running - but it will buy you an enormous inrush current at turn-on - and maybe nuisance tripping of your input fuse or circuit breaker. As a couple of quick notes; you can expect around 15 VDC raw rectified output from a 12 VRMS transformer running through a FWB rectifier using a capacitor filter - but bear in mind that transformer manufacturers design for full-load output. This means that you should plan for higher raw voltage at light loads, and rate your components accordingly. With commercial transformers, regulation can vary widely. At only 40 degrees C temperature rise, this can mean up to 50% regulation (higher no-load voltage), if the manufacturer has gotten aggressive with his design margins. Best to test the transformer on the bench before making any assumptions. You should allow around 3 VDC compliance for your regulator (15 VDC raw voltage for a 12 VDC regulated output). Hope this helps.

• 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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