Power supply filtering and BFC values

[zpyder]

When filtering a PS, as in http://aronnelson.com/gallery/Burnt-Fingers-stuff/power_supply_filter_with_led_res_board, I'm wondering if any capacitor is TOO big.  I've read R.G. refer to this as a BFC (big-f'ing-capacitor I assume)... this makes me think that the bigger the better.  Burnt Finger's layout uses a 100uF cap.  I've got a bunch of 1000uF, 330uF, and 220uF.  Too much??

thanks,
zpyder

[R.G.]

Any of those will work. Put a 0.01uF ceramic disk in parallel to have low impedance up at RF too.

[zpyder]

The schematic above also uses a reverse-biased diode between +9v and ground... what is the purpose of this?

zpyder

[R.G.]

Polarity protection.

[shanter]

Howabout when your dividing the voltage... ie in the bluesbreaker schem  ( http://www.tonepad.com/getFile.asp?id=75 ) does the 100uf have to be placed in series with the 47k or can it be removes and placed somewhere else??

and whats the difference between having the polarity protection diode in series with the power source or having it connected to ground?

[R.G.]

Howabout when your dividing the voltage... ie in the bluesbreaker schem  ( http://www.tonepad.com/getFile.asp?id=75 ) does the 100uf have to be placed in series with the 47k or can it be removes and placed somewhere else??

It's not in series with the 47K - it's from the junction of the two 47k's to ground. That cap is not filtering the 9V power supply, it's filtering the bias voltage and has to be connected where it is. See http://geofex.com/circuits/Biasnet.htm.

and whats the difference between having the polarity protection diode in series with the power source or having it connected to ground?

When it's in series, current can only flow through it one way - into the effect. So when the supply voltage is accidentally reversed, no current flows at all. But you lose one diode-drop's worth of voltage from your battery voltage this way, all the time. Batteries last less long.

When it's in reverse parallel, again, current can only flow one way. But the way it flows is from ground to the +9V supply. So if the power supply is accidentally reversed, this diode conducts heavily, and clamps the reverse voltage across the effect to only -0.7V or so. But it eats all the current that the external supply can produce, looking like almost a dead short like it does. If the external supply is a battery, the battery gets hot and can spew electrolyte. If the external supply is a wall wart and not well current limited, both the wall wart and the diode get hot. Which one lives longest determines how well this all works out. Sometimes the diode burns the PCB black underneath it, shorts from the overheating, and then the effect won't work at all, even if the power gets right.

See http://geofex.com/Article_Folders/cheapgoodprot.htm and  http://geofex.com/Article_Folders/mosswitch/mosswitch.htm.