Author Topic: Vcc Vref? Reverse polarity protection  (Read 4072 times)

endorphin

Vcc Vref? Reverse polarity protection
« on: January 27, 2012, 01:03:50 AM »
Hey smart people! I've been studying some of the circuits from beavis audio research and have decided to build the power supply filter/ reverse polarity protection and my question is

What does Vcc and Vref mean? My guess is Vcc is + and Vref is ground. I planning on putting this on pad per hole and putting into a Namelco Fuzz Wa Vol so I could use a DC 9v adaptor.

 http://beavisaudio.com/techpages/BlocksFragments/Power_Supply_Filtering.jpg
60% of the time it works everytime...

Seljer

Re: Vcc Vref? Reverse polarity protection
« Reply #1 on: January 27, 2012, 03:49:48 AM »
Vcc is your +9V power supply

Vref is a voltage reference, R2 and R3 from a voltage divider so that output is roughly at 4.5 volts. This is used in pedals where you have opamps and other components that need to be biased to a voltage above 0V so AC signal can pass through.

Mike Burgundy

Re: Vcc Vref? Reverse polarity protection
« Reply #2 on: January 27, 2012, 07:41:34 AM »
Just to clarify:
most opamps we use want to have a dual supply - so +xV AND -xV. inputs and outputs sit at ground level.
For use with a battery we fool the opamps into believing it has a +/- 4.5V supply by introducing a *virtual* ground at 4.5V. That's Vref. Stick that into the + input of an opamp (through a resistor), and the rest of the pins will follow - have a look at some schematics and you'll see. The opamp thinks Vref is "ground", and the battery's poles sit at -4.5 and +4.5V.
This does create the need for coupling caps - they pass signal while blocking any DC offsets - the opamps ins and outs can now sit at Vref without any DC troubles for the rest of the circuit where signal is referenced to real ground.

Edit: read everything you can find on polarity protection and designing bias (or Vref) networks at GEOfex.com.
This scheme is the more or less standard one, which will protect you pedal from reverse polarity - briefly.
Reverse polarity will result in the diode shorting out the reversed voltage, keeping it from the rest of the pedal. This will immediately turn into a battle to the death between diode and supply, either frying the diode (subsequently letting the reversed voltage rampage through your circuit) or frying the supply. There are better ways.
Also - depending on the wah circuit, this may not be the circuit needed. Do you even need Vref?
« Last Edit: January 27, 2012, 07:47:19 AM by Mike Burgundy »

endorphin

Re: Vcc Vref? Reverse polarity protection
« Reply #3 on: January 27, 2012, 12:02:37 PM »
Ok thanks ill go and find out more about this Vref and opamps but I don't think that would apply to this pedal being it doesn't have them. I figured that I could build it and use by drilling a hole for it in the enclosure and adding the battery clip onto the DC jack. Yes geofex is a good resource.

Heres the schematic

http://aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=261&g2_serialNumber=2

I wouldn't be able to tell myself if reverse polarity is needed in the circuit.
60% of the time it works everytime...

DavenPaget

Re: Vcc Vref? Reverse polarity protection
« Reply #4 on: January 27, 2012, 12:17:58 PM »
Yes geofex is a good resource.
IS THE BEST RESOURCE .
Hiatus

ashcat_lt

Re: Vcc Vref? Reverse polarity protection
« Reply #5 on: January 27, 2012, 02:18:13 PM »
You can leave out everything to the right of C2 for your circuit.

Of course that's the wrong way to do reverse polarity protection...