Adding power filter and polarity protection diode

Frequently asked questions regarding powering your pedal.

Adding power filter and polarity protection diode

Postby halfmassive » 24 Jan 2013, 13:38

Hi,
I've been really getting into building circuits on vero board lately and its really the first time I have done anything that doesn't have complete wiring layouts and such. So far so good, but I have a question regarding a power filtering cap and polarity protection diode. I have noticed these in the GGG fuzz face projects and on a few other projects that use fabricated pcbs. I am in the process of building a clone of a colorsound wah fuzz and the perfboard layout I'm following doesn't include these things for dc power. I'm clear on how to wire the dc jack and all but not sure how would I go about adding these components. The method and value seem to vary from pedal to pedal so I'm a little confused. Any help would be greatly appreciated.

Thanks!
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Re: Adding power filter and polarity protection diode

Postby halfmassive » 24 Jan 2013, 13:54

And if I posted this in the wrong area I apologize - I'm new to this forum.
Thanks.
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Re: Adding power filter and polarity protection diode

Postby jmraney317 » 24 Jan 2013, 14:12

That's a completely optional bit of circuitry, but it's handy if you think you might accidentally use the wrong 9 volt adapter or something.

The two components you need to add are a diode (1N4001 works well) and an electrolytic capacitor (value not critical; voltage rating should be something greater than 9 volts, obviously). Basically, you'll break the wire from the positive terminal of the power jack or battery. The anode end of the diode (the end away from the stripe) connects to the positive battery/jack wire. The cathode end (has a stripe) connects to the positive terminal of the capacitor. The negative end of the capacitor connects to ground (usually the negative battery terminal). The point where the positive capacitor terminal and the diode's cathode lead meet is then connected to the effect's positive voltage point (the wire that used to run straight to the positive battery terminal).

Do keep in mind that the diode drops around 0.7 volts, which may make a difference in how the effect operates. If you have a power Schottky diode in your junkbox, those only drop around 0.17 volts.
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Re: Adding power filter and polarity protection diode

Postby halfmassive » 24 Jan 2013, 16:26

This clears things up greatly. Thanks very much. You mentioned the value of the cap isn't too critical but is there a standard value or range I should stick to? I have seen 470 uf and 10 uf used before. Thanks again
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Re: Adding power filter and polarity protection diode

Postby deltafred » 24 Jan 2013, 17:20

There isn't a standard value but 100uF is what I would use as long as there is enough space, but 10 - 470 is fine.

If you want to be technically correct it is also worth adding a low value disc ceramic in parallel with the electrolytic to attenuate any high frequency rubbish that gets through.

This is because at high frequencies electrolytics are not effective at shunting the interference to ground due to their internal impedance, disc ceramics are good at this and 0.1uF is an accepted value.

Whether you will notice any difference is debatable but it will give you a warm feeling that you have at least tried to do it right.

(If you have anything digital on your board and digital noise is getting into the audio chain then the liberal use of 0.1 disc ceramics across the power supply filter caps can sometime reduce this. There are recognised techniques employed in digital electronic design for reducing this, not all pedal designers are that well up on design techniques.)
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Re: Adding power filter and polarity protection diode

Postby halfmassive » 24 Jan 2013, 21:41

Thanks again!
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