Protecting a stompbox

[jeroen_verbeeck]

Hi,

I've read somewhere that when you accidentally switch the positive and negative on your adapter, you can damage the pedal.

I've heard somewhere that you can put something in your chain to prevent this, something that only allows current to come from 1 direction.

I went to the shop and the guy gave me 2 doides.
And when I looked at my prefboard, I saw that I allready used 1 exactly the same as I got from the store.
It's a 1N4002 diode.

I'm making myself a "Rodent", a ProCo Rat Clone.
http://www.generalguitargadgets.com/index.php?option=content&task=view&id=63&Itemid=95

How can I make shure I don't damage things ?

[cd]

http://geofex.com/Article_Folders/cheapgoodprot.htm

[jeroen_verbeeck]

This will probably sound stupid but what if I stick 1 diode to the positive side and 1 to the negative side, would it work ?

[brrt]

It'll shortcircuit the positive and negative sides, so don't do it! Just connect one diode between positive and negative side with the cathode (the side with probably a black mark) at the negative side.

greets,
Brrt

[Samuel]

Only one series diode will be necessary, bt do keep in mind that you will sacrifice 0.6V by doing this. If you know you're only going to be using DC adaptors or batteries, the 1n4002 strapped across the + and - terminals should do, look at the layout for the PT-80 delay on generalguitargadgets.com for a functional example of this setup.

[jeroen_verbeeck]

If you look ate the layout of the Rodent, you see 1 is installed on the bottom in the centre.
Does this have the same function than when between the + and - terminals ?

Would it hurt if I put in 1 extra like you discribed or not ?
(Ohw and it will be in my pedal chain with a DC adaptor)

[stm]

For simple protection with easy to find components you can try one of the aproaches below:

Code Select Expand


      1) Series Schotky Diode


              1N5819
  9 Vdc in o---->|-----o 8.7 Vdc aprox.



       GND o-----------o GND





 2) Shunt Diode with Fuse Resistor

               2.2 ohms
               1/4 W
                ___
  9 Vdc in o---|___|-+-o 8.9 Vdc @ 45 mA
                     |
                     -
              1N4007 ^
                     |
       GND o---------+-o GND

(created by Andy´s ASCII-Circuit v1.27 beta www.tech-chat.de)


Circuit 1: Instead of using a standard silicon diode in series, you can use a 1A schotky diode like 1N5819 for reduced forward drop.  This circuit is failsafe, except for negative voltages above -30 to -40V (the schotky's breakdown).

Circuit 2: The idea of the 2.2 ohms resistor is that it will burn in case power is applied reversed for too long (i.e. several seconds) if current flowing through the diode gets close to 1A (hence the name of "fuse resistor").  Usually these resistors are mounted floating around 6 mm (1/4") above the PCB so if they burn they don't damage the PCB.  The PCB trace should withstand currents around 1A for a short period.  40 to 50 mils is more than enough for this purpose.

The above options can be implemented with easy to find or readily available parts.

PD:  Man, I love the ASCII graphics editor!  :mrgreen:

[Mark Hammer]

Once more for good luck....

If a pedal uses a barrel jack for an adaptor and also uses batteries, then the adaptor plug MUST be almost always outside positive and inside negative, because that's what barrel plugs need to be able to switch from battery to adaptor and keep a negative ground.  Because the inside of the barrel plug doesn't touch anything else besides the pin in the middle of the jack, the only electronic risk posed is that of either using AC by mistake instead of DC, or using an adaptor with too high a supply voltage, or a high current adaptor with a pedal that lacks sufficient current limiting, or plugging something into a pedal that has the opposite ground from what you thought.  (Come to think of it, that's not such a short list, is it?).

If a pedal uses a phone-type miniplug/jack for an external adaptor, the tip connection is almost ALWAYS positive.

If you use one adaptor of the right type/polarity, and simply daisy chain your power connector cable to power up several pedals that all use barrel jacks with the same polarity/ground, then there is no real need to put in the kind of protection you describe.  It *would* be necessary, or advisable, however when:

1) You are using pedals with phone plugs/jacks for external power since the plug touches a few things on the way in.

2) You have some pedals with negative ground and others with positive ground in your collection and you either use them only when you've had a few beers and your reflexes are slow, or loan them out to friends with similar beverage/gigging habits.

3) You have no idea what adaptor you're going to use next.  Could be yours, could be a friend's, etc.

Proper adaptors used properly pose no risk.  So if you do that, don't worry.  If you step outside the perimeter of those conditions, use the diodes.

Just note that the slight voltage drop may make some things sound better with a new battery than they sound with an adaptor (e.g., unrestrained new batteries can give you 9.6V sometimes, in contrast to an adaptor that gives rock steady 9vdc and has 0.6v chopped off by the diode; not a huge difference, but until proven otherwise I'll assume your hearing is good enough to notice).