Protecting pedals from reversed polarity

[Bill_F]

I know I can protect pedals from reversed polarity with a diode. Now the question. I have built so many pedals already without this and want to find the easiest way to protect them. Can I simply solder a diode to the input and ground of the jack? If not is there any other simple solutions?

Thanks,
Bill

[Thomas P.]

If you mean the DC-jack I'm sure you can. But look at GEO - R.G. got a more secure solution with a BS170.

[Boofhead]

It's best place across the +V and 0V of the effect circuit itself.  This protects against both battery and DC jack reversals.

[Paul Perry (Frostwave)]

It's best place across the +V and 0V of the effect circuit itself.  This protects against both battery and DC jack reversals.

I still say it is better to have the diode in series with the power supply, although many disagree with me! My main argument is that if you have the diode in parallel, in a reversal situation there is a big current thru the diode, which may 1. fry the diode, leaving the voltage across the circuit, or 2. burn out the wart.
OK, the argument AGAINST a series diode is, you lose .6 volts, and yu have a small series resistance added to the power source.

[Marek]

Actually, I agree with you, after having a discussion with my father on the subject. He came up with the same agruments as you.

But, I mean - look at the old BOSS schematics. All of them have V+ to GND protection! There must be a reason to it.

Greetings,
Marek

[R.G.]

The issues are simple, as everyone notes:
-- The best protection is provided by a series diode, but that loses you a diode-drop in supply voltage.
-- You can avoid losing the diode drop from your 9V battery by using a reverse-shunt diode to effectively short the power supply. This works as long as the shunt diode *can* short the power supply without burning out either itself or the power supply.

The reverse shunt diode works GREAT for protecting against momentarily reversed batteries, and I suspect that its cheapness and simplicity for primarily-battery-powered effects is what accounts for its wide use. However, I have rescued many effects from pawn shops that are "broken" that merely have the reverse shunt diode shorted. That's what happens when the power supply (usually a wall wart) can cook the diode until it gives up.

The MOSFET polarity protector at GEO sidesteps all that. It's a slightly-nonstandard use of a MOSFET that provides series protection against reverse voltage so you don't cook the protection element, and also uses the MOSFET to keep the series losses low. At the risk of sounding smug, the MOSFET protector is literally the perfect polarity protector for battery powered effects. It has no series voltage drop and no conduction when reverse polarity is applied. What keeps me from being smug is that I didn't conceive of this - I found it in Robert Pease' book "Analog Troubleshooting".

Its only drawback is that you buy a MOSFET and a resistor instead of a diode. That used to be a problem when BS170's were US$0.50 to US$0.75, I guess, but I've seen them recently for US$0.20 to US$0.25. If you've ever repaired a burned out pedal, the extra quarter seems pretty cheap.

The article shows how to stitch it into effects. I think it ought to be part of the standard effects "wrapper". By the way, I think it's worth reading about wrappers - see the article at GEO.

[Mark Hammer]

The whole issue of diode protection emerges largely (though not solely) because barrel-plug wall-wart cables have two choices as to which contact carries hot.  If the "right" adaptor with the "right" plug polarity is used, this provides an advantage in that nothing can be accidentally shorted during plug insertion.  You can stick a plug in or pull it out any time you want.

Unfortunately, because there are two choices, it is also possible to have the "wrong" plug polar orientation and do grievous damage.  Typically, if the pedal can also use a battery, inserting the adaptor plug bumps a contact outof the way and power switches over from the internal battery to the  external source.  Naturally, the only contact that can do the bumping is the outer one, which is why BOSS and other similar pedals employ the outside-hot/inside-ground convention.  Of course, if the device does not use batteries, and if the jack is isolated from chassis ground (which most are) it then becomes the perogative of the wallwart manufacturer as to whether they use outside for hot or ground, and that is where your 50/50 chance of error comes into play (which is also why pedal makers make a big stink about you using THEIR adaptors, since it is easier and less prone to error than trying to explain what you need to look for in an adaptor and why).

I contrast this with the older-style mini phone plug/jack like E-H and many others used for years.  Here, the jack is always connected to chassis ground, and the only possibility is for the tip to carry V+.  Really, the only thing to look out for is that the wallwart provide a suitable voltage and be DC not AC.  Of course, the problem here is that there IS risk of shorting contacts during insertion so the user has to always be careful to plug into the pedal first and THEN plug into the wall or enable the wallwart transformer in some other manner

I suppose the other reason for a protection diode is the inadvertent contacting of the battery to the wrong connectors on the snap (generally after the beer starts working, or before the coffee starts working).

I have seen ads on TV for "miracle" booster cables for cars that sense errors in battery-to-battery connections and make appropriate connections regardless of how poor lighting conditions are or how little one knows about giving one car a boost from another.  Anyone know how those work?[/u]

[Bill_F]

Thank you everybody for your help. Now to pour through this info and read that article on your site R.G.

Bill

[gez]

If you go for the series diode method Paul mentioned (I'm a fellow heretic!), you can 'get round' the issue of reduced headroom by using op-amps with rail-to-rail swing at their outputs.  

Although Maplin no longer stock it (gits!), ESR still do the CA3130 which is MOSFET input, CMOS output and (unlike many CMOS amps) has pretty decent slew rate figures.  One of my favourite amps!

Unfortunately it's only a single .  Dual CMOS amps aren't exactly thick on the ground in the UK, I use various ones but they're not exactly ideal for audio.

Just a little more food for thought...

[Nasse]

So shunt diode protection is crude "crowbar" method. For devices taking more power than pedals there is normally a fuse or some current limiting circuitry, but I doubt this kind of thing is cclever with pedals. Costs and part count and size... But must look that Geofex thing...

For mobile radio and R.C applications there is few tricks used, but again we are talking much more power than 9V batteries. One "idiot proof" method is to connect dc supply trough bridge rectifier. But voltage drop is trough two diodes... Another spiffy thingie I saw in one electronics book was a relay arrangement. Relay is normally closed, but when wrong polarity is added, relay contact opens and protects the circuit. Big question is does this happen so quickly that your chips dont fry, most likely not, so youll need a diode or something anyway...
But if the voltage drop and current consumption is not a problem then it can be done

[Paul Perry (Frostwave)]

One "idiot proof" method is to connect dc supply trough bridge rectifier. But voltage drop is trough two diodes...

Someone once said there is no such thing as "idiot proof" because idiots are so ingenious. In this case, the idiots have to remember not to share a supply between a + and - earth units.