Protection Diode in FTM

[Phend]

On any 9v battery operated effect, specifically the FTM, is is a good idea to add a diode from battery plus to ground as protection against "touching" the battery terminals backwards to the battery clip?

[antonis]

I'm not sure for what you're thinking about but a diode from +9V to GND shouls make an almost(*) perfet short-circuit..!! 

(*) Perfect for an equivalent source of about 8.3V..

[patrick398]

I think the 'classic' protection diode set up, i.e in parallel with the supply is a relic from times when pedals were almost always battery powered and it provides protection for exactly that scenario. For times when people plug in the wrong power supply and then spend a good 5-10 minutes staring at the pedal wondering why it isn't working i don't think it provides good protection. Hence why using a series diode (1n5817) is better in applications where the voltage drop doesn't matter.

This is my understanding anyway...

[Phend]

FuzzCentral's Axis Face incorporates this diode protection with a better explanation.

http://fuzzcentral.ssguitar.com/axisface.php

[antonis]

Thanks for attaching a "better" explanation..
(although, partrick's explanation is more pithy but let it be..)

As you might noticed in attached schematic, diode is facing upside down (cathode facing +9V) where your proposal was for the opposite:

a diode from battery plus to ground..

The above arangement results in a parallel to battery poles forward biased diode..

[Phend]

So getting back to the question, is this type of diode protection a "relic" or is it a good idea to incorporate in to "any" or almost any Battery operated effect.

[patrick398]

I don't think we ever got away from the question did we?
Yes for battery only builds a parallel diode will protect against momentary reverse voltage. In cases where the voltage drop doesn't matter i'd always go with the series diode though, battery powered or otherwise

[Phend]

Thank you Patrick398. Being the relic that I am I can be slow at times.

[ElectricDruid]

I don't think we ever got away from the question did we?
Yes for battery only builds a parallel diode will protect against momentary reverse voltage. In cases where the voltage drop doesn't matter i'd always go with the series diode though, battery powered or otherwise

Another vote for series diode from me. Nothing is perfect, but it's simple and cheap, and that's close enough.

I started off using parallel diodes, reckoning that it was up to the designers of power supplies to protect their power supplies from short circuits, but ultimately that seemed a bit unfair. Who am I to smoke some poor power supply, just because the polarity is "back to front" according to me? The volt drop across a 1N5817 at the sort of currents we're talking about is minimal, and if it mattered, you can just use a 12V supply. So series wins, and these days that's what I use.

[Rob Strand]

Back in the day of when Plug-packs/Wall-warts came out they had low ratings and used a transformer + rectifier + cap.   Like most small transformers they can handle shorts for quite a long time before actually failing.

One point about the parallel diode method is the diode has to be rated high enough not to fry itself.    That usually means a 1N4004 etc.  but you still see 1N914 used.   I've seen many modern bass preamps which use parallel diode protection and they use tiny SMD diodes.  These things regularly fry with the current from an alkaline battery.  Hopeless!

In the modern day, switch mode supplies can provide high currents and can take out even high-rated parallel protection diodes.

A low drop Schottky is probably the best solution for today.   If you didn't know power Schottky diode were not around in the old days.

[amptramp]

Another vote for series diode protection.

It doesn't overload the power source and Schottky diodes are readily available that don't drop much voltage.  Parallel diodes always have a race condition - is the diode going to blow open or short before the power stops?  If short, everything is protected.  If open, nothing is protected.  The 9 volt battery is a relic that should have been scrapped long ago for music service.  We can use wall wart 12 volt supplies that would offer more headroom even with a series diode and 15 volt supplies with a series diode and a regulator so we can get rid of power line sensitivity.  Two 9 volt batteries in series would allow for polarity protection and a regulator.

[duck_arse]

I have a phlosophical question - in a battery only powered pedal, how easy is it to change the battery with the in/out cables still plugged in? I only ask because a battery powered pedal would [shirley/shorely] have a switched-by-the-jack-plugged-in battery  switching  arrangement, and you'd need to unplug to battery access, and so there would be no current flow, battery ded or alive, fitted right way or wrong way, and so no protection needed. and if you could plug it wrong in the snap, and you could get the back on and plug it all in, well, good luck to you.

[antonis]

I have a phlosophical question
..............

Looks more like a realistic declaration to me..

[Phend]

Thanks for the conversation. Stephen has a good point, I will be using a stereo jack to connect the battery to the circuit. The FTM will be unplugged when the battery is hooked up.  If I had used two mono jacks and a "on off" switch to activate the battery then different scenario. But let me ask this hypothetical case, if No diode protection, what is the possibility of doing damage to a component if the battery is quickly touched the wrong way to the clip? Is it really an issue, I have read "better safe than sorry" on this topic.

[ElectricDruid]

But let me ask this hypothetical case, if No diode protection, what is the possibility of doing damage to a component if the battery is quickly touched the wrong way to the clip? Is it really an issue, I have read "better safe than sorry" on this topic.

It would depend entirely on what "the component" is in this case and how it's been wired up. Some things are sensitive, and reversing the voltage like that even briefly will be the end of them. Other things are far more robust and will survive a lot of abuse, so there isn't really a definitive answer. Hence "Better safe than sorry" is not a bad policy.

It's also worth saying that there's a difference between building a one-off pedal for your own use and designing something commercial for a mass market. If you build something and leave off the protection and then blow it up, you probably (a) know what happened, and (b) can fix it, as soon as you stop swearing at yourself. If you're designing a commercial pedal, you can bet that users will try out using the transformer from the answering machine to run the pedal, so it'd better be protected, and if it dies when they do, they'll think it's your fault. So the need for protection is that much greater.

[Rob Strand]

I have a phlosophical question - in a battery only powered pedal, how easy is it to change the battery with the in/out cables still plugged in?

Excellent point.  I have no idea how people fried the diodes on their in-board preamps.   Modern stuff often have battery compartments but old stuff you had to unscrew the back plate.    For good or bad, people are able to find the most obscure cracks.  How remains a mystery,

[PRR]

> you'd need to unplug to battery access

I don't recall unplugging to change battery. (Days before way-short cords, pedal-boards, and nailed-on pedals.) Flip the box over, open, change battery. YOUR pedalboard situation may prohibit that, but the pedal-maker can't control that, yet gets all the blame if you manage to poof the pedal.