About polarity protection (when things go wrong)

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Duckman
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Post by Duckman »

DSC00513.JPG
Ok, here's what a good friend can do with a chinese wall wart (those 9v to 12v w/ polarity selector)
It's a TS808 I built for him as birthday gift and now is working again, but I can not forget that I lost a nice sounding 4558 for failing to put a simple diode, so I need to learn more.
I saw the 1N400x's are the most common diodes for polarity protection in so many stompboxes , so I assume to be the most appropriate, but what about the configuration: is better to connect the diode from the 9v line to ground, as in the Rat, or added in series in the 9v input, as in the Blues Breaker (just to give a couple of examples) or is exactly the same? What about 18v or higer voltages? Same 1N400x's or need something different?

Thanks to all who are interested in this subject and want to help :wink:
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Post by salocin »

Very interested to hear peoples thoughts on this. I've just been wondering the same thing.
I've not really bothered with it in the past but have recently had friends wanting to buy/trade pedals I've built so I've had to add it in so they don't come back to me with the pedal looking like the one in your post.
To this point I've just been sticking a 1N4001 in series with the voltage input. The first issue that comes to mind with that is the voltage drop.
The polarity protection in the Barbershop intrigues me, but what are the merits of it?
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Post by Duckman »

The MOSFET is indeed for polarity protection, works like a charm. D1 is a 15V zener diode, it shuts off Q4 when the user plugs in the wrong voltage power supply.
Nice trick! Seems like the MOSFET does the work, but without burning any component, just disables 9v, uh?
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Post by mictester »

Duckman wrote:
DSC00513.JPG
Ok, here's what a good friend can do with a chinese wall wart (those 9v to 12v w/ polarity selector)
It's a TS808 I built for him as birthday gift and now is working again, but I can not forget that I lost a nice sounding 4558 for failing to put a simple diode, so I need to learn more.
I saw the 1N400x's are the most common diodes for polarity protection in so many stompboxes , so I assume to be the most appropriate, but what about the configuration: is better to connect the diode from the 9v line to ground, as in the Rat, or added in series in the 9v input, as in the Blues Breaker (just to give a couple of examples) or is exactly the same? What about 18v or higer voltages? Same 1N400x's or need something different?

Thanks to all who are interested in this subject and want to help :wink:
The 1N400X series are chosen because they're rated at 1 Amp, and can therefore carry quite heavy "wrong way" current.

I prefer a series diode - the effect just does nothing with a reversed supply - but there is a voltage drop. If you avoid the use of a conventional silicon diode, and use a Schottky type instead, you can keep the voltage drop down to 0.2V. With a modern Alkaline battery, you'll still have 9 Volts! I mostly use NiMH rechargeable batteries in my effects, and can just put in enough cells for whatever voltage I want, though I tend to design for 9 Volts so that users can use ordinary Alkaline batteries, in a pinch.

Remember - most effects are designed to work from batteries, so their current draw will be very small (just a few milliamps), so almost any Schottky diode will do. However, for compatibility with all circuits, it's a good idea to choose one capable of (say) 1 Amp.
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Post by Duckman »

However, for compatibility with all circuits, it's a good idea to choose one capable of (say) 1 Amp.
Something like that, I guess... any preference?
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Post by Hides-His-Eyes »

I tend to use the parallel 1n4001 in the opposite direction- is the main disadvantage to that the fact that if someone doesreverse it you lose the diode and have to put a new one in?
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Post by IvIark »

I've found that classic shorting diode protection going between +9V and ground in many non effect circuits often seems to be used in conjunction with a fuse, which I've never seen in an effect pedal. The series diode acting as a blocker seems like a much better option to me than putting in a component that you expect will blow.
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Post by Duckman »

But if you have the chance to use a diode in series is not going to blown and has a lower voltage drop as the Schottky, why are most popular silicon diodes? Schottky Diodes cost the same... :scratch:
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Post by Duckman »

Duckman wrote:
The MOSFET is indeed for polarity protection, works like a charm. D1 is a 15V zener diode, it shuts off Q4 when the user plugs in the wrong voltage power supply.
Nice trick! Seems like the MOSFET does the work, but without burning any component, just disables 9v, uh?
Perhaps the secret of using a MOSFET is that it does not cause voltage drop? :hmmm:
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Post by mictester »

Duckman wrote:
Duckman wrote:
The MOSFET is indeed for polarity protection, works like a charm. D1 is a 15V zener diode, it shuts off Q4 when the user plugs in the wrong voltage power supply.
Nice trick! Seems like the MOSFET does the work, but without burning any component, just disables 9v, uh?
Perhaps the secret of using a MOSFET is that it does not cause voltage drop? :hmmm:
You can get a very low drop with an FET, but a MOSFET wouldn't be my first choice. I sometimes use the HexFETs from International Rectifier. These have very low RdsOn.
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Post by Duckman »

So what's the advantage of the MOSFET used in the Barbershop? Why not to use a simple Schottky instead of the MOSFET-Zenner-resistor combination?
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Post by Hides-His-Eyes »

Duckman wrote:But if you have the chance to use a diode in series is not going to blown and has a lower voltage drop as the Schottky, why are most popular silicon diodes? Schottky Diodes cost the same... :scratch:
Schottky diodes cost 20p each whereas 1n4001s cost about 2p each, at least from one of my main smaller suppliers.

Even on tayda schottky are 5c each compared to 1c.
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Post by Duckman »

:lol: Wow, such a diiference!!
I just check it at Small Bear! :wink:
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Post by mictester »

Hides-His-Eyes wrote:
Duckman wrote:But if you have the chance to use a diode in series is not going to blown and has a lower voltage drop as the Schottky, why are most popular silicon diodes? Schottky Diodes cost the same... :scratch:
Schottky diodes cost 20p each whereas 1n4001s cost about 2p each, at least from one of my main smaller suppliers.

Even on tayda schottky are 5c each compared to 1c.
Exactly! You have to remember that commercial effects manufacturers always use the cheapest components available. That's why the "legendary" 4558 was used in the Tubescreamer - not because it was best for the job, but because they cost almost nothing when bought in big quantities!
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Post by RnFR »

Duckman wrote:So what's the advantage of the MOSFET used in the Barbershop? Why not to use a simple Schottky instead of the MOSFET-Zenner-resistor combination?
here's an explanation of the MOSFET protection. nothing blows up, and very little (.15V) series voltage loss with the BS250P. best way i've found to do it yet. of course i don't have a clue about HexFETs. :wink: (hint, hint.)

http://www.geofex.com/article_folders/m ... switch.htm

there is also this way to do polarity protection without having to use expensive MOSFETs. it takes a few more components, but should be easier to get a hold of the parts for the average DIYer.

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

now if you can deal with the .3V drop, just use a series schottky or Ge diode. many people will remark that there will be little difference in the sound of a distortion circuit anyways.

if you haven't yet, i would read everything that GEOFEX and AMZ have to offer. they are really excellent resources.
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Post by Duckman »

RnFR wrote:if you haven't yet, i would read everything that GEOFEX and AMZ have to offer. they are really excellent resources.
:oops: I usually do, but not on this occasion. Thanks for your comments :wink:
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Post by RnFR »

no biggie. i'm still finding things on RGs page. there is all kinds of stuff hidden all over that site. :D
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Post by Duckman »

:lol: Yes! It's like a bottomless pit. Should give the prize to the site more useful-for-other-but-less-recognized-by-them
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Post by Hides-His-Eyes »

I tried a pedal using a 9v1 zener as protection today and it was no harder than my old hopeless way except for a 220R series resistor where the power joined the board. So I think zener is the future for me? Or is there a special reason to use schottky instead? I mean zener offers overvoltage protection as well as reverse protection right?
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Post by mcaviel »

as i can see from your opinions, there is no love for the 1400x parallel way?
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