About polarity protection (when things go wrong)

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Re: About polarity protection (when things go wrong)

Postby Hides-His-Eyes » 24 Jul 2010, 12:18

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


I used to use it, but I'd recommend you try out a different way next time- I even found extra benefits such as zener siodes are smaller...
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Re: About polarity protection (when things go wrong)

Postby Duckman » 24 Jul 2010, 22:49

Hides-His-Eyes wrote: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?


Sorry 'bout my noobness :oops: maybe looks like I'm triyng to win the TISQA ("That Incredible Stupid Question Award") but, I still don't get it how a zener can protect you against reverse polarity... What about voltage drop? 9v1? Sorry, I'm lost :(
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Re: About polarity protection (when things go wrong)

Postby Hides-His-Eyes » 26 Jul 2010, 11:22

Duckman wrote:
Hides-His-Eyes wrote: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?


Sorry 'bout my noobness :oops: maybe looks like I'm triyng to win the TISQA ("That Incredible Stupid Question Award") but, I still don't get it how a zener can protect you against reverse polarity... What about voltage drop? 9v1? Sorry, I'm lost :(


A zener diode is, in the forwards direction, not dissimilar to a 1n4001 or something- it'll conduct and provide the 'self destruct' kind of polarity protection, although i put a small resistor between it and the +ve supply to soak that up and hopefully save it in the case of reverse polarity.

The voltage drop is zero because you're using it in parallel, but the special thing about a zener diode is it'll pass anything greater than 9.1v (9v1 using the european labelling method) directly to ground.

I think. But electronics lessons where a long time ago, so if someone says I'm wrong, believe them.
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Re: About polarity protection (when things go wrong)

Postby earthtonesaudio » 27 Jul 2010, 18:45

Forward and reverse can get confusing when talking about zener diodes. When in doubt, use anode and cathode. A zener conducts like a normal Si diode when the potential is higher on the anode compared to the cathode. It conducts at its rated zener voltage when the polarity is reversed, but the V-versus-I response curve is much different (usually much more abrupt change in conduction).

As polarity protection, the zener's cathode would point toward the positive supply and the anode would go to ground. This means when you apply the proper polarity and the proper magnitude voltage, the zener does nothing at all. If the power supply exceeds the zener voltage, it starts conducting suddenly sharply and will conduct as much current as necessary to maintain that zener voltage across its terminals. ...Which means you need a current-limiting resistor or a fuse to keep things from releasing their magic smoke.

If you apply the wrong polarity voltage, the zener diode conducts in the other direction and is now doing the same thing as the "parallel" diode protection scheme. Again, a current limiting resistor or fuse is required (if you don't install one, the diode will assume the role of the fuse itself). This resistor can be the same one as above, but generally the zener voltage is larger than the conventional diode conduction voltage, so the current will be higher in the reverse direction.

In both cases, to protect the circuit the resistor must provide a lower impedance path than the effect circuitry, and the components should be placed such that the resistor is in series with the parallel combination of (zener+effect circuitry).
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Re: About polarity protection (when things go wrong)

Postby Duckman » 27 Jul 2010, 19:58

Hides-His-Eyes wrote:A zener diode is, in the forwards direction, not dissimilar to a 1n4001 or something- it'll conduct and provide the 'self destruct' kind of polarity protection, although i put a small resistor between it and the +ve supply to soak that up and hopefully save it in the case of reverse polarity.

The voltage drop is zero because you're using it in parallel, but the special thing about a zener diode is it'll pass anything greater than 9.1v (9v1 using the european labelling method) directly to ground.

I think. But electronics lessons where a long time ago, so if someone says I'm wrong, believe them.


earthtonesaudio wrote:Forward and reverse can get confusing when talking about zener diodes. When in doubt, use anode and cathode. A zener conducts like a normal Si diode when the potential is higher on the anode compared to the cathode. It conducts at its rated zener voltage when the polarity is reversed, but the V-versus-I response curve is much different (usually much more abrupt change in conduction).

As polarity protection, the zener's cathode would point toward the positive supply and the anode would go to ground. This means when you apply the proper polarity and the proper magnitude voltage, the zener does nothing at all. If the power supply exceeds the zener voltage, it starts conducting suddenly sharply and will conduct as much current as necessary to maintain that zener voltage across its terminals. ...Which means you need a current-limiting resistor or a fuse to keep things from releasing their magic smoke.

If you apply the wrong polarity voltage, the zener diode conducts in the other direction and is now doing the same thing as the "parallel" diode protection scheme. Again, a current limiting resistor or fuse is required (if you don't install one, the diode will assume the role of the fuse itself). This resistor can be the same one as above, but generally the zener voltage is larger than the conventional diode conduction voltage, so the current will be higher in the reverse direction.

In both cases, to protect the circuit the resistor must provide a lower impedance path than the effect circuitry, and the components should be placed such that the resistor is in series with the parallel combination of (zener+effect circuitry).


First of all, thanks guys for helping me understand zener protection :roll:

Just a few questions to finish the matter in my head:

1) What about voltage drop?... No voltage drop at all or just a little?
2) 220R can survive wrong polarity?

Thanks!
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Re: About polarity protection (when things go wrong)

Postby mictester » 27 Jul 2010, 20:12

Duckman wrote:
Just a few questions to finish the matter in my head:

1) What about voltage drop?... No voltage drop at all or just a little?
2) 220R can survive wrong polarity?

Thanks!


220 ohms is probably a bit high - 100 ohms is probably more realistic to keep the voltage drop down. In the event of reversed polarity, the resistor will usually vaporise!
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Re: About polarity protection (when things go wrong)

Postby Hides-His-Eyes » 28 Jul 2010, 11:19

call it 10v backwards: 10V^2 / 100R = 1 Watt!

So if you wanted the pedal to survive a serious polarity reversal, you'd need a 1 watt resistor (or perhaps 2 parallel .6W metal film 220Rs?)
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Re: About polarity protection (when things go wrong)

Postby PmCimini » 21 Jan 2011, 01:07

Instead of making a new topic, I just wanted to confirm if this would be correct (see attached pic).

Would it protect against both reverse polarity and larger voltages?

Thank you :horsey:
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Re: About polarity protection (when things go wrong)

Postby Duckman » 21 Jan 2011, 02:11

PmCimini wrote:Instead of making a new topic, I just wanted to confirm if this would be correct (see attached pic).

Would it protect against both reverse polarity and larger voltages?

Thank you :horsey:


Just to reverse polarity. A larger voltage but w/correct polarity will pass direct to your circuit.
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Re: About polarity protection (when things go wrong)

Postby Hides-His-Eyes » 21 Jan 2011, 10:08

Zener...
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Re: About polarity protection (when things go wrong)

Postby mictester » 21 Jan 2011, 12:09

PmCimini wrote:Instead of making a new topic, I just wanted to confirm if this would be correct (see attached pic).

Would it protect against both reverse polarity and larger voltages?

Thank you :horsey:


No. The 100R resistor should come from the +ve battery terminal (or power socket). The zener then connects across the supply terminals of your board. Anything more than 9V1 is dissipated as heat in the 100R resistor and the zener, because the diode will start to conduct. If you connect the supply "backwards", all the current will flow through the 100R resistor. I usually use a 1/4 Watt resistor there, which will vapourise if the supply is reversed. This will release smoke, and will smell bad. The repair is just to replace the resistor, but it serves as a useful lesson to the owner of the pedal to be more careful next time!
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Re: About polarity protection (when things go wrong)

Postby Hides-His-Eyes » 21 Jan 2011, 14:05

If you want it to survive reverse polarity collection you could use 5 470R resistors in parallel, or find a 1W resistor (£3 for ten on ebay)
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