Polarity Protection for Effects

[Ashura]

If you have found it yet, there are a number of articles on powering pedals, including reverse polarity protection, at geofex.com.

Hi R.G..
Looking for an intelligent way to protect the circuits of pedals, I've been reading the article "Advanced Power Switching and Polarity Protection for Effects" ( http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm ). I imagine this article is own your. If I'm wrong, correct me please.

Thinking that the article mentioned is yours, I would ask that you answer me a question, since I am not a great electronics expert and I am still learning about it.

In this article it is suggested to use the mosfet BS250 which is no longer produced. In substitution, it is suggested to use the SO223 SMD, among others. It happens that here in Brazil the suggested components are not easily found. One that I found that seems to me to be comparable in something with the SO223 SMD is the A1shb ( Si2301DS https://www.vishay.com/docs/70627/70627.pdf ).

My question is: what features for the P channel mosfet should I consider as important to build this circuit of the above article?
Reading the article, it seemed to me that I should give attention of Drain-Source ON Resistance (Rds on). It seems to me that the smaller, the better. But, would only be it? What other important features does someone inexperienced in electronics like I should observe in a mosfet candidate to replace the BS250 or SM22 SO223?

In advance, thank you for your attention.

[POTL]

The mosfets mentioned in the article should be available in stores in 2pak / dpak irfr9024 cases

[Ashura]

This message that introduces this subject here in this topic, comes from a replica that I made to a message written by R.G. (https://www.diystompboxes.com/smfforum/index.php?topic=120730.msg1133581#msg1133581) in a topic I created about the Ibanez Tube Screamer.

My replica has been moved from the Ibanez Tube Screamer topic here. So, this message that introduces this subject here in this topic may seem a little pointless.
Thinking about clarifying something, I am writing this message you read now.

As the topic suggests, I have a question about reverse polarity protection on guitar pedals. My question is what would be the features for the P channel mosfet that I should consider as important in building the article protection" circuit mentioned above. What other important features someone inexperienced in electronics like I should observe in a mosfet candidate to replace the BS250 or the SM22 SO223.

Originally, as you can see, these doubts were directed to R.G., but I believe that anyone who feels interested in the subject can comment here. So, feel (averyone) encouraged to comment. Maybe R.G. may come up and give us some hint against the central doubt of this topic. That would be very good!

[GGBB]

Have another read of the article - specifically the section "Update 8 October 2005." You don't want BS250 - you want BSP250 or one of the suggested alternatives. The "important feature" is rdson.

[bluebunny]

FYI, the IRFD9024 alternative still appears to be available (Mouser, Rapid, ...).

[R.G.]

The important features are
1) enough Vds voltage rating to actually stop any projected reverse voltage; cheap and easy P-channels go to 30V or more, so this one is easy
2) enough current rating to supply the projected current in the circuit; likewise, even little P-MOSFETs usually have 100ma or more, so this is easy
3) low Rdson, as noted; you don't want this to introduce a lot of DC voltage drop when it's working normally; at the low currents of most pedals, a few ohms is fine, and is also easy to find in an inexpensive MOSFET
4) threshold voltage (Vth) less than the normal power supply voltage, even with low battery voltage; most small MOSFETs have Vth of 0.5V to 3V, so this is easy, too; but check to be sure
5) For extra points, the P-MOSFET would have internal bidirection zener gate protection. This makes it simpler to use as you don't have to worry as much about protecting the gate.

Most small-package P-MOSFETs will work, but these are the things to consider. At the time I wrote that article, P-channel MOSFETs in TO-92 packages at reasonable prices were rare. Today, over a decade later, we have a veritable wonderland of easy, cheap choices for devices.

[POTL]

https://www.infineon.com/dgdl/irfr9024npbf.pdf?fileId=5546d462533600a401535635f91d2132
https://www.digikey.com/product-detail/en/infineon-technologies/IRFR9024NTRPBF/IRFR9024NPBFCT-ND/812529
what do you say about them?

[R.G.]

1) enough Vds voltage rating to actually stop any projected reverse voltage; cheap and easy P-channels go to 30V or more, so this one is easy

Vds = 55V or more; just fine

2) enough current rating to supply the projected current in the circuit; likewise, even little P-MOSFETs usually have 100ma or more, so this is easy

Idmax = 11A (!!) plenty, but wow, that's a big sucker!

3) low Rdson, as noted; you don't want this to introduce a lot of DC voltage drop when it's working normally; at the low currents of most pedals, a few ohms is fine, and is also easy to find in an inexpensive MOSFET

Rdson = 0.175; that'll do fine, but it's probably two orders of magnitude smaller than it really has to be

4) threshold voltage (Vth) less than the normal power supply voltage, even with low battery voltage; most small MOSFETs have Vth of 0.5V to 3V, so this is easy, too; but check to be sure

Vth = min 2, max 4; that's OK.

5) For extra points, the P-MOSFET would have internal bidirection zener gate protection. This makes it simpler to use as you don't have to worry as much about protecting the gate.

Missing this. Using it will require external gate protection parts like zeners, etc. Note that in this application, you're likely to need two zeners, one for positive and one for negative, not just one zener acting as a diode one way and a zener the other way.

All in all, it ought to work. It's also a largish overkill, but that's fine if you have the part or just want to overdo it. The cheapest SOT-23 P-MOSFET looks to be $0.24, TO-92 for $0.49 (although both are much less expensive in any kind of quantity. The 9024 runs for $0.86 in ones (which surprised me: dang, parts are good and cheap these days!).

Works, bigger and more expensive than it has to be.

[PRR]

BTW: If anybody clicks on "Quote from: R.G." in your message, it takes the reader to the context.

As R.G. seems to be saying-- it is very hard to find a MOSFET which is NOT "big enough" to power a pedal or defend against any likely bad voltage. Rgss is important to us, but FAR more important to the Big Users of MOSFETs (switch power supplies). I remember over 10 Ohms, and that was no big deal to a pedal load. Today near everything is under 1 Ohm.

Your Si2301DS seems to be fine. 1+ Amps is more than most pedals eat. Resistance <0.2 Ohms is less than 0.2V at full 1 Amp and that should not matter to any pedal. It will do this with 1.5V gate voltage, fine. 20V breakdown might be marginal if you have a lot of 24V power on your pedalboard; most folks don't. I do not see gate protection diodes; I don't know if I would spend a lot of time shopping for a protected part or just throw on a Zener.

[Ashura]

Have another read of the article - specifically the section "Update 8 October 2005." You don't want BS250 - you want BSP250 or one of the suggested alternatives. The "important feature" is rdson.

Yes, indeed. My intention was to refer to the BSP250, but I was wrong to write. Thanks for the correction.

FYI, the IRFD9024 alternative still appears to be available (Mouser, Rapid, ...).

I do not find this component here in Brazil. One of the reasons for my question to R.G. is precisely this: the difficulty of finding the components suggested by him.

https://www.infineon.com/dgdl/irfr9024npbf.pdf?fileId=5546d462533600a401535635f91d2132
https://www.digikey.com/product-detail/en/infineon-technologies/IRFR9024NTRPBF/IRFR9024NPBFCT-ND/812529
what do you say about them?

Also not found here in Brazil. Thank You for suggest.

The important features are
Most small-package P-MOSFETs will work, but these are the things to consider. At the time I wrote that article, P-channel MOSFETs in TO-92 packages at reasonable prices were rare. Today, over a decade later, we have a veritable wonderland of easy, cheap choices for devices.

Many thanks R.G.! That will be of great help! I'll keep you posted on what I do.
Yes, the fact that we have many options makes me a little confused.

Your Si2301DS seems to be fine. 1+ Amps is more than most pedals eat. Resistance <0.2 Ohms is less than 0.2V at full 1 Amp and that should not matter to any pedal. It will do this with 1.5V gate voltage, fine. 20V breakdown might be marginal if you have a lot of 24V power on your pedalboard; most folks don't. I do not see gate protection diodes; I don't know if I would spend a lot of time shopping for a protected part or just throw on a Zener.

Thank you for your opinion, PRR. It has value to me. 

[amptramp]

Designing for a 9 volt supply is something that belongs back in the 1960's.  If you use a 12 volt supply, an ordinary diode will provide the protection you need and still give you sufficient headroom.  I like the Geofex solution but a little more voltage will make it a lot simpler with just a series diode.

[merlinb]

Designing for a 9 volt supply is something that belongs back in the 1960's.  If you use a 12 volt supply...

My cell phone runs on 3.7V and yet it can reproduce analogue audio, including guitar...

[bool]

You can use a ordinary small-signal BJT as a mosfet gate-protection zener. Most b-e junctions (collector open; not connected) will "zener" at 6, 7, 8 or so volts reverse voltage, and most small-signal bjts will cost less than a regular zener diode - especially smt and in quantities.

[~arph]

How about just using a bridge rectifier with low drop diodes (eg. https://nl.rs-online.com/web/p/rectifier-diodes-schottky-diodes/1703478/). Then your pedal will work with whatever polarity you throw at it.

[diffeq]

How about just using a bridge rectifier with low drop diodes (eg. https://nl.rs-online.com/web/p/rectifier-diodes-schottky-diodes/1703478/). Then your pedal will work with whatever polarity you throw at it.

Nice solution... until you daisy chain a pedal like this with a "normal", 0V ground. Rectified "ground" will have an offset of 150mV above 0V, causing troubles (hum, noise and other malfunctioning).

[~arph]

Yes there might be an issue there, but how are AC powered pedals solving this then?

[idy]

Guess: AC powered pedals don't have a problem with this because you can't daisy chain them? Each has its own power supply... If you tried to hook several AC effects to one power supply you might?

[POTL]

1) enough Vds voltage rating to actually stop any projected reverse voltage; cheap and easy P-channels go to 30V or more, so this one is easy

Vds = 55V or more; just fine

2) enough current rating to supply the projected current in the circuit; likewise, even little P-MOSFETs usually have 100ma or more, so this is easy

Idmax = 11A (!!) plenty, but wow, that's a big sucker!

3) low Rdson, as noted; you don't want this to introduce a lot of DC voltage drop when it's working normally; at the low currents of most pedals, a few ohms is fine, and is also easy to find in an inexpensive MOSFET

Rdson = 0.175; that'll do fine, but it's probably two orders of magnitude smaller than it really has to be

4) threshold voltage (Vth) less than the normal power supply voltage, even with low battery voltage; most small MOSFETs have Vth of 0.5V to 3V, so this is easy, too; but check to be sure

Vth = min 2, max 4; that's OK.

5) For extra points, the P-MOSFET would have internal bidirection zener gate protection. This makes it simpler to use as you don't have to worry as much about protecting the gate.

Missing this. Using it will require external gate protection parts like zeners, etc. Note that in this application, you're likely to need two zeners, one for positive and one for negative, not just one zener acting as a diode one way and a zener the other way.

All in all, it ought to work. It's also a largish overkill, but that's fine if you have the part or just want to overdo it. The cheapest SOT-23 P-MOSFET looks to be $0.24, TO-92 for $0.49 (although both are much less expensive in any kind of quantity. The 9024 runs for $0.86 in ones (which surprised me: dang, parts are good and cheap these days!).

Works, bigger and more expensive than it has to be.

Good
What are the optimal parameters for the transistor, I have a large selection, I want to find something in SMD, if there is an opportunity to take the opportunity in the case less or less, I will do it.
What do you think about this?
http://www.aosmd.com/pdfs/datasheet/AO3401A.pdf

[POTL]

In fact, I am a big fan of modern builders and I like their modern approach and use of modern components.
If earlier small producers offered copies of old or popular effects, now they are developing modern and interesting solutions.
Here is the Walrus 385 board, most likely this component next to the voltage converter is a field effect transistor.
I'm also attracted by small diodes near the max1044 voltage converter, they are very compact, I'm wondering which parameters for schottky diodes are important so that you can use them when working in a voltage doubler.

[POTL]

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