Reverse polarity protection circuit. Help with component values?

[frascati]

EE is not my forte. Although this little circuit is so simple that I could get myself 90 percent there, it performs such an important function that I would like to get it more like 99 percent correct.

I want to add the circuit to this board
http://www.ebay.com/itm/Finished-board-New-TPA3123-2-1-digital-power-amplifier-board-subwoofer-output-/251210310744?

I already fried one board. Another is on order and I'm "bench testing" it with a number of SMPS ranging from 15vdc@3.0amps to 20vdc@5.0amps.

Below is the source of the protection circuit I want to add.
Is it possible to calculate the correct value/capacity for diode/resistor/pmos fet from the description I've given of the power sources and amplifier used?

P-Channel FET

For the ultimate in low voltage drop and high current capability, replacing the PNP transistor with a P-channel MOSFET as shown in this circuit, can’t be beat. Please note that the FET is actually installed in the reverse orientation as it would normally – the drain and source are reversed. This orientation is necessary so that the slight leakage current through the FET’s intrinsic body diode will bias the FET on when the polarity is correct and block current when reversed, thus shutting off the FET. Here is a real nice video tutorial of how the magic works.

If the supply voltage is less than the FETs maximum gate to source voltage (Vgs), you only need the FET, without the diode or resistor. Just connect the gate directly to ground. I have found that most smaller FETs maximum Vgs is 12 volts or less, which can be a problem for 12 volt (or higher) supplies. If after checking your FET’s spec sheet, you find that Vcc could exceed the maximum Vgs, then you must drop the voltage between the gate and the source.

The circuit shown does exactly that by a very clever means. By inserting a zener diode with a voltage less than the maximum Vgs, it limits the voltage to a safe level between the gate and the source. You will need to calculate the resistor value so that it will provide enough current to properly bias the zener diode chosen. The zener diode’s spec sheet will provide the minimum current required to achieve the zener breakdown voltage, and you can then calculate your resistor value from that.

 

[frascati]

Guess that's a "no" then?

What if I take a less permanent approach? I'm really only using this for the testing phase, it won't be part of the final project since it will be hard plumbed to an internal xformer.

So would this temporary circuit protection circuit work ok?

The concern about diode overheating at 5.0 amps and voltage wasted on the diode wouldn't matter since this module would only be temporary.

Just a little board between the amplifier and the power source with two terminals in and two terminals out. Will this work to protect against temporary reverse polarity? Would the LED light up to indicate the wrong condition?

 

[dlgoff]

Reverse polarity protection ...

Just curious why you don't just use one of these?

 

[frascati]

Just curious why you don't just use one of these?

It adds one more possibility for error between the power source and the amplifier. I mentioned I'm bench testing. A few amplifier modules and half a dozen various SMPS.

 

[Windadct]

Why not just a full wave diode bridge? 1.4V Drop - but pretty much fool proof.

 

[sophiecentaur]

A reversed power diode and a fuse would probably suit you. (Or lots of red and black paint on all your connections and the right coloured wires. lol)

 

[dlgoff]

(Or lots of red and black paint on all your connections and the right coloured wires. lol)

You think the OP would reconsider polarized connectors if they were red and black?

 

[frascati]

Screw it. This turned out to work perfectly as a temporary "bench-testing" failsafe on a separate little board. Correct polarity... all's good. Reverse polarity... led warning and nothing touches the board (amplifier).

Once I get everything sorted out and tested on the bench and have decided on connections, power supplies, physical layouts, etc... then the board is no longer necessary and I just hardwire. Set the little reverse polarity protection module aside for the next project.

Thanks for all the, ahem, help here.

 

[davenn]

You think the OP would reconsider polarized connectors if they were red and black?

ahhhh the awesome Anderson Powepole connectors. They are used extensively in my radio work and for general use around the workshop
"Best thing since sliced bread" ;)

Dave

 

[sophiecentaur]

Thanks for all the, ahem, help here.

Caveat emptor.
What exactly were you after, in any case?

 

[frascati]

"quia si nunquam petere"
(if you must ask you may never know)

Sophie, do you honestly misapprehend what I've been "after"? Have you, or anyone else responding to this thread taken a moment to Google "reverse polarity protection" to see "what I'm after"? It's not that obscure or esoteric. DC voltage plus and minus into an amplifier board. The board has no protection against reversed polarity input whatsoever. Some boards incorporate this protection via discrete circuitry or within the pre-opamp. The boards I'm testing have neither. One little "oops" and it's bricked.

I merely want to eliminate the possibility, via the implementation of a single 1.50 cent IC, of encountering one of those "oops" moments while bench testing various power supplies and destroying a board that took two and a half weeks to get here from China.

What don't you understand Sophie? Have you bothered to absorb the whole thread? How could you have misapprehended my need?

Bench testing, over the course of time, wide varieties of power supplies and boards. It is ridiculously impractical as seems to have been suggested here), to utilize one polarized type of inline plug set for all circumstances.

Aw hell. Never mind. We're getting nowhere here really.

End of thread.

 

[sophiecentaur]

What I don't understand is how you can get upset that your question didn't receive the rapt attention you obviously expected. You seemed to be complaining that no one happened to be as interested in it as you were or that no one took it very seriously.
If the safety of your boards is that important to you then write yourself a check list to follow every time you connect them up.
Is a simple protection diode not sufficient for you? Just feed them using a bridge rectifier on every power rail.
What do all the thousands of other circuit builders do?

 

[dlgoff]

Have you, or anyone else responding to this thread taken a moment to Google "reverse polarity protection" to see "what I'm after"?

I brought up polarized connectors in a question thinking that maybe you hadn't considered them; as lots of designers use them in all sorts of equipment. However I'll bite. Here's the first hit from your suggested Google "reverse polarity protection" search.

http://www.edn.com/design/analog/4368527/Simple-reverse-polarity-protection-circuit-has-no-voltage-drop


And the first two sentences;

Common methods of reverse-voltage protection employ diodes to prevent damage to a circuit. In one approach, a series diode allows current to flow only if the correct polarity is applied ...

Isn't that what sophiecentaur suggested in post #6?

 

[frascati]

Look folks, before you get your panties in a knot, know I consider myself a b- level electrical engineer. I post to a forum, a forum called "Physics forums" with the expectation of fielding advice from something more than b minus level enthusiasts. Maybe I'm just expecting too much. Series diodes and polarized plugs are high school level stuff and I really must apologize if I misapprehended the level of expertise here.

I was well beyond such suggestions before posting. I wanted advice on implementing a Pmos fet or dedicated reverse polarity IC protection device. I was pretty explicit about that. With all respect, and with apologies if I appear to be insulting anyone by suggesting so, I was hoping for more than the color-coded, polki-yoked, approach. There are many very good reasons for wanting a more integrated and fail-safed approach, or a company like Fairchild Semiconductor, a company that is arguably smarter than the combined lot of us here, would not have decided to invest R&D and tooling into manufacturing such a dedicated IC.
http://www.fairchildsemi.com/pf/FR/FR014H5JZ.html

So they're discussing the need for this chip based on consumer response and field/failure data and you're sitting at the conference table and raise your hand and say "what about just using a damn diode? Or a color coded plugs?"

And the tumbleweeds roll slowly across the conference table in utter silence from the gallery as all eyes turn toward the most likely to see a layoff in the next crunch.

Why don't I ever learn. They end up in forums like this with breakfast coffee and a danish, sitting in their sweatpants, still trying to "help".

 

[sophiecentaur]

Look folks, before you get your panties in a knot, know I consider myself a b- level electrical engineer. I post to a forum, a forum called "Physics forums" with the expectation of fielding advice from something more than b minus level enthusiasts. Maybe I'm just expecting too much. Series diodes and polarized plugs are high school level stuff and I really must apologize if I misapprehended the level of expertise here.

I was well beyond such suggestions before posting. I wanted advice on implementing a Pmos fet or dedicated reverse polarity IC protection device. I was pretty explicit about that. With all respect, and with apologies if I appear to be insulting anyone by suggesting so, I was hoping for more than the color-coded, polki-yoked, approach. There are many very good reasons for wanting a more integrated and fail-safed approach, or a company like Fairchild Semiconductor, a company that is arguably smarter than the combined lot of us here, would not have decided to invest R&D and tooling into manufacturing such a dedicated IC.
http://www.fairchildsemi.com/pf/FR/FR014H5JZ.html

So they're discussing the need for this chip based on consumer response and field/failure data and you're sitting at the conference table and raise your hand and say "what about just using a damn diode? Or a color coded plugs?"

And the tumbleweeds roll slowly across the conference table in utter silence from the gallery as all eyes turn toward the most likely to see a layoff in the next crunch.

Why don't I ever learn. They end up in forums like this with breakfast coffee and a danish, sitting in their sweatpants, still trying to "help".

That's just offensive and quite uncalled for.
You get out what you put into a forum.You put nothing in (not even civility) and you got nothing out.

 

[jim hardy]

Well, maybe we didn't answer your original question.

which was:

Is it possible to calculate the correct value/capacity for diode/resistor/pmos fet from the description I've given of the power sources and amplifier used?

I saw your post but honestly I thought it was one of those asking everything be done for you.
And it seemed overkill for simple reverse polarity protection.
So I didn't reply.

The smart-a** reply would have been: "yes".

I think just a few more words in your request for help would have averted all this unpleasantry.
You asked the readers to go back and parse out for themselves what were your several questions.
And to guess why you want to introduce such sophistication into such a simple task.
Then you blast them for suggesting simpler alternatives.

.....................

The notes below the diagram in your first post completely describe how and why the circuit works.
I don't think there is anything about circuit values unanswered.

If you wanted help with picking a Mosfet, which is not calculating values,
here's IR's selector guide
http://www.irf.com/product-info/hexfet/

clicking "P channel" takes me here
https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841671

clicking on IRF9520N , the only one with TO220 because that's a handy package for breadboarding, takes me here
http://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=IRF9520N

and the top one is only one still available.
Clicking its "datasheet" link takes me here
http://www.irf.com/product-info/datasheets/data/irf9520npbf.pdf

and we see that it has adequate voltage and current capability for the supplies you mentioned.
But its Vgs is only twenty volts which your supply can exceed.
So you'll need to include the zener and resistor as described.

..................

"A question well phrased is half answered"
and it wasn't clear just what help you wanted.

This is generally a very civil forum. When I fail to get something I am after, I ask myself "what should I have done different ? "

I hoped typing all this would make me feel better, but it hasn't.
What should I have done different?

Just typed "Yes".
Now I feel better.

So here's an appnote with advice on using mosfets.
http://www.irf.com/technical-info/appnotes/an-936.pdf

regards, and lighten up

old jim

 

[sophiecentaur]

Well said Jim.
You should work for the UN!

 

[jim hardy]

Thanks sophie.