Power Supply Filter Anatomy

[POTL]

No.     The 47R can burn out.    It will create an open circuit.

The best solution is to use 47R  2W resistor.   

2W resistor will not burn out.

Thanks Again!

[GibsonGM]

Reverse polarity:

9V / 47R = 191 mA

.191A  *  9V = 1.7W

Use 2W resistor for 47R, and it will not burn out.

You're welcome! 

[antonis]

2W resistor will not burn out.

Maybe it SHOULD burn out...
(a resistor is by far the cheapest fuse..)

[EBK]

Reverse polarity:

9V / 47R = 191 mA

.191A  *  9V = 1.7W

Use 2W resistor for 47R, and it will not burn out.

You're welcome! 

(9 - 0.7)²/47 = 1.5W
Same result, but different number. 

[GibsonGM]

Reverse polarity:

9V / 47R = 191 mA

.191A  *  9V = 1.7W

Use 2W resistor for 47R, and it will not burn out.

You're welcome! 

(9 - 0.7)²/47 = 1.5W
Same result, but different number. 

?  Accounting for diode's Vf?

"2 watt is REQUIRED!!!"   LOL

[EBK]

?  Accounting for diode's Vf?

yes

"2 watt is REQUIRED!!!"   LOL

I'd never buy a 2W resistor for this purpose personally.  Just use a series Schottky and be done with it.  I'd advise against using a resistor as a fuse too.  Sure, it would work to protect the circuit, but then you have to open up the pedal and do some soldering before you can rock and roll again (with the corrected polarity, of course). 

[duck_arse]

I'm with my old mate antonis on this one. use a low wattage 47R in series, with the reverse-biased diode following it. short time backwards supply shouldn't burn anything, long time backwards should burn the resistor open. that will stop any further damage - to power supply or to pedal internal. just the diode can burn holes in the board if left conducting backwards long enough, it's not hard to replace a single resistor.

or use the series schottky (and the series 47R and a bypass cap).

[GibsonGM]

I know it adds to the parts count...but why couldn't you just place a 1k resistor in series with the diode (as shown, diode is parallel)?  Then nothing at all burns or needs to be replaced, current is very limited if reversed, but neither the 1k nor diode are involved in normal circuit operation...no need to make the 47R higher rated either.    Just thinking (which gets me in trouble).

If you were making many units (commercial), the single extra R would add up, but for us, making 20 or 30 (or 80...) pedals, I don't see it is a big deal...

[EBK]

I know it adds to the parts count...but why couldn't you just place a 1k resistor in series with the diode (as shown, diode is parallel)?  Then nothing at all burns or needs to be replaced, current is very limited if reversed, but neither the 1k nor diode are involved in normal circuit operation...no need to make the 47R higher rated either.    Just thinking (which gets me in trouble).

If you were making many units (commercial), the single extra R would add up, but for us, making 20 or 30 (or 80...) pedals, I don't see it is a big deal...

Voltage across diode and 1k resistor combo would be around 8.6V....
You would have your reverse polarity problem back again.

It's ok to think like this, by the way.  Even an unsuccessful design results in useful data (and in community like this, no judgment).

[GibsonGM]

Yes, I suppose you're looking at a 1K and a 4, 5 meg resistor, essentially...good call - nothing is free    Tho I don't see a reverse polarity PROBLEM, per se...if polarity reversed, you'd have a conducting diode and 1k limiting resistor, if I'm not missing something?    I don't like the voltage drop; no!

I hope POTUL doesn't get confused by all this English chit chat!! 

[EBK]

Tho I don't see a reverse polarity PROBLEM, per se...if polarity reversed, you'd have a conducting diode and 1k limiting resistor, if I'm not missing something?   

That ~8.6V is across the rest of your circuit too. That's 8.6V of reverse polarity, rather than 0.7V of reverse polarity without the 1k resistor.  (For the time being, I'm ignoring the loading effect of the rest of the circuit)

[antonis]

Just thinking (which gets me in trouble).

Plz Sir, don't do it..!! 
(We can't stand the insight of Your Highness disturbance..) 

Now for the record:

Reverse polarity voltage will momentarily be across the circuit despite diode's V_fd and resistor's value (if any)..
(current is willing to go everywhere exists a convenient voltage difference..)

In case of single diode, it will be lowered to individual V_fd by extemelly loading PS out..
(current is now much less susceptible to go into the circuit..)

Placing a series resistor to diode we oppose to PS loading in a degree of our taste..
(yes, it will be reverse voltage placed across the circuit but it will not be enough taste dependable current to go through it.. - for 9V even MosFets pay no mind..)

Taking in mind PS maximum current capability, we have to decide which part we want to load enough to burn-out.. 
(or just use a p-channel MosFet in power path or even a n-channel one in ground return path - if we don't mind for a slightly floating circuit in the second case..)

[EBK]

Taking in mind PS maximum current capability, we have to decide which part we want to load enough to burn-out.. 

I'd say pick whatever part would look or sound the most impressive as it fails. 

[antonis]

Taking in mind PS maximum current capability, we have to decide which part we want to load enough to burn-out.. 

I'd say pick whatever part would look or sound the most impressive as it fails. 

If you don't want a failure prone situation DON'T connect your PS with reverse polarity..!!!   

[GibsonGM]

If you don't want a failure prone situation DON'T connect your PS with reverse polarity..!!!   

Personally, I only toss a diode, reversed, across my power terminals.  That's all.   I've never needed it (fingers crossed), but it is there just in case.  It might gain a couple of seconds of reverse connection...

Good discussion, however...I had failed to see the voltage divider action under reverse bias with the idea I presented a few posts back, for example, ha ha. 

[amptramp]

If you insist on using a shunt diode with a series resistance to limit the current, make the series resistance a light bulb.  The resistance when cold is about 1/4 of the hot resistance so you have only a small resistance in series under normal circumstances but in the event of a fault, the higher resistance provides added protection.  There is something in my other hobby, restoring antique radios, called a dim bulb tester.  It is an electrical outlet with a light bulb in series.  If the radio is OK, it works but at reduced line voltage.  If it has an internal short, the current is limited to what the light bulb can pass.

All you have to do is find the right rating for the bulb.

[GibsonGM]

If you insist on using a shunt diode with a series resistance to limit the current, make the series resistance a light bulb.  The resistance when cold is about 1/4 of the hot resistance so you have only a small resistance in series under normal circumstances but in the event of a fault, the higher resistance provides added protection.  There is something in my other hobby, restoring antique radios, called a dim bulb tester.  It is an electrical outlet with a light bulb in series.  If the radio is OK, it works but at reduced line voltage.  If it has an internal short, the current is limited to what the light bulb can pass.

All you have to do is find the right rating for the bulb.

Nice application of the 'current-limiting bulb cord', tramp.   They're a very nice thing to have for testing tube amps (and radios of course).

[antonis]

@Ron: You're absolutely right but EBK will come soon and yell at us that, despite lamp's hot/cold resistance, there will be a reverse voltage applied to circuit.. 
(but we like him and don't tell off..)

[EBK]

Nah, lamps aren't my thing (which means I won't yell at anyone using them ).  I was going to suggest a vibrating motor or piezo buzzer might make a good reverse polarity deterrent.  Perhaps even whatever fog stuff model railroad enthusiasts use for realistic-looking steam engines.  Maybe that is the way to address the problem.   

[ElectricDruid]

I like it. Pedal backwards? CHOO CHOO!! CHUFF CHUFF!! (Clouds of "smoke"/"steam")

T.