Reverse polarity protection

[DeusM]

Hi guys. I have this circuit which is power supply. I though that it was protected against reverse polarity but two exploded caps tell me otherwise. It works woks 12V 2A, so I think 1N4001 or 07 won't work. Also, I have some 1N5817 and 19, but the data sheet say it can only hold up to 1 to 10 mA. Can you guys help me?

[GGBB]

1N581x is 1A same as 1N400x.

[antonis]

It works woks 12V 2A,

You mean 12V/2A input..
('cause LM317 current rating is 1.5A maximum..)

D2 is reverse polarity protection from OUTPUT..!!
(like a  reverse connected charging battery..)

D3 helps C2 discharging in (the rare) case of V_IN<_VOUT..
(like shorting input supply..)

AS it is, there isn't any INPUT reverse polarity protection..

[imJonWain]

Is that reference actually -12V or is it GND?

The standard polarity protection diode setup is "designed" to conduct then fail before other parts.  There's also a much better but more complex mosfet based reverse polarity protection floating around on here you could try.   

[DeusM]

1N581x is 1A same as 1N400x.

How do you find out the maximun reverse current? I'm not good at reading datasheets and what it says about 1n5829 is "max reverse current (Ir) 2.0mA"

[GibsonGM]

Yes, max reverse current is 2.0mA, you are reading that correctly...

[ElectricDruid]

Separate issue, but what's the point of R1 and R2 in parallel? Sometimes that would be done to get a very specific value (although in series is much easier to work out!), but that seems a bit unlikely when there's a 2K variable pot underneath them. You could just tweak that instead, right?

[DeusM]

Yes, max reverse current is 2.0mA, you are reading that correctly...

Yes. And the input has 2A, so I need a diode with 2A of max. reverse current. Right? Or am I missing something?

Separate issue, but what's the point of R1 and R2 in parallel? Sometimes that would be done to get a very specific value (although in series is much easier to work out!), but that seems a bit unlikely when there's a 2K variable pot underneath them. You could just tweak that instead, right?

Yeah. I though the same. I just grabbed some circuit and tweaked it. I'ts not necessarily finished.

[imJonWain]

No, the reverse current doesn't really matter based on the second schematic you show.  That diode will only conduct nA in "reverse" until you exceed it's reverse breakdown voltage (20V). 

[PRR]

...I though that it was protected against reverse polarity but two exploded caps tell me otherwise.

It protects the *regulator* against a specific but odd event: short on the input while the output cap is charged.

It never was an all-purpose reverse protector.

You can use a series diode and suffer 0.2V-0.8V of voltage drop.

Or a shunt diode to blow-up a cheap part instead of an expensive part.

I doubt "diode reverse current" is a spec you need here.

[DeusM]

No, the reverse current doesn't really matter based on the second schematic you show.  That diode will only conduct nA in "reverse" until you exceed it's reverse breakdown voltage (20V).

I'm not sure I understand. Max reverse current is the current that will flow if the diode "backwards" Or is is the maximun current the diode can block?

[DeusM]

It works woks 12V 2A,

You mean 12V/2A input..
('cause LM317 current rating is 1.5A maximum..)

D2 is reverse polarity protection from OUTPUT..!!
(like a  reverse connected charging battery..)

D3 helps C2 discharging in (the rare) case of V_IN<_VOUT..
(like shorting input supply..)

...I though that it was protected against reverse polarity but two exploded caps tell me otherwise.

It protects the *regulator* against a specific but odd event: short on the input while the output cap is charged.

For What I read D2 is for protection from discharging capacitors in pedals that are connected to the power supply.
D3 is in the case of capacitance between C1 and ground.
Interesting stuff

[imJonWain]

No, the reverse current doesn't really matter based on the second schematic you show.  That diode will only conduct nA in "reverse" until you exceed it's reverse breakdown voltage (20V).

I'm not sure I understand. Max reverse current is the current that will flow if the diode "backwards" Or is is the maximun current the diode can block?

Max reverse current is how much current the diode can handle flowing "backwards" through it.  However a diode will only conduct significant current "backwards" when you exceed it's reverse breakdown voltage (Cathode is more positive then the Anode).  See the graph and link below.  Zener diodes exploit this quality.



https://www.electronics-tutorials.ws/diode/diode_3.html

[GGBB]

Separate issue, but what's the point of R1 and R2 in parallel? Sometimes that would be done to get a very specific value (although in series is much easier to work out!), but that seems a bit unlikely when there's a 2K variable pot underneath them. You could just tweak that instead, right?

The reference implementation in the datasheet calls for 240 ohms - which is 270 || 2200 - more readily available values - in order to achieve the required voltage drop of 1.25V between the output and adjust terminals. See page 11 figure 9 https://www.ti.com/lit/ds/symlink/lm317.pdf. The posted circuit is a straight copy of that with increased output capacitance.

[Rob Strand]

It protects the *regulator* against a specific but odd event: short on the input while the output cap is charged.

It never was an all-purpose reverse protector.

You can use a series diode and suffer 0.2V-0.8V of voltage drop.

Or a shunt diode to blow-up a cheap part instead of an expensive part.

I doubt "diode reverse current" is a spec you need here.

Yes the intent of that diode is *very* different to the true reverse polarity fix in GGBB's schematic.

[PRR]

....Max reverse current is how much current the diode can handle flowing "backwards" through it........

https://www.vishay.com/docs/88526/1n5820.pdf

Maximum average reverse current IR specifies that the reverse current will not be higher than these numbers at these voltages and temperatures.

The way it rises 10X from 25c to 100c strongly suggests it is a measured leakage, not a "how much it can handle" rating.

If you turn to Fig 4 you see it "typically" changes slowly with reverse voltage (not a breakdown phenomena) but more rapidly with temperature than implied by specs (however the leakage is typically much-much lower than spec) (however if it isn't you can't complain). There's a lot of CYA in this because leakage is unpredictable.

[merlinb]

I think 1N4001 or 07 won't work.

So buy a bigger diode? (3A)...

[Rob Strand]

Yes. And the input has 2A, so I need a diode with 2A of max. reverse current. Right? Or am I missing something?

No.   For GGBB's circuit, with a protection diode series diode, under normal conditions the diode is forward biased and the diode current is the same is the circuit current.     For a pedal that's going to be a small current, say less than 100mA.   The forward diode current need to be at least that big to handle the current.

When the circuit is under reverse polarity the diode protects the circuit by blocking the current.    The whole idea of a series protection diode is very little current flows in the fault condition of reverse polarity.

[duck_arse]

Hi guys. I have this circuit which is power supply. I though that it was protected against reverse polarity but two exploded caps tell me otherwise.

Is that reference actually -12V or is it GND?

we need an answer to this, still. are you putting rectified DC in, or AC from a transformer?

[Ice-9]

I don't know if it has been said in a post above but D2 is not there for reverse polarity protection of the circuit, D2 is there to protect the regulator from a higher voltage on the regulated side when the power is disconnected. When power is disconnected C3 470uf can hold the voltage higher on the output of the regulator than the voltage is at the input of the regulator which can damage the reg.