Reverse Polarity Protection

POTL · Post by **POTL** » 07 Sep 2017, 20:48

Hi
In the world of stompboxes, there are many ways to protect the circuit from the wrong polarity
But the most common are circuits using diodes 1N4001-1N4007 or Schottky diodes (usually 1N5817).
I attach the schematics below

Advantages of the scheme with 4001 - no loss of voltage
Lack of 4001 - the circuit does not always save, the diode can fail and the circuit will burn, it can also burn the power source
The drawback of the circuit is from 5817 - it gives a voltage drop of 0.45 volts.
Are there any more disadvantages?
What are the advantages?
I noticed that both schemes are often found in stoppboxes, but the scheme with 4001 is considered unreliable.
Tell us what schemes you use and what their pros and cons are.

Hideki · Post by **Hideki** » 10 Sep 2017, 00:53

For simplicity, no voltage drop to speak of, and no shorting out the power supply, I usually go for a single P channel MOSFET in a SOT-23 package. See for more information.

Dirk_Hendrik · Post by **Dirk_Hendrik** » 10 Sep 2017, 08:02

Please explain why the "4001" scheme (I'd say, the scheme with a diode to ground as opposed to a series diode) is considered unreliable.

Other than that,
In the top circuit C4, R5, C5 and R6 do not have anything to do with reverse polarity protection.
In the bottom circuit anything left from D3 does not have anything to do with polarity protection.

alexradium · Post by **alexradium** » 10 Sep 2017, 08:12

diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.

Timpanogos Slim · Post by **Timpanogos Slim** » 17 Sep 2017, 07:03

alexradium wrote:diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.

You mean DC voltage input.

I think the general idea is that the resistor in series with the DC voltage before the diode in parallel should be small enough that it acts as a fuse. Or large enough that it's almost a circuit breaker.

The 5817 in series is more likely to work, i think, there's just the question of whether dropping a fraction of a volt is ok with you.

plush · Post by **plush** » 17 Sep 2017, 09:04

Timpanogos Slim wrote:
alexradium wrote:diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.
You mean DC voltage input.

It happens both with reversed DC and AC.
----

IMO the best and cheapest protection is a polyswitch (resettable fuse) in series at the input and then a 1n4001 or any other type of diode in parallel. Once the dc voltage is reversed (or AC is applied) the polyswitch rises it's internal resistance and takes all the pain from the diode (since his basic resistance is somewhat higher than the diode's).
"Same" polarity protection comes in most EHX pedals, but instead of a polyswitch they use a simple 10-100 Ohm resistor, that burns itself, breaking the connection and saving the circuit from the AC.

POTL · Post by **POTL** » 17 Sep 2017, 12:19

plush wrote:
Timpanogos Slim wrote:
alexradium wrote:diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.
You mean DC voltage input.

It happens both with reversed DC and AC.
----

IMO the best and cheapest protection is a polyswitch (resettable fuse) in series at the input and then a 1n4001 or any other type of diode in parallel. Once the dc voltage is reversed (or AC is applied) the polyswitch rises it's internal resistance and takes all the pain from the diode (since his basic resistance is somewhat higher than the diode's).
"Same" polarity protection comes in most EHX pedals, but instead of a polyswitch they use a simple 10-100 Ohm resistor, that burns itself, breaking the connection and saving the circuit from the AC.

[ Image ]

Hi
1) Can you share a link to specific fuse models? What parameters to look at?
2) What is the advantage over the 1N5817 - I measured the drop, it was 0.1-0.12V that is the same as the fuse

Timpanogos Slim · Post by **Timpanogos Slim** » 17 Sep 2017, 19:52

plush wrote:
Timpanogos Slim wrote:
alexradium wrote:diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.
You mean DC voltage input.

It happens both with reversed DC and AC.
----

IMO the best and cheapest protection is a polyswitch (resettable fuse) in series at the input and then a 1n4001 or any other type of diode in parallel. Once the dc voltage is reversed (or AC is applied) the polyswitch rises it's internal resistance and takes all the pain from the diode (since his basic resistance is somewhat higher than the diode's).
"Same" polarity protection comes in most EHX pedals, but instead of a polyswitch they use a simple 10-100 Ohm resistor, that burns itself, breaking the connection and saving the circuit from the AC.

[ Image ]

I think we're tripping over nomenclature -- there is no such thing as reversed AC since it's alternating current. I think you are referring to battery and external power.

Polyswitches and other PTC devices are good, but which one? Have you tested them to verify that they protect well enough?

A 5817 typically has a Vf of between a quarter and a third of a volt. The lowest trip current rating available for a PTC is 100ma and that comes with a nominal resistance of 11, 20, 50, or 55 ohms. since pedals typically consume very little power, perhaps one of these in the very lowest range will work well enough.

But i think the 5817 will also work well enough. *shrug*.

plush · Post by **plush** » 17 Sep 2017, 20:31

Timpanogos Slim wrote:
plush wrote:
Timpanogos Slim wrote:
alexradium wrote:diode in parallel with an AC voltage in input,already tried that,diode gets hot and burns out and then the circuit gets damaged.
You mean DC voltage input.

It happens both with reversed DC and AC.
----

IMO the best and cheapest protection is a polyswitch (resettable fuse) in series at the input and then a 1n4001 or any other type of diode in parallel. Once the dc voltage is reversed (or AC is applied) the polyswitch rises it's internal resistance and takes all the pain from the diode (since his basic resistance is somewhat higher than the diode's).
"Same" polarity protection comes in most EHX pedals, but instead of a polyswitch they use a simple 10-100 Ohm resistor, that burns itself, breaking the connection and saving the circuit from the AC.

[ Image ]
I think we're tripping over nomenclature -- there is no such thing as reversed AC since it's alternating current. I think you are referring to battery and external power.

Polyswitches and other PTC devices are good, but which one? Have you tested them to verify that they protect well enough?

A 5817 typically has a Vf of between a quarter and a third of a volt. The lowest trip current rating available for a PTC is 100ma and that comes with a nominal resistance of 11, 20, 50, or 55 ohms. since pedals typically consume very little power, perhaps one of these in the very lowest range will work well enough.

But i think the 5817 will also work well enough. *shrug*.

I'm talking about reversed DC and AC. Not reversed AC.

I have tested polyswitches, they do the job.
If you look into okayish power supply, T-rex Fuel Tank Chameleon for example, you'll find bunch of resettable fuses, one per each channel.

Timpanogos Slim · Post by **Timpanogos Slim** » 19 Sep 2017, 06:48

Can you share which exact polyswitches you used? Or some rough specs for them even.

plush · Post by **plush** » 19 Sep 2017, 11:52

Timpanogos Slim wrote:Can you share which exact polyswitches you used? Or some rough specs for them even.

Sorry, it won't work this way, since my applications differ quite a bit.

You'll have to invest some time and make your own research, read through numerous datasheets and find the one that suits your design the most. Especially, if you are plannyng to run another boutique pedal brand or something. Otherwise, this fuse thingy is quite an overkill for your home experiments.

I guess, you need to start with something that has
I(hold) around 100-200ma (or more, depending on your circuit),
I(trip)/I(max) should be determined by your power supply, (i guess I(max) of around 2-3 amps will fit most wall warts)
V(max) - around 30-50 volts

There is also a lot of of things that should be mentioned, that i can't focus on right now.