Why are diodes used in electrical protection? - OneStep Power

A quick review of diodes, what are they?

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A diode is an electrical component that acts as a one way valve. They can be made to conduct current in one direction or the other depending on the diode’s intended use. If they are not electrically and thermally rated for their intended operating environment, diodes can fail. Failure of a diode causes current to flow in an undesired direction, which could damage any components normally protected by the diode.

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Why are diodes used in circuit protection?

Diodes are used in circuit protection due to their ability to restrict electrical current to flow in only one direction. This trait is useful because some electrical components and devices will be damaged or malfunction if current flows in the wrong direction. For example, current problems can cause microprocessors to experience erroneous calculations or simply short out and fail. One of the reasons that radiation damages microprocessors because it induces stray currents across the microprocessor's contacts.

The diode can be used to not only protect from stray currents, but can be electrically routed to redirect the stray currents into useful current. This is known as shunting, which is dumping current down a very low resistance path. Rerouting the current ensures that the stray power is not simply wasted through grounding.

In systems where multiple power supplies are present, diodes play an extremely important role. The diode can be used to protect against transient overvoltages from the first or second power supply, depending on the system setup. The diode can also protect against transient overvoltage on the load side of the diode or even short circuit and/or overload on the power supply side. Additionally, if current supply to a diode protect load must be shut off, it can be done without taking other loads offline.

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Other uses of diodes

Diodes can be used to convert AC current to DC current. DC current flows like water from a hose, meaning it follows a linear path towards the end of the hose. Devices that function on DC circuits need protection against stray back-flow currents because they are most susceptible to damage from such currents. Other devices function on AC current, which flows back and forth like a zig-zag. Devices use AC current for many reasons, the primary being that it is more power dense, meaning that an AC current flows in and out of both the negative and positive contacts of any device in the system.

This current conversion is known as AC to DC rectification. Because AC flows like a zig-zag, diodes can be used to prevent the negative cycle from flowing down the circuit. Therefore, the top of the wave is allowed through and is electrically converted to the linear flow of DC current. It is also for this reason that diodes are only used in protecting DC circuitry - they only allow current to flow in that single direction and thus do not allow AC current to flow normally.

Sources:

https://en.wikipedia.org/wiki/Diode

If you use a diode to ground for polarity protection I would highly recommend replacing the positive wire coming from the DC jack with a ~100 ohm resistor.

Please note that this has a potential to get destroyed with application of reversed polarity power. You also add a LPF to the power supply (i.e., beyond what is typically formed by the source impedance and the filter caps). I'm suggesting the resistor come before the diode, specifically to limit current and protect the diode (and nearby PCB traces) from failure in a reverse polarity situation. Like you mentioned, it will also improve the filtering quality of C1.



I say suggesting and not recommending because I would rather just avoid the diode to ground method altogether. To the average pedal user with no soldering iron a shorted protection diode is just as broken as a defective opamp or transistor. Thanks for all the great feedback!

Is this Diode strictly for reversed polarity concerns or will it protect against other issues. i.e. using 18v instead of 9v. Does it affect the circuit at all? Not that I am advocating this approach but, if I knew I was always going to use the correct power supply is this diode really needed? Could I just jumper it? (knowing full well that if my buddy brings over his Nintendo wall wart I'll be plugging straight into the amp!)

The reason for my original question was mostly because I've seen other projects use 1N and I had this surplus of 02 and 03s I never use. I thought maybe I could put them to use as substitutes. I think I'll likely save them for something else and stick with the proper diode.

Apologies for all the questions. I did read up on the "crow bar" method and I think I get the difference now and why protection is better. Thanks for that info! Neither of these methods protect against overvoltage, only reverse polarity.

If you use a zener diode in the place of D1 above it will protect against overvoltage and reverse polarity simultaneously, but it still has the same shortcomings of a regular diode to ground.

You can jumper the diode when it is in series to bypass polarity protection and the circuit will work fine, but will have no protection against reverse polarity.

You can't jumper the diode to ground, but you can remove it (or omit it) completely. Again, the circuit will work fine but will have no protection against reverse polarity.

Neither of these methods protect against overvoltage, only reverse polarity.

If you use a zener diode in the place of D1 above it will protect against overvoltage and reverse polarity simultaneously, but it still has the same shortcomings of a regular diode to ground.

You can jumper the diode when it is in series to bypass polarity protection and the circuit will work fine, but will have no protection against reverse polarity.

You can't jumper the diode to ground, but you can remove it (or omit it) completely. Again, the circuit will work fine but will have no protection against reverse polarity.

Awesome thanks! That all makes sense and partly what I suspected. Thanks @PedalPCB and @benny_profane!!!