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PCB Design Guide for Reverse Polarity Protection

reverse polarity protectionWhen user safety depends on equipment functioning reliably -- as is the case with many medical devices and handheld mining equipment, for example -- electronic components must be trustworthy.

That’s why reverse polarity protection becomes critical for printed circuit boards (PCBs) in so many applications. Keep reading for a little PCB design guide for keeping your product protected.

Why Do You Need Reverse Polarity Protection?

Anything that is tied to DC (direct current) power is a candidate for protection from reverse polarity. Anything battery-powered were there is a risk of connecting a battery backwards, using a AC/DC wall power supply with a different polarity connector, or other potential mess-ups.

If you’re not the resident whiz on your staff, it helps to think about reverse polarity in terms of daily-use items like car batteries. Reversed poles occur when you connect the negative cable with the positive and vice versa. This can damage the battery and other electrical components.

Any battery-powered product you use while it’s plugged in will be energized through its full circuit and thus a potential source of electrical shock.

Reverse polarity can cause PCB damage and even PCB failure, yet the damage can be hard to see. If this happens to, say, a laptop computer, the results can be highly unfortunate but not fatal. If it happens to a life-sustaining medical device, it can mean death.

Reverse polarity protection does not apply to AC (alternating current) connections.

What Can You Do to Prevent Reverse Polarity?

The circuit has to be able to get through a reverse polarity voltage onslaught without taking damage. The fact that most equipment today is designed to prevent users from inserting a plug the wrong way doesn’t completely solve the issue. Damage to the PCB can occur during the board’s development and testing.

To protect PCBs from the effects of reverse polarity, you can opt for a couple of approaches:

  • Insert a diode
  • Insert a Schottky diode

Neither option is perfect, but both can minimize harm and give users peace of mind.

Adding a Protection Diode

Placing a reverse polarity protection diode in series with the electrical supply line provides a “shut-off” mechanism that will halt the voltage flow. It doesn’t fix the reverse polarity, but it does stop it from doing more harm.

Essentially, the diode separates what remains of the circuit from the reverse polarity. When the anode voltage is lower than the cathode voltage, the diode won’t carry current.

There are a few negatives to the reverse polarity diode approach, even though it’s effective and inexpensive. For example:

  • By dropping voltage, the diode can cause the device to stop before it should.
  • The diode consumes power and can shorten the life of batteries.

Adding a Schottky Diode

A Schottky diode presents neither of the problems associated with a reverse polarity prevention diode and is therefore the better solution. However, there is at least one shortcoming.

The Schottky diode allows much greater reverse current and voltage across the load. On the other hand, the reverse current is low enough that it shouldn’t be a big concern.

Are You Designing for Safety First?

Polarity protection diodes are quite effective, and they offer a very economical solution. But with a medical device, for instance, you don’t want premature stoppage. The consequences could be disastrous for the patient and for your company’s reputation.

For equipment as critical to life and safety as medical devices and mining equipment, it seems clear that a Schottky diode is the right choice. However, it’s also important to install the diode correctly, so you should consult with your electronics contract manufacturer before designing the PCB. Your joint goal should be to produce intrinsically safe equipment when the job calls for it.

Of course, the best diode protection strategy is to avoid creating reverse polarity in the first place. Diodes are for backup and should not be taken for granted as a perfect solution.

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Author: Matric