Bridge Rectifier DC Polarity Correction


Photo credit: User 'Guam' on Wikimedia Commons

Let's face it: The world is a very unstandardized place, especially when it comes to electronics. The world is split between 120V and 240V, 50Hz and 60Hz. We have different TV standards and everything. But here's a problem that annoys me a lot: it's that plug-in AC adapters with the typical barrel jacks use either center-positive (most common) or center negative (less common, but out there somewhere). Therefore, it bugs me when I can't power one of my projects or boards because the polarity of the adapter is wrong. It's even worse when you don't pay attention and end up frying the board.


Two different polarities

So what's the the solution to this? Well, you could use a singe diode to prevent accidental damage, but that only protects the circuit and you would still need to correct the polarity by flipping the jack or finding a different transformer. What I really wanted was something that would automatically compensate for the change in polarity and work no matter what.


Using a diode will protect the circuit, but you still need to fix the supply polarity.

The solution came to me when I was wondering why my AVR32 NGW100 board had a bridge rectifier chip on it next to the power barrel jack. The silkscreen layer clearly says it only works with DC, so what is the point of the rectifier? Usually they are used to convert AC into DC. I read later in the manual that the board is polarity insensitive. Could the rectifier be used to correct the the polarity?

Testing It

To find out, I decided to set up a simple test using an off-the-shelf bridge rectifier, a power supply, and a bi-color LED with current limiting resistor.


Rectifier and bi-polar LED

The two inputs to the bridge rectifier (which would normally be the "AC" input) are connected to a bench supply set to 6 volts to compensate for the voltage drop through the diodes. The rectifier's output is hooked to a bi-color LED. Because the LED is bi-color, it has two light diodes with opposite polarities so only one can be lit at a time depending on the polarity and therefore the direction the current flows. Basically, if the color of the LED stays the same, then the polarity hasn't changed.

In the pictures below, the DC power supply is attached through the red and yellow jumper cables. The LED is connected is via the green and black cables. Switching the polarity of the input (second picture), the polarity of the output is unchanged according to the LED. It works!!

 

Conclusion

The bridge rectifier polarity correction circuit is extremely useful. Not only does it add more flexibility for power supplies, but also passively corrects the circuit if you accidentally connect the power supply wrong. The only possible drawback is that the input voltage has to be a few volts higher than needed to overcome the voltage drop of the diodes.

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