# Is it possible to reverse the polarity of DC using diodes?

From my humble understanding of a bridge rectifier, it "flips over" the negative cycles of AC, turning it into DC. Is it possible to modify this circuit so it can be used to reverse the polarity of DC as an alternative to using switches or transistors. I am trying to work it out, but can't grasp how to exploit the bridge rectifier's "flipping" for this purpose.

• What exactly do you mean with "reversing polarity"? Swapping two wires requires zero diodes... – Vladimir Cravero Feb 13 at 20:12
• Diode doesn’t flip over the negative cycles, it is just a peak detector – MarkU Feb 14 at 3:12
• if you want to flip DC back to AC you need a "H bridge" which uses transistors instead of diodes, and needs input signals to tell it when to flip. – Jasen Feb 14 at 5:23

No, the bridge doesn't turn around the current, it only steers the positive to one side and the negative to the other side. If it isn't already going in one direction it can't turn it the other way.

In fact if you apply a DC voltage… a DC voltage comes out (less the diode drops). Sorry but diodes can't do magic

• Huh? Really confused. – user148298 Feb 13 at 19:53
• Turning around isn't the same as reversing directions? – user148298 Feb 13 at 19:54
• A diode is quite simple in principle: it the current want to pass in the right direction, it let it pass; otherwise it blocks. If you look on wikipedia under "diode bridge" there is an animation showing how exactly the bridge works and how the current flow – Lorenzo Marcantonio Feb 13 at 19:56
• A rectifier only works because the current is going where we want it to, albeit half the time. The other half its in the opposite direction and that's what we use diodes for - blocking this reverse direction of current. So the final result is only in 1 direction, then we use capacitors to smooth this out etc... – QuickishFM Feb 13 at 21:39

The diode doesn't do any 'flipping', the AC voltage coming in is already 'flipping'.

All the diodes do is steer the appropriate polarity to the relevant terminal.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Diodes can only conduct in the direction of the arrows. (b) and (c) are the equivalent circuits with the reverse biased diodes removed as they have no contribution to the circuit in that state of AC polarity.

• (a) shows the full-wave rectifier connected to the AC supply. Current flows through R1 from top to bottom.
• (b) shows what happens when the upper input is positive. D5 is "forward biased" and current flows through R1 top to bottom and returns to the supply through D8.
• (c) shows what happens when the lower input is positive. D10 is forward biased so current flows through R3 from top to bottom and returns to the supply through D11.

Is it possible to reverse the polarity of DC using diodes?

You don't need any more diodes. Just swap the wires on the output of the bridge rectifier and current will go the opposite direction through the resistor.

Figure 2. Diodes are electrical non-return valves. Image source: What is an LED.

The diode check-valve analogy.

If you look at the check-valve in the figure above, it should be clear that the spring normally keeps the ball in position and prevents back-flow. When “forward-biased” the ball shut-off can be moved against the spring but it will take some initial pressure to move the ball. This results in a pressure drop across the valve: the pressure downstream will be less than the inlet pressure.

Diodes are similar in that they only conduct in one direction and you'll see a 0.7 V drop across the diodes.

It isn't possible to "reverse the polarity", but you can make a protection circuit so no matter what polarity is on the terminals, the load always sees a positive voltage (load like a microprocessor, which would be very unhappy and burn up if the polarity was reversed). This comes at a cost of power and voltage drop in the diodes.

If someone were to swap the vin and the GND (there are some idiots that can't tell the difference between red and black no matter how many times you tell them), the load would still be the same.