# Why not use diodes in parallel but opposite polarity to make a full wave rectifier

Discussion in 'General Electronics Chat' started by ElectricityStudent, Nov 2, 2011.

1. ### ElectricityStudent Thread Starter New Member

Nov 2, 2011
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Hi Guys,

Why not use diodes arranged in parallel but opposite polarity to make a full wave rectifier. This way in the positive alternation, one will resist and the other will allow, and vice versa for the negative alternation. Am i missing something here or am i on to something. I strongly feel like i am missing something because i do not see such a model being used to make a full wave rectifier; therefore your insights will be much appreciated

Electricity Student.

Jul 9, 2011
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3. ### ElectricityStudent Thread Starter New Member

Nov 2, 2011
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Wow this is awsome! this is my first time ever writing on a forum, and my first reply ever. Thanks for the prompt response!! i have checked out the site you gave and i liked it, however that is a different topology compared to what i have inquired about, although it seems to be working in a similar manner; however, further elucidation will be much appreciated. I have attached a drawing as per your recommendation.

PS: this is the secondary transformer circuit with the alternating current. My assumption for the circuit depicted in my drawing is that current in the cycles will take the path of least resistance, which leads me to believe that this circuit would work as long as AV < BV (i.e breakdown voltage).

• ###### drawing.JPG
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Last edited: Nov 2, 2011
4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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That topology isn't really doing much at all - other than preventing current flow when the applied emf is below the diode forward bias threshold voltage.

ElectricityStudent likes this.
5. ### praondevou AAC Fanatic!

Jul 9, 2011
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As TNK said , this circuit passes current in both directions doing nothing to the polarity of the signal at the output (which I assume to be on the right )
So assuming the voltage on the winding is higher than the forward drop of the diode, the voltage at the load resistor will be the same as on the winding itself (-forward drop), i.e. it will be AC.

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6. ### ElectricityStudent Thread Starter New Member

Nov 2, 2011
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Oh man!! i see what you're saying. LOL how could i have overlooked that, i mean this circuit is pointless because it's supposed to be converting from AC to DC. Mine still maintains the AC!!! Thanks guys for all your help

7. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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That would pass AC with the voltage drop of the diodes. On the positive swing of the AC, the top diode would conduct clockwise, on the negative swing of the AC, the bottom diode would conduct counterclockwise. (using conventional current flow)

You can do it with two diodes if you have a center tapped transformer.

8. ### ElectricityStudent Thread Starter New Member

Nov 2, 2011
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Am i supposed to close this thread now?
i mean the question has been answered thanks to you guys and now it'll just be wasting space

Last edited: Nov 2, 2011

Jul 9, 2011
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You can't

10. ### ElectricityStudent Thread Starter New Member

Nov 2, 2011
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oh ok lol, i see so this is how this becomes a platform for learning... it's like a reservoir of questions answered.
thanks again.

11. ### SgtWookie Expert

Jul 17, 2007
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Actually, your circuit is not useless.

Change the diodes to light-emitting diodes, and put a resistor in series with them to limit the current. You can then use the circuit to detect what polarity the voltage is across the circuit; if DC, just one will be lit; if AC, both will be lit.

Like this:

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12. ### EB255GTX Active Member

Apr 30, 2011
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To be *really* pedantic, add to SgtWookie's excellent reply that if both LED's are lit, the signal is AC of frequency higher than about 20HZ.

Human persistance of vision means that you see both LEDs lit if they swap being ON and OFF quickly enough.... but the circuit could be put across an AC ignal with f=0.1HZ and you might mistake it for DC.

As I said - I'm being a pedant in the extreme, but IMHO part of being a good engineer is thinking of EVERY possible case not just the common ones

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13. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I actually have built one of these, I ripped up a "Live AC Warning Cheker" that didn't work and put the LEDs in it, and used the power supply in the pen-like tester. Ran a couple probes out the front, and I can actually say it's in the top 10 items I grab when doing a quick check, such as "is this battery in the right way" without digging out the multimeter.

For the curious, the other 9 are: Fluke 87 meters, DSO Nano portable scope, Fluke Clamp hall effect clamp on meter for measuring both AC and DC current with no contact, IR Thermometer with laser aim for finding hot components, Capacitor ESR tester, and the $M^3$ Semicondcutor Testor.