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Circuit Design Help: LED Circuit that can be powered if positive and negative are reversed? Answered

Hi there,

I am looking to design a circuit with:

• Has 10 LEDs
              o 5 LEDs in Series, with two of these sets connected in parallel.
• I want the capability of the LED array to illuminate if one end of the PCB is powered by the positive and the other end is in contact with the negative.  And vice-versa, meaning the first end is now touching negative with the other end in contact with positive.
               o I was told that I can use diodes, but I’m not sure by what configuration.

I've attached something I've found online.  Will this work?  if so, can you provide an explanation of how it works?

12 Replies

user
icengBest Answer (author)2013-07-31

These circuits show how the four diodes of a full-wave bridge function
when a battery is reversed...

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ThompsonGenie (author)iceng2013-08-01

Great diagram iceng!

I have a question, what is the purpose of having the resistors in the middle of the LED series? Would it be better to have the resistor in series before the group of 4 LEDs?

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iceng (author)ThompsonGenie2013-08-01

Each group of 3 LEDs is connected across 11 volts
and each needs a current limiting resistor.

(That is 12V  less two of the bridge diodes forward voltage drop)

The relevant point is to realize that where the resistor is placed Does Not Matter  .

In addition if you place  only one resistor in series  with all the LEDs
see the uneven lighting about the badly designed cube pics and
that is the least undesirable effect that would occur .

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user

Doesn't make any difference

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user
jduffy54 (author)2013-08-01

just look up "bridge rectifier" on google, anything with four diodes in a diamond shape is it. The schematic you attached would only need a diode attached from the "+ input" to the wire between the BAS70 diode and 820 ohm resistor. Also, you don't need BAS70 diodes for this to work. Practically any diode (except zener diodes) will do the trick.

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framistan (author)2013-07-31

You can MAKE a full-wave rectifier assembly with 4 diodes but it is easier to just purchase one that comes in a small package with 4 wires coming out. 2 of the terminals have a little "S" symbol on it which indicates those 2 pins are your INPUT which can be PLUS or MINUS voltage coming into it. No matter which way the voltage comes INTO the bridge rectifier... the OUTPUT will come out with PLUS on the "+" pin and MINUS will come out on the "-" pin. That is what you need.

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iceng (author)2013-07-31
user
Josehf Murchison (author)2013-07-31

Ok so you want to run LEDs from an AC source after running it through a rectifier.
No problem so what is your difficulty.

Joe

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user

I'm running the LEDs off of a 14 VDC Power Source.

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user

I don't know what you are building or why but mpilchfamily has it right a full bridge rectifier is best the half bridge rectifier in your schematic will do that circuit will run on 14 VAC also.

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mpilchfamily (author)2013-07-31

You need 4 diodes set up as a bridge rectifier or just get a bridge rectifier. Then no matter what way the polarity is the positive voltage will go to the anode and the negative will be on the cathode of the LEDs.

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MDheliMech (author)2013-07-31

I have built an alternating flashing circuit using a 555 timer. The output of the 555 was connected to the positive of one led and the negative of the other. I have never had a problem this way. The way it works is when the signal is high one led has a high and a low and the other has a high and low. When the signal is low it is opposite one led has two lows and the other has a high and low.

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