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Rectifier Attempt : FAIL

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theunfrailhale

New Member
so i made my first attempt at a full wave rectifier circuit, I built it off a circuit diagram and some advice from an older thread

http://www.electro-tech-online.com/threads/24-vac-12vdc-rectifier.92441/

i soldered the whole thing up and it looked pretty good in my opinion, i checked for accidental bridges in my soldering etc...

to the point anyways, I plugged it in today and when I powered it up the "housing" on the 7812 Voltage regulator exploded off of it. besides being slightly entertaining it was a failure.

this is what I *think* could have went wrong.

1) the diagram had values next to the capacitors, but no reference value. i.e. it said cap one was 3300. i am guessing that that is 3300uF, so thats what i put in. Radio shack didnt have a 3300uF cap so i put in a 4700uF cap, but i figured that would just give me extended discharge time, and not effect my circuit negatively.

2) the 7812 had a reference picture telling me what my leads are, but it is symmetrical, and so i assumed that the picture meant i was looking at my chip from the front, or the side with the writing... maybe i put it in backwards?

3) i did not attach the center lead of the 7812 to the machines ground, or anything for that matter...

so, thats what i know, let me know what you think, I really appreciate it. this is the first circuit i have ever built and im stoked to learn more.

if you think a picture of my board would be usefull i can get some and post them. Can i just replace the 7812 and try again, or do you think the other components are messed as well?

thanks in advance,
Dave
 

crutschow

Well-Known Member
Most Helpful Member
What is your power source?

3) Why would you not connect the center lead of the regulator? It clearly states that this is the GND pin which needs to go to your circuit ground. How do you expect a circuit to work if you don't connect it as the schematic shows?

Please post your schematic.
 

Sceadwian

Banned
Check your diodes, the first time I built a bridge rectifier I had it hooked up wrong, instant dead short, very common, and there's no telling what else you blew, hopfully just the regulator. That was a big nono not hooking up that center lead though.
 

Boncuk

New Member
Another point of interest: What is the regulator's input voltage?
 

theunfrailhale

New Member
I had the ground lead of the chip wired into my circuit but i didn't attach it to the machines ground, so it was effectively not used. The machine is a shear that cuts 10 foot sheets of sheetmetal in a shop i work at, and I am converting the machines incandescent lighting which fails constantly due to vibration. The machine feeds the incandescent lights at 24VAC, and I am purchasing some prepackaged LED assemblies to install on the machine. I appreciate the feedback, the importance of the ground lead didn't strike me since the machines i wire up for the most part dont use their ground leads at all... Circuits are a whole different game i'm learning.

the regulators input voltage to my understanding is 12 volts DC, and it is supposed to act as a smoothing "regulating" chip for the rectifier output. im building the circuit based off the schematic i was referred to on a previous thread, except for the 4700uF cap instead of a 3300uF cap. the caps that i bought are labeled 4700uF 35V. that 35V means that they can handle up to 35V correct? but since i am feeding them at 12V that is their output?
 

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Hero999

Banned
With 18VAC in the voltage across C1 will be 25V.

The regulator has a drop-out voltage of 2.5V so it requires 14.5V to regulate properly. The 4700µF capacitor is large enough to filter the ripple so the voltage never falls below 14.5V.

Is the input voltage 24VAC?

If so the voltage on the C1 will be 34V which is pushing it and the regulator close to their maximum ratings.
 

theunfrailhale

New Member
i have left out the power transformer, because the machine is already reducing my lines to 24VAC. the capacitor can charge to a greater voltage than the input?

i know my questions may be kind of ridiculous. I have no formal instruction on any of these principles, and im just trying to put together what i am learning from the internet and books...
 

crutschow

Well-Known Member
Most Helpful Member
i have left out the power transformer, because the machine is already reducing my lines to 24VAC. the capacitor can charge to a greater voltage than the input?
AC voltages are normally given as RMS. A rectifier capacitor will charge to near the peak of the AC waveform which is 1.4 times the 24VAC (RMS) value.
 

ronp849

New Member
If you are only driving LEDs, do you really need a regulator and capacitor? A series resistor after the bridge rectifier should be sufficient to drop the excess voltage.
 
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