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# A Basic Question About Rectifying 110VAC

Discussion in 'The Projects Forum' started by Tom Kay, Feb 17, 2009.

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1. ### Tom Kay Thread Starter Active Member

Feb 10, 2009
35
0
Hi;

I'd like to make sure on something.

I was told long ago, that when you rectify AC with a full wave bridge, the voltage goes up 1.41 (root 2) because of the RMS concept.

Example, say you have a transformer. It goes from 110VAC primary to 24 VAC secondary. You full wave rectify it, then smooth the output with a properly sized electrolytic capacitor. Rather than having 24 VDC, you actually end up with 33.8 Volts DC, minus about 2 volts for the full wave bridge. (24 x 1.41)-2=31.8 VDC

So far, correct?? I think I verified this with a volt meter on one of my power supplies.

Next example. Let's say we don't use a transformer at all. Let's simply rectify and smooth the 110VAC to DC.

Would we get (110 x 1.41) - 2 = 153 VDC ??

Or am I missing some concept that is different because we didn't use a transformer to reduce the VAC?

Just to inform you, where I am going with this, is to create a DC voltage source for large numbers of LEDs. If the voltage didn't have to be reduced, there may be advantages to this (such as avoiding small wall-warts that operate at around 85% efficiency, and therefore waste power).

The 2 GU-10 type 72 LED bulbs I have, do have a full wave bridge, a smoothing cap, but no step-down transformer. Therefore, they avoid wasting power on an unwanted component, ie, transformer. Since there are 36 LEDs in series (and 2 of these "banks" in parallel) I assume the voltage is shared by the 36 LEDs and current limiting resistor, which is about 500 ohms.

I like the fact that there is no small transformer because of the power waste issue.

Thanks. You'ave all been very helpful this far. Tom.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
You would even more likely get dead. Messing with non-isolated and -limited AC is simply suicidal. Surely your life is worth more than a transformer? Wasting a life is the real issue.

3. ### Tom Kay Thread Starter Active Member

Feb 10, 2009
35
0
Beenthere;

Your answer was not helpful at all. I would expect more detail from someone who is trying to assist hobbiests with questions, or more importantly, play a part keeping them safe. And yes, thank you, my life is worth more than a transformer.

We deal with 110 VAC all day, and it doesn't kill us if we take precautions. Why (and this is the part where waaaaaay more detail would be more helpful then a reply like your last one) would handling the electrical power in a standard home-wiring way be more dangerous than any other home wiring job?

From a standpoint of, at least, layman's logic, there seems to be no obvious violation of safety rules with this. If I'm wrong, enlighten me. That, I had hoped, was the entire point of internet forums.

As far as the project is concerned, household cabling would be used. All circuits would be contained within standard metal boxes. All circuits would be carrying no more voltage than normal, and far less current.

If you would like to provide a reasonable level of detail to support your the warning in your response, I would have a lot more respect for your position. If, however, you can't justify explaning what you mean, I respectfully ask you to ignore all of my further posts, and allow helpful people such as Bill Marsden or Bertus furnish useful replies.

Regards, Tom Kay.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Mea culpa - and I will continue to close threads that propose to accomplish dangerous and foolish things in the name of "saving" money.