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Concerning Real World output of rectified AC, is it really ACx1.4??

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MikeT1982

Thu Apr 05 2007, 06:54AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

I apologize ahead of time if this is a dumb post, I honestly am trying to understand this though. I have been told and have read that "the square root of 2, x the AC voltage, will give you the DC voltage you get out of a Rectifier Bridge. Now for example, say I have a 4,000 volt Neon Sign transformer, and I put a Multimeter on it (in theory as at least my Fluke doesn't go that high LOL). Lets say I got 4,000 volts exactly on the Fluke. Now according to the formula, the rectified DC would be 5656.854 Volts DC.......I am puzzled, WHERE does the extra volts come from....maybe I am mistaken and this is only what the capacitor bank I am charging will rise too due to the "peak"?? but the amplitude of the sine wave stops at Positive 4,000 and Negative 4,000 no???so why and how?.....Would my multimeter ACTUALLY read 5656.854 if I had no capacitor bank and just the transformer, into a 4 diode rectifier bridge, then into my Fluke meter?

Thanks ahead of time,

Mike

haulinick

Thu Apr 05 2007, 07:03AM

Registered Member #619 Joined: Sat Mar 31 2007, 05:26AM
Location: San Jose, CA
Posts: 16

You did something wrong. I myselft don't know the exact equation but I do know that your rectified dc voltage will be the same as the rms ac voltage rating. Here's an example: mains voltage is 120v ac RMS. It is actually peaking at somewhere around 170 but the average voltage on the alternating way (the rms value) is 120v.
What I am not sure of is if the rating on the nst is rms or a peak voltage reading.
Please don't take my word for this. Make sure you hear it from someone else too, because I am not 100% positive...I'm just giving you my understanding from what I have read.

uzzors2k

Thu Apr 05 2007, 07:12AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The AC voltage rating is always RMS, Root mean square. ((Square root of 2) / 2) * AC peak voltage = RMS voltage, which is what is on the label of the transformer. When rectified the capacitor will charge to the peak voltage, and therefor to Square root of 2 * RMS. RMS is used because the amplitude of a sine wave isn't peaked all the time like a square wave, so using the peak value would give a poor representation of the power available. Your multimeter would read close to 5657 volts if the NST was rectified and filtered.

EDIT: Good point haulinick, if the NST rating isn't RMS, but peak voltage then you'll get 4kV.

Dr. Slack

Thu Apr 05 2007, 07:17AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

H'mm, so much to learn, so little time to type it (I'm off for Easter hols in a few hours, I'm supposed to be checking the traffic on the route

)

Unfortunately, Wkikpedia doesn't do a good job on the "rectifier" page, but there must be some electronics tutorials around if you Google. But the things to look for when you find the articles are ...

AC supply volts is rated at the RMS value, so gives the same heat output if placed across a resistor as the same rating of DC voltage. Thus 100v DC gives the same heating power as 141.4sin(2pi.freq.t), where freq is 50 or 60Hz. So an ideal "100v AC" supply peaks at 141.4 volts.

However, lots of minor losses and approximations will bite you in the bum if you try to get exact, a few of them are...

1) mains supply isn't sinusoidal, it's contanimated with harmonics, and with a "chunk" taken out of the peak of the waveform by all of the recitifier/capacitor input devices in your neighbourhood drawing heavy current at the peaks.

2) Only "true RMS" meters read true rms, most read rectivifed average, then scale the display by a small factor to compensate, so read slightly wrong on non-sine waveforms.

3) Diodes have a nearly fixed forward drop, which in your case is negligable compared to 4kV.

4) "115v" mains is rarely that, but varies by a few % up or down

5) A "4kV" transformer is nominal rather than exact, and especially an NST, the output varies with load

If you can measure your transformer AC output, then multiply by sqrt(2), and measure that on your caps +/- 10%, you are doing very well!

Steve Conner

Thu Apr 05 2007, 08:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sinusoidal AC voltages are always specified as RMS. So your "120V" wall socket puts out 120V RMS, and the peak amplitude from it is + and - 169V.

Likewise, when it says 4000V on the NST nameplate, that is RMS, and it does in fact put out 5660V peak.

These relations may not be exact if the waveform is not a pure sine wave, which it hardly ever is. Usually, in the case of line voltage, the peak is squashed a bit.

MikeT1982

Thu Apr 05 2007, 08:11AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

OK thank you so much everybody. So if I rectify a 4kV France NST (i know France makes solid ones), I can pretty much hands down gaurentee I will get my cap bank past at least 5kV DC realworld right?

uzzors2k

Thu Apr 05 2007, 12:57PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Yes.

...

Thu Apr 05 2007, 02:20PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There is a little info about it on our wiki (near the bottom)

But most of what is there has aleady been stated...

BTW, for an application like this, a normal cheepo meter that average peak is the best type of meter tp use, because as touched earleir mains isn't actually a sine wave, so the peak voltage isn't excatly rad2*rms. But it you have a meter that just reads peak/rad2, you will always get the actual peak value

Simon

Fri Apr 06 2007, 11:16PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

wrote ...

It is actually peaking at somewhere around 170 but the average voltage on the alternating way (the rms value) is 120v.

I'll just point out that the average voltage of one side of the AC output is actually more like 107V (2/PI times 170V).

RMS voltage is something a little different from the arithmetic mean. You almost never have to use the average, though, so don't get confused.

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