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Old 9th February 2009, 10:39 PM   #1
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Default What does this notation mean?

Hi peeps,

I have seen a number of posts recently about Power Supply design and I must conclude that I have the wrong idea about Power Supplies?

My Sensai taught me (15 years ago!) that +- 50 volts meant +50volts on the positive rail and -50volts on the negative rail. This would mean using a transformer with a primary of 240V (coz I'm in the UK!) and twin secondaries of 70 volts each; therefore after smoothing would result in plus and minus 50 volts RMS (0.707?)

Diagram:

Click the image to open in full size.


Yet recently I have seen a number of websites that say if you want to supply an amp with +-50V, you should use a transformer with the (twin) secondaries rated at 35 volts; so after smoothing would result in +25volts and -25 volts (50volts gap)

I want to build a new amp soon and I don't want my inference of Power Supply notation to kick my *** when it comes to the power-on stage! Any help much appreciated!

Rich.
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Old 9th February 2009, 10:54 PM   #2
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Your Sensai was correct.
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Old 10th February 2009, 12:03 AM   #3
CBS240 is offline CBS240  United States
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70VAC X sqrt 2 = 99V peak, or 99VDC after smothing caps. You divided when you should have multiplied.

Should have about 35VAC + 35VAC secondary if you want +/- 50V.



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Old 10th February 2009, 12:27 AM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by CBS240
70VAC X sqrt 2 = 99V peak, or 99VDC after smothing caps. You divided when you should have multiplied.

Should have about 35VAC + 35VAC secondary if you want +/- 50V.



I can confirm, you multiply volts rms by 1.414 to get volts pk - this minus the diode drop and any resistive losses in the transformer's secondaries (and primary too) gets you the rectified dc voltage. Alternatively you can divide by 0.707 to arrive at the same result. - you used the equation for Vpk to Vrms which is where your confusion lay.

More here: http://en.wikipedia.org/wiki/Root_mean_square

Note crudely speaking that rms voltage and current values are intended to relate the heating power of an AC voltage or current back to a value of DC that results in the equivalent heating power in a resistor.
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Old 10th February 2009, 01:14 AM   #5
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Uh, yeah.

I was thinking 70 VCT, which is how they end up specifying the winding. I neglected to see each winding was 70V.

Sorry.
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Old 10th February 2009, 01:51 PM   #6
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Ah, I see! Fantastic, thanks peeps for clearing that one up.
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