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Old 25th January 2014, 07:17 AM   #11
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You set you had a few SMPS - haul out one transformer and analyse it, make some mods and then replace it. The primary is normally wound first around the bobbin. nIf the secondary that you want to modify is 107V and you want it to be 50 - 0 - 50 then just count the windings, halve the number and wind two from the same wire and into the same direction as before. The secondary could have as few as 10 turns on it.
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Old 25th January 2014, 10:13 AM   #12
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Yes, good tip.

Jan
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Old 25th January 2014, 10:14 AM   #13
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Quote:
Originally Posted by crazifunguy View Post
So I took some measurements and the SMPS is putting out a square wave with some overshoot and the rising and falling edge. A DMM reads ~48V ac on both taps when half rectified it reads 107VDC no load and ~77VDC with a 100W load. I suppose something is dooable but I will need an amp that can support some serious rails. Any final ideas or feedback on the part that I listed? This is mainly for fun but I do have 9 of these supplies in my parts box. They were cut off from damaged Klipsch Sub 10 amplifiers.
The DMM is calibrated for RMS of sine waves so will underestimate if you use it on square waves. That's why the big difference between the DMM and the rectified voltage.

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Old 25th January 2014, 10:36 AM   #14
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Most DMMs use averaging. Take lots of samples and average the resulting voltage.

A square wave using averaging gives a good result.
A sawtooth or triangle wave using averaging gives a good result.
A sinewave using averaging gives a good average. BUT it is a bad result as far as informing us of the energy contained in the waveform. The manufacturer scales (multiplies) the average result to give a good equivalent to the heating effect (rms) of the waveform.

But that scaling from average to sinewave destroys the usefulness of the triangle and squarewave results. If you know the scaling factor used by the manufacturer, you can apply that in reverse to find the true average of those straight edged waveforms.

The scaling factor is roughly sqrt(2). Can anyone confirm the actual scaling factor?
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Old 25th January 2014, 12:59 PM   #15
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Quote:
Originally Posted by AndrewT View Post
Most DMMs use averaging. Take lots of samples and average the resulting voltage.
That's not the point - the point is they are calibrated for an indication assuming the input is a sine wave. That has nothing to do with whatever technology they use to sample the input.

Jan
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Old 25th January 2014, 01:06 PM   #16
AndrewT is offline AndrewT  Scotland
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Understanding how they work allows one to decide if they can be used for non sinewave measurements.

Trying to identify the extent of ripple on a DC voltage is just such a use.

All my DMMs, both hand held and bench give a good approximation of ripple voltage on a supply rail without reading the rms effect of the dominant DC voltage.

Using a scope confirms that the DMM mVac reading is very approximately 1/3rd of the peak to peak voltage shown on a scope.
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Old 25th January 2014, 02:55 PM   #17
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Quote:
Originally Posted by AndrewT View Post
Understanding how they work allows one to decide if they can be used for non sinewave measurements.

Trying to identify the extent of ripple on a DC voltage is just such a use.

All my DMMs, both hand held and bench give a good approximation of ripple voltage on a supply rail without reading the rms effect of the dominant DC voltage.

Using a scope confirms that the DMM mVac reading is very approximately 1/3rd of the peak to peak voltage shown on a scope.
Andrew, they give 'a good approximation' in as far as the ripple comes closer to a sine wave. You still confuse 'the way they work' with the calibration. It doesn't matter how they work. They are calibrated to show the correct RMS value of a sine wave with a certain peak value. They are calibrated to show 0.707 of the peak value as RMS value. If you measure anything else than a sinewave they are off because a non-sinewave does not have the 0.707 ratio between peak value and RMS value.

Of course they can read the AC on a DC supply - coupling cap! That said, many DMMs have the option to include the DC component at the flick of a switch.

Giving 1/3rd of the ripple peak to peak value is to be expected. The DMM 'measures' the peak value which is 1/2 of the peak-to-peak and then indicates 0.707 of that, which leads to - surprise! - 1/2 * 0.707 = 0.35.
That does not mean that the RMS value of the ripple is 1/3rd of the peak-to-peak value, although that is what the meter is telling you.

Jan
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Old 28th January 2014, 02:10 AM   #18
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It has been a long time since I have torn one of these transformers apart. If my memory is correct the windings are staggered on the ER35 bobbin. Re-winding would be a pain. I suppose this would be a good learnign expierence on how this SMPS actually works and the transformer ratios.
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Old 13th March 2014, 02:22 AM   #19
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I learned a few more things. The main supply operates in a push pull method. The transofrmer then has a couple different tapps. It has a low current +-35V winding that is dropped down to +-15v with linear regulators(they do get pretty hot). The second winding measures between the suplies unloaded are around 90-100V. Would it be easier to just add a DC/DC buck to drop it down to 48V?
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