|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
63.156.217.195
In Reply to: RE: ok, what is happening here???? posted by vinnie2 on December 03, 2016 at 14:29:15
One needs to be a bit careful because of the shape of the waveform. If it were a precise square wave, then 12V rms would require 24V peak-to-peak. If your rather shaky looking scope trace is accurate, then the sides that should be vertical in a true square wave actually have quite a slow rise and fall time. This means the rms voltage is somewhat less than simply half the peak-to-peak voltage.That is why I was saying that your 35V peak-to-peak is maybe corresponding to something of order 15V rms, rather than the 17.5V rms that it would have been for a pure square wave.
(It would have been about 12.3V rms if it were a pure sinewave with 35V peak-to-peak. But the waveform you showed in that photo of the scope trace looks as if it would have a bigger area under the square of the curve than for a sinewave, but certainly less than for a pure square wave.)
It might help if you could get a steadier scope trace by fiddling with the sync level, as TK suggested. One could then more easily estimate the rms voltage.
If I understand correctly, your "square wave" is from a switching power supply operating at about 140KHz or so; is that right? (I'm guessing your statement of the horizontal axis being 1ys> per division was meant to be saying 1 microsecond per division?)
A typical cheap "true rms" meter might very well give a more or less meaningless reading at such a frequency. (I saw some discussion of this issue higher up the thread.)
Chris
Edits: 12/03/16 12/03/16 12/03/16Follow Ups:
"If I understand correctly, your "square wave" is from a switching power supply operating at about 140KHz or so"
I'm pretty sure Vinnie has mis-stated the sweep time. All the lighting transformers I've purchased operate closer to 20 kHz out of the box. That's why the transformer must be modified. As for the waveshape, I did notice that on the later transformers. My earlier units were much more square. This issue definitely changes the method needed for ensuring that the tubes have the correct power applied.
Hate to say it, but I'm not sure I would use this method again. Pulling the square wave from a standard DC switcher and filtering it to a sine wave now seems to make more sense.
--------------------------
Buy Chinese. Bury freedom.
I may have misstated the name, but the number was correct, it was 1. Like I said the symbol looks like a backwards y.
We are talking about the halogen AC transformer right, not the switching transformer?
I don't know what that could be. I've never seen a scope with any markings for time other than S, mS, uS and nS. I also can't think of any "1" setting for these categories that corresponds with 7 divisions, unless the frequency is *much* higher or lower than I've seen in these units. 7uS is roughly 140kHz. 7mS is 140Hz. Yes, we're discussing the lighting transformers with (supposedly) 12VAC output.
--------------------------
Buy Chinese. Bury freedom.
I just went and check my scope. On the sec/div dial there are 3 sections, S, mS and the backwards y with a tip on the short end which probably equates to your uS, and the setting I was using was 1 uS in that case.
If you really have a 140 kHz lighting transformer, it would be very helpful to know the model number. A pic of the inside when you have time would also be of use. Thanks!
--------------------------
Buy Chinese. Bury freedom.
I will take one in the morning and post it. It's bed time for this old fart.
This trace is from a halogen light transformer. I am not sure what the frequency is. I will have to look and see it it says the on the transformer. The y was as close as I could get to the symbol, which looked like a backward y. I am going to hook it up to an 813 or two in the morning and then try fiddling with the scope and see if I can get a better trace.
"The y was as close as I could get to the symbol, which looked like a backward y."
The backward y sounds like it is probably a Greek mu, which is why I thought you probably meant 1 microsecond per division.
But the frequency is not really important, as far as the rms voltage is concerned, except that the higher the frequency is, the less likely a cheap "true rms" voltmeter will give an accurate figure.
I just tried an estimate based on the shape of the approximately trapezoidal waveform in that scope trace, and it looked to me as if the rms voltage is coming out to be about 15.4V.
Chris
Well that is closer at least.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: