Scope Waveforms of Transformer Has Me Confused... - Page 1
Forum section: electronics / beginners

author: Shredhead
author_rank: Regular Contributor
timestamp: August 24, 2016, 04:57:24 pm
content: Can anyone help me understand what is happening with the waveforms of the secondary out of this transformer under load?I know that I am not exceeding the max output out of this CT transformer. The max voltage out of each tap is 14V and I'm reading 16.6V. The max current is rated for 214mA and I'm reading 200mA.1. -When switched on, the voltage ramps up for more than 300ms while delivering a current spike of 2.02A.2. -When at full output, the waveform of the voltage looks not just distorted which I understand is normal but it looks clipped as if it's reaching a hard limit somehow. Is this normal and why?3. -I have been told that the spikes in the waveform are from the diodes switching but I don't really understand why it looks like a momentary dropout in voltage out of the transformer.The output of this transformer is going to a full bridge and then to a split rail supply with regulators. There is a load on the output of the regulators which consists of op amp circuits.

author: Audioguru
author_rank: Super Contributor
timestamp: August 24, 2016, 05:25:57 pm
content: The rectifiers conduct a very high current near the peak voltage for a short duration which causes the sinewave from the transformer to appear to be clipped which is normal. The peak voltage of a sinewave is 1.414 (the root of 2) times higher than the RMS voltage.The spikes are caused by switching of the rectifiers and poor high frequency filtering by the main filter capacitors. In the olden days a 0.01uF capacitor was connected parallel with each rectifier.

author: PChi
author_rank: Frequent Contributor
timestamp: August 24, 2016, 08:48:01 pm
content: That does look like a long flat top, I guess that you have got some large smoothing capacotors and it looks like the transformer is being used heavily. If you have got an rms measuring multimeter it might be worth checking that the transfomer is being used within it's ratings.If you don't like the look of the spikes you could substitute some faster turn off diodes and / or add snubbers as suggested by Audioguru.

author: StillTrying
author_rank: Super Contributor
timestamp: August 24, 2016, 09:29:27 pm
content: It looks odd to me, usually the flat tops still have some small voltage rise / curve, and the 300ms ramp seems very slow, often 2 or 3 cycles are enough where the load hasn't started drawing current yet.It looks like the transformer is struggling just to charge the caps!

author: K5HJ
author_rank: Regular Contributor
timestamp: August 24, 2016, 09:57:52 pm
content: The flat portion is when the diodes are conducting as others have stated. If the capacitors are increased in value, this flat region will be shorter as more energy will be stored in them.The spike is the inductive "kick back" when the diodes stop conducting and there is no longer a load on the transformer. The magnetic field collapses and produces a reverse voltage.This "old school" method of linear rectification, which is in a lot of old devices, creates harmonics on the input power lines that makes the power companies unhappy especially in Europe.Many commercial installations have a limit on harmonics on the power line. Not so for residential, yet.Randy

author: StillTrying
author_rank: Super Contributor
timestamp: August 24, 2016, 10:31:59 pm
content: Quote from: K5HJ on August 24, 2016, 09:57:52 pmWhat's the new school way? Using a smaller capacitor and a switch mode circuit after it is all I can think of.

author: electr_peter
author_rank: Unknown
timestamp: August 24, 2016, 10:46:36 pm
content: Quote from: StillTrying on August 24, 2016, 10:31:59 pmPower factor correction circuits is the new way. PFC can be thought as a voltage boost circuit which allows to draw current from lower points of AC waveform (traditional rectifier + cap draws current at the top only).

author: K5HJ
author_rank: Regular Contributor
timestamp: August 25, 2016, 03:47:25 am
content: Quote from: StillTrying on August 24, 2016, 10:31:59 pmWell, electr_peter beat me to it but, yes power factor correction "PFC" is used on most commercially available switching power supplies.Randy