Potential fault with Transformer Secondaries - Page 1
Forum section: electronics / beginners

author: NTB
author_rank: Newbie
timestamp: December 07, 2018, 12:05:50 pm
content: Hi,This is my first post here. I am gradually learning about electronics. I have an audio amplifier and decided to hook up my oscilloscope, which I'm fairly new at, to the transformer secondaries that powers the main output stage. These secondaries have a neutral center tap. Each of my probes are set to 1:1 and the earth clip to the metal chassis which is earthed via the power cord to the power point.I noticed that the expected AC sine waves were not really uniform. Please refer to attached pics. Sorry about the unintended Rigol....aspect?So, I have 2 questions, 1. could this indicate that the transformer could be faulty? or 2. could the circuit following the point where the scope was attached be reducing the appearance of a sine wave? Hopefully the later!Thanks in anticipation of your patience and assistance,Nigel Brown

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 12:46:34 pm
content: hi,first, you need to watch some videos about oscilloscope probes, try to use mainly 1:10 probes.unless you want a bad frequency response.most likely transformer is ok.usually the common point of transformer is't tied to earth, only case is(1'st mistake).correct those and we'll talk again after.regards,pierre

author: Benta
author_rank: Super Contributor
timestamp: December 07, 2018, 01:49:49 pm
content: Looks absolutely correct. What you are seeing is the current peak when charging the electrolytics after the rectifier. This current is so high that it depresses the output voltage from the transformer.

author: NTB
author_rank: Newbie
timestamp: December 07, 2018, 11:44:34 pm
content: Thanks very much, for your advice. I'll look into learning more about probes, ie: using 10:1 setting instead of 1:1.Ciao,Nigel

author: vk6zgo
author_rank: Super Contributor
timestamp: December 08, 2018, 01:23:49 am
content: Quote from: perieanuo on December 07, 2018, 12:46:34 pmBecause both channels are showing similar waveforms, it rather looks like this device uses a 2 diode full wave rectifier as was common in the days of valve/tube rectifiers using power transformers.They did use the centre tap of the secondary connected to chassis, either directly, or commonly in radios,via a low-ish value resistor to provide " back bias".

author: spec
author_rank: Frequent Contributor
timestamp: December 09, 2018, 09:46:12 am
content: Hi NTBQuote from: NTB on December 07, 2018, 12:05:50 pmA1. No- quite normalA2. Yes- most definitelyWhat you are seeing is a perfectly normal waveform from a transformer secondary winding feeding a rectifier/reservoir capacitor, as the other members have explained. The waveform is actually a distorted sine wave. A diode will only pass current in one direction, that is when its anode is more positive than its cathode by about 0.8V. A reservoir capacitor is like a huge bucket that can store billions of electrons (in the case of an audio amplifier PSU, about 1018electrons). By the way, your scope measurements are absolutely fine too. There absolutely is no need for a 10:1 probe.The flat tops on the sine wave are where the rectifying diodes start conducting and connect the transformer secondary to the reservoir capacitor, which is a very low impedance (resistance). At that time a large current flows into the reservoir capacitor to charge it up, as indicated by the small ramp on the extremities of the waveform. The large current, coupled with the output impedance (resistance) of the transformer secondary winding, causes a voltage drop, and hence the flat tops on the sine wave.If you put your scope probe on the reservoir capacitor, you will see a saw-tooth voltage waveform (ripple voltage) where the reservoir capacitor is being charged and then discharged, a bit, when providing current to the amplifier, until the next peak of the sine wave comes along to charge the reservoir capacitor once again.(if you don't mind, I would like to steal your image showing an actual equipment transformer secondary waveform, as it will be very useful for explaining PSUs in the future)The attached image below shows what your amplifier power supply probably looks like.