Need help understanding the output of a Transformer - Page 1
Forum section: electronics / beginners

author: sgkini
author_rank: Contributor
timestamp: February 23, 2014, 08:55:32 pm
content: Hi,I was trying to learn about transformers. So I purchased one from ebay. I received it yesterday . The detail of the transformer is attached along with the message. I connected the primary (blue + brown) to the mains (230 V 50 Hz) and connected the secondary to my Siglent DSO and captured the output .Then I connected a 6.8 K power resistor across the secondary and then captured the image again.The output looks like an output from a bridge rectifier. I thought transformer converts from AC to AC. Is this transformer custom made and has the bridge rectifier inside in the center somewhere or is my understanding totally wrong ?RegardsSatish

author: xygor
author_rank: Regular Contributor
timestamp: February 23, 2014, 09:12:28 pm
content: Yes, it is rectified according to the label. After SEC it says ... DC.According to the line after that, it also appears to be fused.I'm not sure what the "T" means. Thermal protection maybe.

author: JOERGG
author_rank: Contributor
timestamp: February 23, 2014, 10:16:22 pm
content: Quote from: xygor on February 23, 2014, 09:12:28 pmThe T behind the 6,3 A Fuse stands for "träge" witch means delayed action or slow fuse.The thermal protection switch at 125 degrees C is insertet in the primary winding, according to the label.

author: tautech
author_rank: Super Contributor
timestamp: February 23, 2014, 10:26:24 pm
content: Now you know it has an integral full wave rectifier, and the giveaway is the waveform in your second image.Also note the frequency of 100 Hz is your line frequency x 2

author: sgkini
author_rank: Contributor
timestamp: February 24, 2014, 11:12:58 am
content: Thanx for helping me understand. I did not read the DC part till xygor pointed it out.I assumed that 25.5 V was RMS voltage so the Vpp would be around 72 V and the output from the bridge rectifier would be around 36 V - 2 * Vf . I connected a 2200 µF cap across the 6.8 K resistor and found the dc voltage to be arounf 51V (2*25.5 ) . Can you please explain how that is possible or is there some thing I understood wrong?ThanX for taking the time to explain it to me.Satish

author: codeboy2k
author_rank: Super Contributor
timestamp: February 24, 2014, 11:30:18 am
content: 25.5 V RMS is (25.5 / 0.707 ) peak = 36.07 peak AC Volts. When full wave rectified it will be 36.07 - (2*Vf) = 34.87, assuming Vf=0.600 .After the 2200 uF it will be 34.87 * sqrt(2) = 49.31VYou are seeing almost 51V most likely because the transformer is unloaded so the voltage is actually higher than 25.5V. The 25.5V is probably the output voltage at the rated 5.4A load.Working backwards from your scope output, which shows 51.2 volts ==> ( ( 51.2 / sqrt(2) ) + 2*Vf) =26.4437.4 VAC RMS

author: sgkini
author_rank: Contributor
timestamp: February 24, 2014, 02:24:03 pm
content: I understood the first part, that the peak voltage of the AC wave would be 25.5 * sqrt(2) = 36.06 and the peak after full wave rectifications will be around 34.6 V. But you lost me after thatWhy are you mutliplying the peak value again with sqrt(2) ? I thought that the smoothing cap will charge to the peak value of the full-wave rectified voltage (34.86 v) and discharge depending on the load and forming a saw-tooth like ripple.Would be great if you could explain why sqrt(2) is multiplied to the peak value ?Satish

author: megajocke
author_rank: Regular Contributor
timestamp: February 24, 2014, 03:50:18 pm
content: sgkini: You are right about that. It should only appear once in the calculation.However, that transformer is marked "ED=20%" which means that it is not rated for continuous operation but for 20% intermittency at 190 VA. (20% of the time on, 80% off) I'd guess it's the transformer for powering the motor of a desk with adjustable height. The intended application would have the mains connected to the transformer all the time, but it would only be loaded for short periods quite seldom.If you only load it to 40% of its rated current it should be okay for continuous operation because this gives 20% of the heating compared to the rated current which it is only rated for to carry 20% of the time.The reason the voltage is quite high is to compensate for the voltage drop at rated load. The voltage drop at rated load can be quite large for a low-intermittency tranformer.25.5V might also be the average rectified voltage (rather than RMS), including the diode drops. The average rectified value is about 0.9 times the RMS value for a sine, so it adds about 10% to the expected peak value.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 24, 2014, 05:45:06 pm
content: Are transformers with built-in rectifiers common in Europe? The only ones I have seen here in the states are the "wall wart" power pack types, which usually have a filter cap as well.Have never come across a discrete transformer that had a built-in bridge. Seems like a pretty stupid idea, actually. If the bridge fails, you need to replace the entire thing, which is going to cost a LOT more than a simple rectifier would...

author: sgkini
author_rank: Contributor
timestamp: February 24, 2014, 05:58:27 pm
content: Quote from: N2IXK on February 24, 2014, 05:45:06 pmI never knew one existed before I got this one. At least learned something newQuote from: N2IXK on February 24, 2014, 05:45:06 pmThat's exactly what came to my mind, what if the bridge fails?

author: codeboy2k
author_rank: Super Contributor
timestamp: February 24, 2014, 06:23:13 pm
content: Quote from: sgkini on February 24, 2014, 02:24:03 pmDoh! my sleepy mistake... I've been awake all night.. I screwed that calculation.. , you're right ... There is only 1 multiplication by sqrt(2), which I already did when I divided by .707

author: megajocke
author_rank: Regular Contributor
timestamp: February 24, 2014, 08:51:52 pm
content: Quote from: sgkini on February 24, 2014, 05:58:27 pmI'd guess you throw it away, just as if the thermal fuse had failed. If this really is from an adjustable height desk I doubt you'd do any component-level repair.Still, it seems highly unusal to pot the bridge rectifier inside the transformer.

author: sgkini
author_rank: Contributor
timestamp: February 24, 2014, 09:48:34 pm
content: @codeboy2k no worries . Every one make mistakes and I know how its after pulling an all nighter .@megajocke Yes seems very unusual as per the very little knowledge of transformer I have. I was thinking that something was wrong with my scope when I saw the no load waveform. But things got a little clear after adding a 6.8K load and a lot clearer here. Got it on ebay for 7 euro + shipping. While buying I thought the output of the transformer was 25.5 RMS VAC . Next time should read more carefully

author: xygor
author_rank: Regular Contributor
timestamp: February 26, 2014, 09:14:22 pm
content: Quote from: sgkini on February 24, 2014, 05:58:27 pmWhy do you think it's a bridge?Measure the output with your ohmmeter on the diode setting (with the primary not energized) and see how many diode forward drops you measure.It could be a center-tapped secondary with two diodes.

author: tszaboo
author_rank: Super Contributor
timestamp: February 26, 2014, 09:27:09 pm
content: Quote from: xygor on February 26, 2014, 09:14:22 pmUsually silicon is cheaper than copper. This is interesting, I dont think I've seen transformer with built in rectifier potted in the middle. But it actually makes sense, less cabling, automatically good size rectifier. I kinda like it.