Weird transformer output waveform distortion / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Weird transformer output waveform distortion

1 2 next

Move Thread

LAN_403

PhilGood

Thu Aug 01 2013, 12:07AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hi all,

When doing some measurements with my scope I sometimes need to display line voltage to compare it with another signal.

So I quickly built a small mains adapter. Basically, it is a switch, a neon bulb with its resistor, a 12V 1VA molded transformer, and an adjustable voltage divider. Resistance across the secondary is around 12k and a potentiometer allows me to vary output voltage from 0 to 5V.

After putting everything in place and connecting the adapter to the scope I was unpleasantly surprised to see a heavily distorded waveform.

I first though it could come form something wrong in my setup, but when I directly scoped the output of the transformer I got the same problem.

I have two of these small transformers so I checked the 2nd one alone, and I got a nice sinus

Then I thought this could be due to the fact my transformer is glued in the aluminium enclosure, so I put my other transformer in the same kind of enclosure, and I still got a nice signal

Any idea on what can cause this problem on my setup and/or how to fix it would be much appreciated :)

Thanks

Mads Barnkob

Thu Aug 01 2013, 02:47AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Did you also check the first one alone without LED etc?

Your distorted signal looks like your seeing the odd harmonics if that is any help to you.

Steve Conner

Thu Aug 01 2013, 09:39AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This is the waveform I'd expect from a cheap transformer that is saturating. There can also be a phase shift due to the RL time constant of the primary winding.

A small toroidal transformer would give a better result.

PhilGood

Thu Aug 01 2013, 11:46PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hi,

@Mads: Yes I also checked the first transformer without anything else than the probe connected to the secondary and I got the same distorded waveform.

The light is not a LED but a neon bulb connected on the primary side, Could this neon bulb cause this problem ?

@Steve: Can the transformer saturate with barely no load ? It is rated 1W and I'm actually "pumping" ~0,012W

It is indeed a cheap transformer, I got them for 3.80â‚¬ each

I was first planning to use a toroidal transformer, but I couldn't find such a small one. My enclosure inner width is 35mm and I'd like to keep this adapter as small as possible.

Well I might try with the 2nd one as it seems to give a decent output, unless I can find a small enough toroidal transformer

Thanks guys for your replies

Dr. Slack

Fri Aug 02 2013, 06:23AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

A transformer saturates with too much input voltage, this effect is worst at no load. As the output current rises, so does the input current and the primary IR voltage drop, which reduces the input voltage the core sees, and so improves saturation.

If you have a variac, first check its output waveform. Then use it to power your suspect transformer. Wind the volts up and down around nominal and check the output waveform.

Fabio

Fri Aug 02 2013, 09:27AM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

A power transformer is different from a potential transformer; they are the same apparatus but the first one works closer to the highest magnetic flux that the core can handle in order to lowering the dimension per watt (and cost per watt) and the second one works with lower magnetic flux in order to improve the stability and to reduce the distorsions of the waveform.
Try to buy a similar transformer with 400V input, it might help.

EDIT:
************************************************* **
You can also try to use two identical transformers (with the primary windings in series and the secondary windings in parallel), every transformer will be energized with only half of his nominal voltage and the magnetic flux will be less.

Ciao!
Fabio.

Steve Conner

Fri Aug 02 2013, 10:47AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Fabio, I have seen two 230V transformers in series used for a line sync circuit in a commercial apparatus for that exact reason.

I've also had some luck running these small cheap transformers in "zero field" mode when I wanted a really accurate line voltage reference with low distortion and phase shift. I was able to get good performance from 16 to 400Hz using more or less the circuit shown here:

I was proud of this piece of engineering, as it made the performance much better while decreasing the cost and getting rid of one of the transformers. But unfortunately it never made it into the final product. Once we realised bringing line voltage into the box would require a lot of expensive testing to the CE Low Voltage Directive, we left out the line sync circuit altogether

PhilGood

Fri Aug 02 2013, 04:56PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks guys for these clear answers

I'm somehow at ease to see it's a common problem taken into account in commercial setups. I will try these suggestions and let you know how it goes

Just one point:

Dr. Slack wrote ...

If you have a variac, first check its output waveform.

How can I do this without an insulation transformer ? I can't connect my scope's ground to mains neutral

Dr. Slack

Fri Aug 02 2013, 09:50PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

PhilGood wrote ...

How can I do this without an insulation transformer ? I can't connect my scope's ground to mains neutral

There are several ways to do this, with different pros and cons

a) If you have a dual channel scope, with a channel 2 invert and add facility, then you can use the two probes as a differential input

b) If you promise not to electrocute yourself, then disconnecting the scope ground breaks any ground loop current

c) You could build a simple 'ground breaker', based on a simple differential amplifier, powered from a battery

d) Put a couple of transformers in series/parallel as suggested by Fabio and endorsed by Steve to make a reasonably linear isolation transformer

PhilGood

Fri Aug 02 2013, 10:15PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Dr. Slack wrote ...

a) If you have a dual channel scope, with a channel 2 invert and add facility, then you can use the two probes as a differential input

Oh true, I forgot I can do that, thanks

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob