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smithy666 27th May 2006 07:07 AM

Elektor Lab Power Supply Oct/Nov 1990
2 Attachment(s)
I built this project some time ago & had problems getting it to work properly. I pulled it out again recently and now have the power supply working. Following an errata I received on the project, I now get both channels working in independent & tracking modes from 0-40V and current up to about 4.5A. There still seems to be a loud buzzing/sort of ticking from the transformer when under a load of 2A or more. The errata said the ticking was caused by overshoot in the pre-regulation circuit and the mods suggested should solve the problem. Monitoring the main filter cap (C26) with an oscilloscope shows the cap is being charged every cycle (actually 2/cycle), which seems correct. My guess is the buzzing/ticking is still occuring from too much amplification in the regulation circuit. Is this correct and what should I do to reduce this or has anyone else still had this problem after implementing the suggested change? The schematic of the change is attached - any thoughts on what could be done to fix this problem? Or has anyone else successfully built this?
Errata attached - see link for schematicPS schematic

rckbrwn 29th May 2006 07:08 PM

Elector PS

Have you looked at the waveforms on the primary side of the transformer while the the problem is occuring? You will need an isolation transformer for the ps or the scope or both. Is the sound a slow ticking or line freq buzz, steady or irregular, changes with load or not. Is there some half cycling taking place? (consecutive pos or neg half cycles on the primary without the corresponding opposite cycle)
Have you posted a question on the Elektor forum?


smithy666 30th May 2006 12:59 AM

Thanks for the reply. No I haven't checked the primary side of the transformer (just getting around to buying an isolation transformer to safely do it).
The sound is more like a ticking/crackling than a mains buzz, is steady and it increases in volume as the load increases. It is exactly the same on both channels. With small loads (say 1 amp) it isn't really audible, but as you increase the load, the volume increases considerably.
I'll do some checks on the primary when I get the isloation transofrmer in the next day or so and see if it is doing what you say.
I posted the problem on the Elektor site some time ago but haven't had any response.
Thanks, Chris

smithy666 30th June 2006 07:01 AM

2 Attachment(s)
Finally got around to hooking up an isolation transformer. The attached photo shows the triac output waveform (leading into primary of transformer). Vertical scale is 50V/div (10:1 probe), horiz is 2ms/div (we are 50hz). The isolation transformer was buzzing when a small load was applied to the power supply. As mentioned before the transformer in the power supply makes a ticking sound - it gets louder as you increase the load. The supply works fine - 0-44V and 0-4.5A, but I think the pre-reg circuit isn't working properly. Any thoughts?

smithy666 30th June 2006 07:20 AM

2 Attachment(s)
2nd photo added - this is the output of the triac with no load

rckbrwn 1st July 2006 10:52 PM

ticking sound
Can you correlate the ticking with a change in the waveform? Use a slower sweep speed and see if you are losing an occasional + or _ half cycle. Can you see any waveform change anywhere in the circuit that correlates to the ticking? What is the P.S. voltage setting for the scope photos? Is the scope probe directly across the xfmr primary? The first photo looks like it's across the triac. It would seem that the buzzzing is normal, given the fast rise times when the triac turns on.
The Nov 91 issue had a dissipation limiting circuit that operated on the secondary side of the transformer and seems much easier to integrate into a p.s.


smithy666 2nd July 2006 11:47 PM

2 Attachment(s)
Thanks Rick. I've attached a CRO photo of the waveform across the primary of the transformer with the PS set at 7V. In the next post I've atached one with the PS set at 20V. The ticking starts when the power supply is set at 1V (with 8R load) and increases in volume as the PS voltage is increased. It probably correlates with the mains cycles (perhaps 100Hz). If you have a look at the photo attached to this post, there seems to be overshoot or a spike at the start of each half cycle - could this be causing it?
Thanks for the tip to Nov 01 - I've had a look at this at it seems like a much better solution, which I could easily retrofit into this PS - ie, dump all the preregulation circuit and insert the disspation limiting circuit between the bridge and the cap. Definitely a serious option if I can't get rid of this annoying noise.

smithy666 2nd July 2006 11:48 PM

2 Attachment(s)
Second photo - PS at 20V (8R load - about 50W load)

rckbrwn 3rd July 2006 02:42 AM

ticking or buzzing?
When you refer to ticking and buzzing, are you refering to the same thing? I thought you were refering to 2 distinctly different sound patterns. Does it happen with the voltage set for maximum with 1-2 amps drain?
I am a bit confused about what the photos represent. You said the first set is the output of the triac and the second set is across the primary. In both cases the reference point should be the low side of the line. (the bottom terminal of PK2 on the schematic) Is that correct?


smithy666 3rd July 2006 03:16 AM

Sorry, the sound is almost like a cross between ticking and buzzing. It's not like a transformer humming under a large load, but has a more distinctive tick/buzz at a quite a fast repetition, which gets louder as the load increases.
I have an 8R load, which at approx 35V & 4.5A still makes the same noise. I would need to hook up another load to test 1-2A at max voltage.
On the CRO photos, the last 2 are measured between the common side of R29 (CRO ground) and the L1 side of R34 (can't easily connect to other isde of inductor).
The first photos I posted were with one connection only from the CRO to the L1 side of R34 (ground of the CRO probe wasn't connected). Does this help?

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