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Is inrush current limiter responsible for quieter power transformer?
Is inrush current limiter responsible for quieter power transformer?
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Old 7th April 2019, 12:27 AM   #1
Evenharmonics is offline Evenharmonics  United States
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Default Is inrush current limiter responsible for quieter power transformer?

I recently built class AB transistor amp without inrush current limiter and it was working with no noise from the speakers but there was buzz coming from power transformer (500VA toroid). Then one day the fuse (4A slow blow) blew upon powering up. So I ordered a kit of inrush current limiter. While waiting for the shipment to be received, I put a temp current limiter using light bulb (150W incandescent). It does its job and no more blown fuse upon powering up. Besides that, it got rid of buzzing noise coming from power transformer. Is it common? If so, the inrush current makes the power transformer buzz during the entire operation?
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Old 7th April 2019, 12:48 AM   #2
Mark Johnson is offline Mark Johnson  United States
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Is inrush current limiter responsible for quieter power transformer?
You could do a very quick experiment to find out more.

Take a test lead with alligator clips on both ends (crocodile clips in Europe) and use it to short out the ICL. Does the unpleasantness return? Remove the short. Does the unpleasantness disappear?

Mechanical buzzing is often blamed upon asymmetric waveshape a/k/a "DC on the mains" and the standard medical treatment is a series capacitor (to block the DC) plus protective super high current diodes in parallel with the cap (to keep reverse voltage small). Maybe the PCB you purchased, secretly includes these components too.
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Old 7th April 2019, 01:18 AM   #3
Evenharmonics is offline Evenharmonics  United States
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PCB was ordered by not arrived yet. What I'm using right now is a temporary ICL using light bulb like the image below.

Click the image to open in full size.
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Old 7th April 2019, 01:43 AM   #4
JMFahey is offline JMFahey  Argentina
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Also think itīs mechanical buzz, "hidden but not really cured" by the series bulb limiter, which does not let transformer reach full magnetizing current.

Not sure a regular inrush current limiter will have much effect, since itīs designed to "put itself out of the way" by reaching a very low resistive value after a couple seconds.
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Old 7th April 2019, 04:30 AM   #5
Ozark HiFi Doctor is offline Ozark HiFi Doctor  United States
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Default Had to comment (I use both)

I have used the series light bulb, for 40~ years... especially for gear that may have been sitting idle for long periods and especially on other peoples amps, to overt potential failure, liability during my use.

It was habit on my Marantz 8b and NYAL Futterman OTL-3. Both of which I struggled to 'update' recap, restore and rebuild. I wanted to save the stock multi-cap in the 8b and finnicky / aged Photo Flash caps in the OTL-3, I was actually 'afraid of them'... another story.

Currently I build, and rebuild all of my amps with IRCL thermistors. I want to try the DC blocking filter, (I have all of the parts) Adding that; to the list of projects...

~ as an aside ~ A buddy, the Rhino'man and I were talking about using IRCL to lower incoming AC line voltage to more design values, in vintage gear. Where I live, I am on a dedicated outdoor power line transformer [a life long dream] me alone, 'q u i e t', and super stiff AC 123.5+ is routine. I like the idea of a slight lowering of the operating voltage ...prospects... along with 'saving' power transformers, rectifiers and caps
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Old 7th April 2019, 05:34 PM   #6
Bonsai is offline Bonsai  Europe
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I think what is happening is that without the lamp in series with the transformer, the core is nearing saturation, or becoming fully magnetized on the mains peaks. When you insert the lamp, even after it has heated up and the filament resistance has dropped, you are getting enough of a voltage drop across it to reduce the peak core magnetization.

I get my transformers wound by a local manufacturer who gave me a bit of advice when I explained that the transformers had to be dead quiet:-

1. specify a 'stabilized core'. This is simply a core in which the long strip of transformer steel is impregnated with varnish before being over-moulded with a plastic insulation case upon which the windings go.

2. Reduce the peak magnetization in the core. This reduces the magneto-restriction which is responsible for the buzzing sound you hear on a lot of transformers. This may result in the core being a bit oversized, but this is audio

3. As Mark indicates, in really stubborn cases, use a DC blocker. See Rod Elliot's site for a very good write-up on the subject here Mains DC and Transformers

I use 500VA and 1200 VA cores in my commercial products - zero mechanical noise.

I have about 10 transformers from various suppliers lying around - some buzz shockingly, others buzz on and off depending on what's on the mains (worst culprit: hairdryer in the middle heat position, or an electric heater, and buzzing seems worse during peak load times like 5:30 pm to 9 pm at night and then in the morning again.
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Old 8th April 2019, 05:50 AM   #7
Evenharmonics is offline Evenharmonics  United States
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I'm in the same situation as far as power transformer buzz goes, sometimes it does and other times it doesn't. After I install the permanent ICL, I'll see if I need to build a DC blocker. Thank you for the info.
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Old 8th April 2019, 09:02 AM   #8
Bonsai is offline Bonsai  Europe
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The ICL per se won’t fix your problem. If it does, it’s through pure luck. You need to do both.
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Old 8th April 2019, 09:22 AM   #9
scottjoplin is offline scottjoplin  Wales
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It's sounds as though he needs a DC blocker if the buzz comes and goes. I had that problem and a blocker fixed it. I have a residual constant low hum from the transformer and I'm tempted to see if a light bulb will cure that, i.e. the core is saturating maybe.
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Old 8th April 2019, 11:09 AM   #10
DF96 is offline DF96  England
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Part of the inrush current is a transient DC pulse. This could magnetize the core, hence making any buzzing worse. Reduce the inrush and you reduce the risk of magnetization.
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