Blocking Mains DC Offset

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mot

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Joined 2012
davidcthomas.co.uk
#1
Hi - This is my first thread on the forum.

I have installed a 4.5KVA toroidal transformer I use to supply my audio system - to reduce the pretty high UK local supply of 240/247 to nearer 230 Vac.

The core hum produced by the transformer can be quite loud, at other times almost silent.

I have read that DC in the mains can exacerbate this noise and it may be possible to block the DC voltage to reduce hum using a circuit such as …..

http://sound.westhost.com/articles/xfmr-dc-f8.gif

Presumably with components capable of handling higher currents.

My question is would this circuit work in my situation and what value components would I use?

I would be grateful for any advice.
 
#2
The capacitor values would be the same, but you would need to make sure you choose ones with a high ripple current rating. I would also change the diodes to higher current devices such as Fairchild FFPF15S60STU or Vishay ETH1506FP which are in isolated TO-220 package.

Are you on a supply with local generation? 247 volts is above tolerance.
 
#10
mot said:
Continuing from the above post..

I use a thermistor to control the inrush current to the transformer at the moment. Would this inrush control work OK with the DC blocking circuit as well?
Click to expand...

Yes.

(in your post above you mention a 4.5kva tranny. Not sure if your using it as an auto-transformer or conventionally taking the output from an isolated secondary. Whatever, 4500va is huge and it will have seriously low impedance of the windings and consequently very large inrush and "shifts" in current with mains fluctuations)
 
#11
Thanks for your reply

Yes it is pretty hefty, 20Kg ish.

Primary: 0-242-247V @ 50Hz
Secondary: 0-230V off load, estimated secondary resistance 0.15 ohm
Regulation: approx 2%
Estimated dimensions: 240mm diameter away from leadouts x 120mm high
Mounting: M12 clearance hole in a potted centre
Audio grade construction

The trans primary is connected directly to the mains via dedicated consumer unit, secondary to audio circuit, so I guess it is the conventional method. I dont think it is an auto-transformer, as it does have primary and secondary windings.

I switch in the thermistor when switching on the supply. I have the option of switching out the thermistor after start up.

going back to my earlier post (8) would the components handle the current for the tranny?
 
#12
mot said:
Thanks for the replies.

Richie00boy
I thought of using these components - as I wanted to ensure I did not limit the current.

MAL210119472E3 - VISHAY BC COMPONENTS - CAPACITOR, 4700UF, 100V | Farnell United Kingdom

FR30G02 - GENESIC SEMICONDUCTOR - DIODE, RECTIFIER, 400V, 30A, DO5 | Farnell United Kingdom
Click to expand...

I wouldn't use a stud mounting package as whatever you mount it to will be at mains potential. Stick with what I have recommended, they are more than up to the job.

I appreciate that you have the transformer, but your mains is well within spec and it is simply not necessary, and as Mooly has pointed out creates more problems than it solves.
 
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#13
mot said:
........... going back to my earlier post (8) would the components handle the current for the tranny?
Click to expand...

I would say a guarded yes. Any thermistor could be the most problematic because of the huge inrush and the time span (whatever that may be) that the inrush lasts for. Seems a pity to have to spend so much on caps to get the current rating. Even a 6.3 volt cap is OK when limited by diodes.

Its all very demanding... a dedicated consumer unit with a low impedance feed makes for worst case. Normally even small wiring losses etc all contribute to limit ultimate surge currents.
 
#14
mot said:
Hi - This is my first thread on the forum.

I have installed a 4.5KVA toroidal transformer I use to supply my audio system - to reduce the pretty high UK local supply of 240/247 to nearer 230 Vac.

The core hum produced by the transformer can be quite loud, at other times almost silent.

I have read that DC in the mains can exacerbate this noise and it may be possible to block the DC voltage to reduce hum using a circuit such as …..

http://sound.westhost.com/articles/xfmr-dc-f8.gif

Presumably with components capable of handling higher currents.

My question is would this circuit work in my situation and what value components would I use?

I would be grateful for any advice.
Click to expand...
What audio amplifier is fed with that transformer?
You know how many amps eat at idle that transformer? If you know, in the next second you sell, but I think you do not know.
 
#16
I take your point richie00boy, however a power supply burnt out on a piece of my equipment, the manufacturer said it was not designed to operate with voltages much over 240. Mine does creep up to almost 250 at times.

Thanks Mooly for your help - it seems difficult to find low cost caps with 15A or above ripple current. Am I wrong in thinking it is a good idea to have a low impedance mains supply?

addressing the other points, the transformer noise is present with and without load, it supplies all the equipment in my audio system.
 
#18
mot said:
Thanks Mooly for your help - it seems difficult to find low cost caps with 15A or above ripple current. Am I wrong in thinking it is a good idea to have a low impedance mains supply?
Click to expand...

😀 Well, not on the face of it.

It was a fashionable and recommended option for many years with reviewers claiming great improvements etc etc. Nothing new there though 😛

And what do we do now... we add a slow start (I'm talking ones that are permanently in circuit) that raises the impedance many hundred fold. I tend to look at PSU's purely from a technical perspective with the view that it doesn't really matter what goes on upstream of the wall socket as long as the supply in the equipment meets the correct requirements.
 
#19
Yes on both counts - CE 230v only.
Thing is, I bought it second hand and although I went and tried it before buying, it started playing up after a month or so. CD mec kept skipping tracks, and it ran very hot. I think the seller knew there was an issue.
Had to send it to Germany to get it fixed.
It is working as it should now but an expensive experience.
 
#20
I thought that the harmonised voltage applicable to all products sold in the EU were rated for correct and safe operation on the full range of 220/240Vac i.e. from 216Vac to 254Vac.
And this would be indicated by the CE mark with the 230Vac rating that you quote.
 
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