Mains filtering for a digital equipment.

AndrewT · 2015-06-04 9:52 am

I post this

AndrewT said:
............................You need a snubber to damp the circuit and minimise the ringing.

A snubber is a resistor to absorb energy (and generates heat) when current passes across it.

Then you ask this

henryspencer said:
.................................Should one avoid very low ESR dielectics like polypropylene?...............

ESR is equivalent series RESISTANCE.
If one adopts polypropylene will it still behave as a snubber?

henryspencer · 2015-06-04 10:01 am

Thanks for the pointers will have a look!
I guess it's hard to avoid being a heavy contributing member since 2004

henryspencer · 2015-06-04 10:06 am

Was just trying to collect info in one post for easy reference to all level users. Sorry if i upset.

mrWagner · 2015-06-04 11:23 am

You may also try to put RC filter in the PSU. That will also reduce the voltage slightly.
Put some beafy 5-10kuF cap after the diodes, then a 1Ohm 5-10W resistor, then connect it to the board.

henryspencer · 2015-06-04 11:27 am

The 7V 15VA transformer is good for this project no need to buy another one. 6V might be slightly more optimal, less power dissipation on the regulator yet table operation but probably not worth the money/effort.

ginetto61 · 2015-06-04 11:39 am

henryspencer said:
The 7V 15VA transformer is good for this project no need to buy another one. 6V might be slightly more optimal, less power dissipation on the regulator yet table operation but probably not worth the money/effort.

Hi and thanks again.
No way to find a 7V or 8V EI transformer ... no way. Values jump from 6 to 9.
It seems that 6 and 9 are more standard value.
Ok ... just caps and maybe some mains filters.
Regards, gino

ginetto61 · 2015-06-05 8:30 am

Hi and excuse me if i reply to myself ...

but i have got an important confirmation from some experimenters that a toroidal placed very close to the digital chips is very very bad.
So the idea of an external PS with umbilical DC cable to the unit is not insane, actually the contrary.
In this way the transformer, that has always some EMI leakage, will be shielded.
I wonder if an aluminum box is a good shield ... do not think so but i am not sure.
Is steel better ?

Thanks a lot again, gino

henryspencer · 2015-06-05 1:16 pm

Stray field from that 7V 15VA can't be very high. If you want external box one tip is to get a cheap aluminium box from ebay or aliexpress.

ginetto61 · 2015-06-05 1:28 pm

henryspencer said:
Stray field from that 7V 15VA can't be very high.

Hi ! there are some people modifying the same unit in order to ger better performance. One has placed some this around the transformer

AB-5100 X 8.26" 3M | 3M5100D-ND | DigiKey

and he says there are improvements in sound.
I still would prefer the only source of "possible" EMI out of the small box with circuits ... ok there will be another box but still ...

If you want external box one tip is to get a cheap aluminium box from ebay or aliexpress

I will do that for sure. I could also buy some of the 3M sheets and place them on the panels of the circuits box.
I will post some pics as soon as i will receive the pieces ...
Thanks a lot again, gino

henryspencer · 2015-06-05 1:43 pm

The tape is not very good emi protection. Distance is what is needed if it affects the circuit, extra box even better. Here is a cheap one, but not very pretty

1pc Retex Box Aluminium Enclosure Minibox RM 14 125x60x205mm 31040014 | eBay

ginetto61 · 2015-06-05 2:50 pm

henryspencer said:
The tape is not very good emi protection. Distance is what is needed if it affects the circuit, extra box even better.

Hi and thanks a lot again.
This confirms my sensation ... those guys are good but low in tech education.
I had this feeling. EMI and maybe even more RFI are disastrous for digital.

Here is a cheap one, but not very pretty
1pc Retex Box Aluminium Enclosure Minibox RM 14 125x60x205mm 31040014 | eBay

thanks a lot for the very helpful advice.
I have to wait for all the pieces i bought to see the needed size for the box.
As i said i still would like to reach the box with a DC umbilical, even not regulated and leave the regulation duty to the regulator on board.
But i have also bought this one ... i do not know if i did right

http://www.ebay.com/itm/371189087840?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

i have been intrigued by the promise of

Output Noise: < 40uV RMS (10Hz to 100kHz)

I do not know if it is just marketing or what.
Thanks again, gino

henryspencer · 2015-06-05 3:18 pm

Since you bought it the best way to find out is try it

If the components are original and good quality you got a decent regulator, easy to adjust via trimpot. There's a lot of fake components/sketchy PCB layouts on ebay, don't know anything about this seller but watch out... LT1963 has low dropout so the 7V transformer will still be good, 6V out it enough..

ginetto61 · 2015-06-05 4:20 pm

Hi again ... yes the fakes are around the corner.
but i have also bought 2 new transformer 9+9 of about 40-50VA
I wonder if using just one secondary and leaving the other open and unused could damage the transformer. The stock one is just 15VA total.
Regards, gino

henryspencer · 2015-06-05 4:40 pm

I doubt that it will shorten the lifespan of the transformer, haven't seen anything to the contrary but can't say 100%.. Why not parallel connect the secondaries? .. The 7V 15VA has plenty power for this application no need for anything larger.

ginetto61 · 2015-06-05 4:48 pm

henryspencer said:
I doubt that it will shorten the lifespan of the transformer, haven't seen anything to the contrary but can't say 100%.. Why not parallel connect the secondaries? .. The 7V 15VA has plenty power for this application no need for anything larger.

Hi ... because i am worried to exchange the + and - of the two secondary

An externally hosted image should be here but it was not working when we last tested it.

Here the transformer has two primary windings and two secondary windings, four in total. The connections to the primary or secondary windings must be made correctly with dual voltage transformers. If connected improperly, it is possible to create a dead short that will usually destroy the transformer when it is energized.

This is enough for me to try with just one secondary connected and the other left open/unused
and see what happens

Looking at the secondaries if their terminals are not market + and - what would happen if i connect the + of one secondary with the - of the other and the same for the other terminals ?
Instead using just one secondary i will not have the least problem for connection.
I have never understood the phase issue ... never.

Regards, gino

henryspencer · 2015-06-05 5:12 pm

The +/- labels are a bit missleading since it's AC. Is it any label on the transformer indicating the "phase dots" to pin number? If not, what brand and model search for datasheet.

ginetto61 · 2015-06-05 5:21 pm

Because this is quite clear

An externally hosted image should be here but it was not working when we last tested it.

but what i have to do if the terminals are not marked + and - ?
How can i identify them ? this is the problem ... always.
Thanks again, gino

ginetto61 · 2015-06-05 5:28 pm

henryspencer said:
The +/- labels are a bit missleading since it's AC. Is it any label on the transformer indicating the "phase dots" to pin number? If not, what brand and model search for datasheet.

Hi ! bought another one little but very nice ... and with terminal marked and very good datasheet. These people are serious !

Chassis Transformer 20VA 7.5V+7.5V

http://www.ebay.co.uk/itm/291216912314

http://www.spiratronics.com/data/4430.pdf

this is exactly how i like it.
EI type with primary and secondaries separated for a good HF mains noise blocking
I do not know why but in lab grade power supplies i have always seen EI type, in some cases exceptional (i remember a HP power supply with a huge EI. Beautiful)
now i have to stop a little and wait for the pieces.
Thanks a lot again.
Kind regards, gino

henryspencer · 2015-06-05 5:33 pm

Check for a label on the transformer, the + in the picture is labeled as a small dot on the transformer. If there are no labels on the transformer search for brand name and model name on the transformer, then google for datasheet that might provide phasing dots to pin number. Please post pictures of the transformers

ginetto61 · 2015-06-05 5:58 pm

henryspencer said:
Check for a label on the transformer, the + in the picture is labeled as a small dot on the transformer.
If there are no labels on the transformer search for brand name and model name on the transformer, then google for datasheet that might provide phasing dots to pin number.
Please post pictures of the transformers

Hi and thanks a lot again for the very kind support.
On the basis of your advice i have decided to buy from now on only branded parts and with datasheets.
As i said above i have bought another one from what appears to be a very serious UK manufacturer and terminals should be clearly marked.

Chassis Transformer 20VA 7.5V+7.5V

I should not miss them.
I am obsessed by transformers and capacitors above any other component.
It is where the energy is transformed and stored ... is like the engine in a car.
Often i have had the impression that very good coomercial equipment are held back but substandard power supplies
This is what i will do when retired. To revamp and modify pieces of hifi in the strategic parts.
On principle i think that any cheap commercial design can be modified with some positive improvements. Any.
Thanks a lot again.
Kindest regards, gino

Mains filtering for a digital equipment.

R.I.P.

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