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I implemented a home brew mains line filter based on a Jon Risch recipe with two transformer and it is absolutely fab. The trannies are both dual primary and secondary, and I wired then in series which allowed me to ground the "center tap" on transformer # 2 ‘s secondary and get balance power. Cannot be happier.
Now I want to repeat but I want to use three identical transformers. All are dual primaries, dual secondary's. 110/220 input, 15v secondary (albeit 7.5 or 30(?))
I want to it this way because when I wire in series my VA rating drops to 50%; hence one amp wired in parallel will handle the same amount of current as two identical ones wired in series. Waste not right?
I'll use the one transformer as the input, which I will wire in parallel as specified for 110v mains. THEN, I want to use wire both trans#2 and trans#3 in series and connect both of their psecondaries (making their secondaries primaries) to the secondary of the first. That way I can "center tap" the outputs of both trans #2 and #3 and get balanced power out of both.
So it's supposed to look like this :
115v parallel input Trans#1, parallel out 15v -> series input Trans#2, series out 115v
-------------------------------------------------------> series input Trans#3, series out 115v
Now in layman's theory it should work fine. I am wondering however if the three transformers may start some shenanigans if they are wired together? Do I need to isolate the two secondary transformers from one another?
I'm planning to use a 5 watt 270ohm resistor and about 10uF capacitance across the secondary's of the first trans. In the two transformer model this is working well for me.
Am I blowing smoke or will this arrangement hold up?
Thanks
Cleet
Follow Ups:
It will work for small current draw.
However, there are three effects that increase the voltage losses:
1) with two transformers in series, the losses add.
2) Operating at half rated voltage, the second transformers still have the same resistances, so to match losses you need to operate at half the current as well as half the voltage, hence 1/4 the VA rating. At full current (perfectly safe) the losses are doubled.
3) Normally a transformer turns ratio is adjusted to maintain rated output voltage at full power - the voltage at low power is normally a bit higher. This makes the losses easy to ignore in simple applications. But the backwards second transformers will cancel that effect.
Transformers with dual 60v secondaries are available; why not use three of them?
Hi PaulI want to put a bit of capacitance across the secondary of the first transformer for filtering, so I want to use lower voltages in the < 25v or so range. I'm using trans. with 15v secondaries mainly because I have them on hand.
Ok, I'm a little electronically challenged. Are you saying that given my application, a trans rated at 300VA would really be safe only to a 1/4 of that? i.e 75VA?
I don't think that would be an issue for me as I will be using a Seduction and a Smash, but it's good to know for the future. I'll make sure I fuse it accordingly.
Regards
cleet
Edits: 08/04/14
It will be perfectly safe with your original plan, it's just that if you try to draw the full 150VA per output, the available voltage will sag significantly.
In most cases, the available power line voltage exceeds the nominal 120v specification, so this may or may not be an actual problem. It is certainly not a problem with low current draw such as you propose!
I understand. Thanks for your response Paul.
I currently have a similar device, two transformer with balanced and on the outputs I a couple of cap - 100uh INDUCTOR - cap. My little seduction absolutely just loves this beyond words. Things are coming out of my vinyl collection that I ahve just never ever heard.
I have a question. If I filter mains with a cap-inductor-cap, what type of cap will work best? Would a mylar cap be a good choice? I used ceramic caps I it sounded better with them out the circuit.
And I do know that on unfused mains one needs to use only main certified caps (X2 and Y2) and nothing else, but I am talking a fused unit and the cap-inductor-cap behind a transformer. So no worries I'm going to burn my house down! :-)
thanks
Cleet
For an RFI filter, you want low ESR even at high frequencies, hence ceramic, mica, or polypropylene. Keep the portion of the cap leads that is shared between input and the choke as short as possible, approximating a "four-pole" cap.
1) Your total VA capacity will still be limited to the VA capacity of T1.
2) You need to make sure windings in parallel are phased. Because if they aren't it's as good as a dead short.
Hi Gusser
The first trans (trans#1) will be wired in parallel (as it designed to be for 110v). It's rated at 300VA so that will stand.
The other two will be wired in series, as it is supposed to be for a 220v mains. Therefore it will only be capable of half the current. Given I will be at 110v, it means they are effectively only 150VA each.
The reason I do this is that by wiring in series I get a center tap which I can ground and then have balanced power. So trans #2 and #3 and sacrifice capacity for that center tap.
Trans #2 and #3 outputs are not going to be connected. Each will go to a seperate outlet on the box. I want the outlets to be isolated from one another.
Thanks
cleet
OK, I do a similar thing myself.
I have a 10kva 480v to 240v transformer that I run with 240v in and get 120v with a center tap. I have all AV electronics in the house on this balanced power sub system. I can get 6kva in theory as building transformers can typically take 150% load all day long.
Looking back I'm not sure if the balanced power really make that much of a difference. Some data I have read is a 10db noise floor drop, which some people don't think is worth the effort.
But the big transformer does make an excellent surge suppressor.
(Yes, I do realize a wired in balanced power system is not legal code wise in a residential setting.)
"Yes, I do realize a wired in balanced power system is not legal code wise in a residential setting."
I could be mistaken, but I thought there was some provision in the code for balanced power.
It requires double-pole breakers, since both current-carrying conductors are live.
Yes, there is, article 642 IIRC. But it also specifically states such an installation only in industry specific commercial facilities.
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