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-   -   rectification and xfmr stress (http://www.diyaudio.com/forums/tubes-valves/64325-rectification-xfmr-stress.html)

beamnet 15th September 2005 06:57 AM

rectification and xfmr stress
 
Hi

I was wondering. Does the way you rectify have influence on transformer heating?

I have a tranny that has 2x 2A 7-3.15-0-3.15-7V winding for heating of output tubes.

With the use of the 6.3v ac for heaters it was very fine.

Now i changed the output tubes to 12,6 Volt types.

I made 4 boards with a diode for rect and a regulator with diode for 12,6 volt

The tranny is getting quite warm now.

Ofcoarse i am asking a little more for it, but it seems really exaggerated.

Now dies the "single diode" rect put great stress on the amp? Would it help if i connected two heaters out-of-phase? that way the tranny really sees a full wave rect.

Anybody?

Geek 15th September 2005 08:43 AM

A schematic with component values would help.

The only time I noticed any tranny heating up nastly, was with an outrageous amount of C immediately after the rectifiers.

amperex 15th September 2005 09:54 AM

With a rectifier is feeding an input filter cap, the high peak to average current charging the capacitor is more stressful on a transformer than a choke input filter or simply an AC load.

ilimzn 15th September 2005 10:15 AM

Re: rectification and xfmr stress
 
Quote:

Originally posted by beamnet
I was wondering. Does the way you rectify have influence on transformer heating?
...
Now dies the "single diode" rect put great stress on the amp? Would it help if i connected two heaters out-of-phase? that way the tranny really sees a full wave rect.
With a single diode rectifier for anything but very low current (eg. bias supply) your transformer sees a net DC current, which saturates the core, increasing considerably the transformer idle current because the effective inductance of the transformer windings becomes quite drastically reduced. You need a full wave rectifier.

You can esily use a full wave rectifier for every tube but as a stop-gap measure, you can put pairs of tubes in antiphase (as long as the tubes in a pair are the same, of course).

Using a transformer with a difference between half wave currents produces core saturation, and also, a much larger stray field, resulting in more hum.

beamnet 15th September 2005 10:42 AM

many, many thanks! I'll drop in some bridge rects immediately!

ilimzn 15th September 2005 03:36 PM

Do not forget that, all other things being equal, peak winding current is a limiting factor for a transformer. This is because the current is what creates the magnetic field, and also, when excessive, saturates the core.

Given an ideal rectifier and a constant load, increasing the filter capacitor after the rectifier increases the peak current, and decreases the conduction angle of the rectifier. This is why you cannot simply increase capacitors to any value - the increased peak current means an increased magnetic field, If the increase is enough to saturate the core, during that angle the transformer becomes ver 'bad' - coupling lowers and ditto inductance. The primary and secondary start looking more like their DC resistance instad of inductive and the heat losses increase a lot.

Often, capacitors are increased to provide better filtering. Speciffically in tube circuits, there will often be a resistive component after the first filter cap, followed by another filter cap. In these situations, when flter caps are increased in value, it is advisable to split the resistance intotwo, and insert one part of it between the rectifier and first filter cap. This will considerably reduce the peak current, and also increase the rectifier conduction angle.

A mistake often made with SS rectifiers involving peak currents is that increasing the peak current increases rectifier recovery. The obvious result is an increase in rectifier heat (often quite dramatic) and a not so obvious is an AC component to the capacitor ripple current, producing a 'crest' of sorts on the ripple voltage, which increases the effective ripple. Also, it stresses the capacitor considerably more. Often there are volumes written about the advantages of vacuum rectification but in reality, if the simple mistake above is avoided by adding a resistance in series with the rectifier (much as a vacuum diode adds it intrinsically), most if not all of the 'objectionable' SS rectification problems will dissapear. The remaining advantage of vacuum rectificiation, that being essentially no recovery time, becomes far less of an issue, if not even a non-issue. In any case, it is then possible to get practically vacuum-like results using fast diodes - with a considerable saving in power that would have been used for a vacuum rectifier's heater, not to mention the saving in price. This is really a simple trick which for some reason I don't see that often. It can also be used to great advantage in DC heater supplies, since these often have a CCS or a series resistor in there somewhere.

beamnet 15th September 2005 04:59 PM

Ok, i'll shovell the ps for the heaters out.

so i have two 7-0-7V windings, 2A rated ( know that's a little underestimated, think it might be able to provide 50% more)

Now i have two issues. Full wave or bridge? Simulating it does not seem to make that much of a difference.

I have 2200uF after rect and a 7812 (with raising diode)

The heater draws 800mA max

I have two heaters.

Is that too much of capacitance?

I'm learning a lot here on topics i have allways seen too easily

Gabevee 15th September 2005 05:52 PM

I am confused...

Are you using rectification for the heaters only? Or for both heaters and B+?

If using for heaters, there should be little to no difference in the power transfer, since 12 volt tubes draw less current than 6 volt tubes, generally.

I think it would be best to use bridge rectfier and not use the center tap at all. Since you are using a 7812 regulator, 2200 uF should be fine. Me, I don't use any regulation and get a nice solid 6.3 volts. I use 10,000 uF for filtering, though.

Besides... this is just simple low voltage AC-DC conversion, done daily by most electronics today. Why would you have trouble there?

However!

For B+, that is another issue. You MUST use a current limiting resistor from the center tap to ground. I use 220 ohms, 20 watts. Without it the current draw from the secondary will not only heat up the transforrmer, but cause it to hum loudly, and eventually burn it out. Otherwise, a 100uF cap for filtering, then a choke, and anything you want after that (I use from 80 to 330 uF) and hum will be just about the same level as noise (about 5mV P-P).

My :2c:

Gabe

beamnet 15th September 2005 06:30 PM

we are talking only heaters here.

i used 4xkt88. 6.3 volts AC, 1,6 amps per tube-> no excess heat

switched to GU50 tubes, 12,6V 800ma

the xformer has 7-3.15-0-3.15-7volt windings, so 7-0-7 yoelds 14 volts ac. Rectified with one diode it drives the xformer incredibly warm.

So which is kind of logical, as the xformer has to produce all it's powe in a half wave. Plus it gets all these nasty spikes from the rect.

The question left was: which is better? full wave or bridge rectification?

what would be the pro's and cons for both systems?

I've build lots of small power supplies, but now i have to consider the max rating of the xformer

Tom Bavis 15th September 2005 06:53 PM

With a full wave rectifier, you'll decrease the RMS current in the secondary and eliminate the DC component flowing as well and considerably reduce supply ripple... Win-win-win.


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