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Old 15th Nov 2018, 12:20 am   #1
bikerhifinut
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Default Another transformer query.

Ok here goes,
Assuming I can get a toroid wound to my specifications, what secondary voltage do I need to attain a DC voltage of 450V at 300mA? Assume a voltage drop of 15 volts across the choke. This will be 5H, DC resistance 105 ohms but using 2, that is one each channel after the reservoir capacitor so its 150mA through 105 ohms.
I will use a standard silicon bridge rectifier using ultrafast recovery diodes.
reservoir capacitance will be 200uF.
My rough calculations suggest that 330V will give 470V off the rectifier. However that is nearly what voltage the mains transformer on my KT88 amp is, rated at 325V @ 750mA and that gets me 420V on the output CT after the choke, but I do not know what the resistance of it is so I guess it could be dropping 30 odd volts at 300mA.
I also think I should specify a current of at least 500mA using the 0.62 rule as i cannot assume the current rating is the DC one?
I need to get this one right. No problems with the lower voltage windings, just the HT.
I thought this would be easy but theres a lot more to it when i dug deeper.
My feeling is 340V is about right, bearing in mind the mains here is usually 250V+.
I can specify the primary for 250V but decided to stick with 240 unless i can get it with an extra tap to be able to choose between 240 and 250 although i have no overvoltage issues with my current 240V mains transformers on the valve heaters.
Ta

Andy.
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Old 15th Nov 2018, 2:11 am   #2
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Default Re: Another transformer query.

Given it some thought and I think I will stick with the 350V secondary.
But what I shall definitely do is wire the reservoir capacitors in series-parallel, i.e. each double capacitor in parallel as they have a commoned -ve terminal and then each pair in series and then parallel 2 pairs. Not forgetting balancing/bleeder resistors. Sounds complicated but that gets me a 100uF reservoir rated at 1kV which will give them an easier life as they are 500V rated and running them at 470 odd volts off the rectifier is probably way too close for comfort. I could try and source higher voltage capacitors but I'd still be a bit iffy about running them that close to max.
From previous experience with the circuit I know the EL34s will work just fine with a bit extra HT as they are still being run well below their maximum limits on both current and voltage.

Andy.
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Old 15th Nov 2018, 8:22 am   #3
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Default Re: Another transformer query.

I'd source higher voltage capacitors. Having large electrolytics with their cans floating at high voltage is dangerous. It's common in kilowatt RF amplifiers, but unexpected in audio amplifiers. The audio world is full of people looking for valved amplifiers and many of them have no understanding of high voltages. If, in the future, it gets into the wrong hands it might be deadly.

You'll need to fit voltage sharing resistors of a value to dominate leakage currents over the lifetime of the capacitors. Less obviously, you need to consider the turn on transient interacting with the tolerance of the capacitor values, and that brings the balancing resistor values down further.

David
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Old 15th Nov 2018, 9:46 am   #4
Ed_Dinning
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Default Re: Another transformer query.

Hi Andy, at these voltages and currents you will need to check the winding resistances of pri and sec. These will give rise to voltage drops on the output which will vary with the current drawn. Your transformer may have had compensating turns added for this; if so the open circuit sec voltage will be higher than expected.
You may also need to include the effects of LV sec VA on the primary volt drop.
Not an easy calculation, but some of the circuit simulators available can do it quite accurately.

Ed
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Old 15th Nov 2018, 9:56 am   #5
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Default Re: Another transformer query.

The 'simple' transformer, diode, capacitor circuit is one which does not yield to analysis. Numerical methods are necessary.

These days, you can at least make an assumption that the capacitor voltage is constant and hardly varies during the cycle. This wasn't so in the past, when significant ripple across the rather-low capacitors available added an extra complexity.

As a rough guide, on-load, with 'realistic' transformer winding resistances for the load expected, you can work on a basis that Vdc = 1.3 x Vac rms.

Do a Google search for O H Schade analysis rectifier circuits - this will take you through by graphical means. But as Ed says, you'll need the series resistances (which play a big part in the DC voltage attained).
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Old 15th Nov 2018, 10:28 am   #6
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Default Re: Another transformer query.

As Kalee20 says you can do a lot worse than reading Schade's seminal paper on rectifier circuits.

"Analysis of Rectifier Operation", O H Schade, Proc IRE July 1943, 341-361

Still copyrighted so difficult to find a free copy.

This uses analysis and a series of nomograms for capacitor and choke filters fed by valve rectifiers. Although it also works well for semiconductor rectifiers too.

That was reprised in Linear Audio Vol 8 "The Otto Schade method - A practical design method for rectifier circuits" by Rudolf Moers. He built circuits and measured parameters to work out if there were discrepancies in the original Schade paper. He found only minor and inconsequential differences. https://linearaudio.net/article-detail/2216

The alternative is PSUD2, which I use to check power supply designs. Free http://www.duncanamps.com/psud2/ .

Craig
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Old 15th Nov 2018, 11:00 am   #7
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Default Re: Another transformer query.

You could consider a voltage doubler with the transformer connected to the capacitors series common and two diodes in series.

You will not need any balancing resistors and will get 100Hz ripple.

A choke input filter is easier on the mains transformer.
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Old 15th Nov 2018, 11:05 am   #8
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Default Re: Another transformer query.

I just chuck things like this at LT spice. Just build a model transformer with winding resistances, inductances and leakage inductance. Diode models of all sorts are on the interweb.

LT spice is free and works well. It's worth learning to drive it, then you can use it for other stuff.

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Old 15th Nov 2018, 11:25 am   #9
Craig Sawyers
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Default Re: Another transformer query.

Most mains transformer specs are silent on leakage inductance. And it is only really mildly relevant for dual bobbin EI units. Toroids have exceptionally low leakage inductance (but very high interwinding capacitance).

Sure you can do things with LTspice, 5Spice, TinaTI etc. But PSU2D is custom designed to do all of that. The only thing to watch is that the winding resistance is referred to the secondary, so you have to remember to reflect the primary resistance to the secondary using voltage ratio squared and add that. Other than that it calculates a table of values and graphs all voltages and currents in the circuit as a function of time - you select which value(s) you want to graph.

Craig
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Old 15th Nov 2018, 12:53 pm   #10
bikerhifinut
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Default Re: Another transformer query.

OK.
Fortunately I already have PSUD2 loaded and a quick simulation, with a load of worst possible scenarios gives me a Voltage off the rectifiers inside their max working voltage.
I hear what RW says, but try and find an electrolytic capacitor with a working voltage over 500V, and when you do they cost rather a lot to get the headroom I'd feel comfortable with. The series wiring of PSU caps is a fairly common practice and as noted in my earlier post, I am up to speed on balancing/bleeder resistors.
I have a couple of 1kV polyprop capacitors at 20uF and something like that would work, and if i need HV caps I will get some polypropylenes at 630V or above.
Bearing in mind I would be buying in the transformer from a reputable manufacturer that specialises in transformers for Valve amplifiers, I'd hope that I'll get in the ball park this time.
No doubt I'll get teething trouble with the new build but as long as I get the mains transformer secondary voltages inside the acceptable tolerances I'll be a happy bunny. I have a fair bit of latitude on the HT to get an acceptable performance from the output pair. My main concern was keeping the voltage across the main PSU capacitors inside their working limits.
Anyway I have a good idea now of what to expect, and a strategy to tackle any issues that arise.

Andy.
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Old 15th Nov 2018, 12:55 pm   #11
bikerhifinut
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Default Re: Another transformer query.

Quote:
Originally Posted by Craig Sawyers View Post
Most mains transformer specs are silent on leakage inductance. And it is only really mildly relevant for dual bobbin EI units. Toroids have exceptionally low leakage inductance (but very high interwinding capacitance).

Sure you can do things with LTspice, 5Spice, TinaTI etc. But PSU2D is custom designed to do all of that. The only thing to watch is that the winding resistance is referred to the secondary, so you have to remember to reflect the primary resistance to the secondary using voltage ratio squared and add that. Other than that it calculates a table of values and graphs all voltages and currents in the circuit as a function of time - you select which value(s) you want to graph.

Craig
Thanks for the reminder Craig, I have that program and it does what it says on the tin so to speak. Just run it up with my parameters and it looks good for my planned circuit design.

Andy
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Old 15th Nov 2018, 1:50 pm   #12
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Default Re: Another transformer query.

I did a run-through of a design some years back using Schade's curves. I wanted 315V 100mA. I used a 5Z4 rectifier; I had available 1" laminations and had a bobbin that could take a 1.75" stack. So calculating winding resistances was possible.

An hour and a half later, iterating twice, I had a 'paper' design. I wound the thing, lashed up connections to rectifier, capacitor, and 3.1kilohm load resistor, wound up the input to exactly 240V, measured the output voltage as the 5Z4 came to life. It settled at 318V.

Less than 1% error. Well done Otto!
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Old 17th Nov 2018, 8:24 am   #13
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Default Re: Another transformer query.

My big amp's HT is at 500v when first SW'd on, then falls to 450v, I'll have a look to see what the AC off the secondary is, think it was 230v.

When I wound my toroid's to be on the safe side and to allow a bit of wiggle room, I added several taps at 10/20v intervals just to be on the safe side.

Andy.
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Old 17th Nov 2018, 10:31 am   #14
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Default Re: Another transformer query.

Instead of say a 350-0-350 winding, you could use 175-0-175 with a bridge rectifier, and the centre tap will give you a stiff 50% voltage to force the voltage sharing in a pair of series capacitors.

David
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Old 17th Nov 2018, 5:34 pm   #15
bikerhifinut
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Default Re: Another transformer query.

I'm sorted i reckon.
had a word by email with Joe bog who made the original toroid.
I just measured it and its got a DC resistance of 7 ohms on the HT secondary and on our home mains it measured 329v so lets say 330 for a round number.

I fed the results in to the PSU software and the voltages come reassuringly and comfortably below the 500V DC rating of the reservoir capacitors, even when i simulated no load across the reservoir. (1 megohm load just to see what the volts rose to off load).

The secondary is CONSERVATIVELY rated at 600mA DC from Joes info i had originally as its designed to supply 8 x EL34 in a parallel push pull stereo amp and I'm only using half that current so it should run very cool. And be a stiff supply.

And Joe has told me I can take some turns off the heater secondaries to get 6.3V rather than the 9V they currently are (measured and confirmed) although I am lairy about getting my unskilled fingers inside the toroid even though the heater windings are on the top. I think they are about 4 turn per volt and I am waiting to find out if winding a few turns in antiphase to the heater windings and put them in series with it will be safe in terms of heat build up etc. i figure 12 turns of thick insulated wire blagged from some hefty ring main cable might do the trick as an overwinding and save me from surgery on it. I am sure there will be something I havent accounted for there if anyone can put me right.

The good thing about this is I have the hardware so Empirical experiments are feasible and I'll be right in the ball park before I actually need to build the amp up.

I have a few more queries but they are less about the electronics and more about cosmetics so will put them in a separate thread.

Andy.

Andy.
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Old 19th Nov 2018, 10:40 pm   #16
bikerhifinut
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Default Re: Another transformer query.

Well I followed the instructions that the winder gave me, and I removed 5 turns off one of the 9V secondaries, the wire gauge is humungous these are designed to heat up 8 EL34 plus a few preamp valves so its ok to only reduce one of the 2 secondaries.
A quick on load test got me 6.5V at the local mains maximum of 252V. That's well inside the tolerance for a valve heater and taking any more off would drop it to below 6V when the mains drops to 240V so I'm happy.
Now all I have to do is wind the Mylar insulation tape back over the thing, easier said than done but has to be done.

Fingers crossed this rework of my favourite power amp, A loosely based take on the Classic Leak TL25+ will be successful. I've learned much over the last few years about layout and reducing noise and hum so am fairly confident that this will be more than acceptable in that respect.

Cheers folks.

Andy.
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