Hammond transformer datasheet - How to read it ?

[kagliostro]

As an example I take the Hammond 1760J OT

there are specs that I didn't know how to interpret

the meaning of some is obscure to me, can someone try to explain ?



- How is to interpret the inductance value ? what means a higher or lower inductance ?

- In the frequency response @35W is also indicated 374.1V, in the primary and secondary impedance are indicated also Voltages

Which is the purpose of this info ? Why they give it, how we must use ?

Many Thanks

Kagliostro

[kagliostro]

Thanks but my questions are about the meaning of the data about:

inductance in H

in the inductance row @240V,60Hz => 90.95H (Open Circuit)

indication in Volt

in the impedance row @35W 374.1V

in the primary impedance row 4000ohm 374.1V

in the secondary impedance row 4/8/16ohm 11.83/16.3/23.66V

why they give the data that I wrote in blue above ?

in which way can I use this data to prefer one transformer from one other ?

Thanks

Kagliostro

[jjasilli]

Re voltage output:  This is different ways of saying the same thing.  Google "ohm table pie chart".  Power Formula:  Watts = volts² / Ohms; 35W = 11.83²/4. 

Re inductance:  Maybe useful for technical design info

[kagliostro]

This is different ways of saying the same thing.

OK, thanks, I can understand

I never remember Ohms Law 



Kagliostro

[FYL]

- How is to interpret the inductance value ? what means a higher or lower inductance ?

Higher inductance = more extended bass response & less current with small or non-existent loads.

Say at 80 Hz, XL = 2 * Pi * f * L = 2 * 3.14 × 80 × 90.95 =  45693.28 ohms

Mo' info with some graphs showing load lines vs. primary inductance : http://www.magnequest.com/tech_article2_voltsecond.htm

- In the frequency response @35W is also indicated 374.1V, in the primary and secondary impedance are indicated also Voltages

Remember Ohm's law?

P = U^2 / R = 374.1 ^2 / 4000 = 34,988 Watts

[kagliostro]

Higher inductance = more extended bass response & less current with small or non-existent loads.

Say at 80 Hz, XL = 2 * Pi * f * L = 2 * 3.14 × 80 × 90.95 =  45693.28 ohms

Many Thanks FYL

Kagliostro

[PRR]

> I never remember Ohms Law 

Maybe you should digest that BASIC relation before looking at the more complicated numbers.

> remember the primary side has dcr of 4000 ohms.  this is DC resistance.

No, the DC resistance of the primary is 185.0 ohms.

4,000 is the suggested working Audio impedance.

OK, the specs. None of these are for design or selection.

The 90H inductance shorts-out the nominal 4,000 ohm impedance at some low frequency, causing bass roll-off. 90H against 4,000 is 7 Hertz! It has more than enough bass response for any audio use.

Why so much? Because before the inductance shorts-out the nominal impedance, there is bass distortion in the iron. If the inductance is high, the bass distortion is less. Push-pull OTs usually have MUCH more "bass response" than we need. What we really might want to know is bass distortion. Hammond does not tell us (though the 70Hz -1dB may be a suggestion not to use this for a 20Hz sub-woofer).

The 90H at 240V at 60H is a Production Test. In US/Canada, 60Hz is handy and 240V is easy to have. This is similar to the voltage and current on the primary of a tube OT. They juice it up on a 240V 60V wall-source and check the inductance. (A check of AC Current will do that.) If it is near 90H then it is healthy. If it is much lower (or much higher) something is wrong. Maybe shorted turn, maybe reversed wires, whatever. Note that the measured inductance of Iron-Core coils varies with both voltage and frequency. Your little 1V 1KHz inductance meter will probably show a much lower inductance.

Why would you prefer this part over any other? Hard to say. This data does not help.

We can see that primary DCR loss of 185 ohms in 4,000 ohms will cause 4.4% loss of audio power. The 0.4 ohms DCR in the 4 ohm secondary is another 9% loss. Overall power loss is 0.86 or 0.6dB. This is fine and normal.

Go by desired impedance (tubes and speakers), weight, suggested use (replacement, super-hi-fi, guitar), quality, and color. Hammond is normally very high quality. This part is generous size for a 35W guitar amp. It might be "too clean"; some well-loved guitar amps have smaller OTs which bend the lowest notes and spray overtones into mid-bass where typical guitar speakers work best.

[kagliostro]

Maybe you should digest that BASIC relation before looking at the more complicated numbers.

Yes, you are right, that is a great handicap I've , I must force myself to memorize the various reports between power, resistance, voltage and current (till now I can remember easily only that I * V = W)

The 90H at 240V at 60H(z) is a Production Test. In US/Canada, 60Hz is handy and 240V is easy to have. This is similar to the voltage and current on the primary of a tube OT. They juice it up on a 240V 60V(Hz) wall-source and check the inductance. (A check of AC Current will do that.) If it is near 90H then it is healthy. If it is much lower (or much higher) something is wrong. Maybe shorted turn, maybe reversed wires, whatever. Note that the measured inductance of Iron-Core coils varies with both voltage and frequency. Your little 1V 1KHz inductance meter will probably show a much lower inductance.

that is an interesting explanation

Why would you prefer this part over any other? Hard to say. This data does not help.

Yes, this data don't help, also because the same Hammond didn't give the same data for all the transformers

see:

1760J  - @240V,60Hz => 90.95H (Open Circuit)

1750T - @120V,70Hz => 6.82H (Open Circuit)

1750V - @240V,70Hz => 90.95H (Open Circuit)

1750Y - @240V,60Hz => 21.22H (Open Circuit)

How can this data be used if they use one time a frequency and another time one other, the same for voltage

I think those data are usable only by very skilled people, don't think everybody can understand the difference between transformers with specs derived from tests with different values

Many Thanks PRR for your help

Kagliostro

[HotBluePlates]

Thanks but my questions are about the meaning of the data about:

indication in Volt

in the impedance row @35W 374.1V

in the primary impedance row 4000ohm 374.1V

in the secondary impedance row 4/8/16ohm 11.83/16.3/23.66V

Start with some assumptions. The transformer you referenced is intended to be a replacement for blackface Pro Reverb, Tremolux, Vibrolux. The assumed output power for these amps is 35w, from a pair of 6L6's.

- From the wheel showing equations, P = V²/R.
- Assume the 16 ohm speaker load, and 35w output. Rearranging the equation, V² = P*R = 35 * 16 = 560. The square-root is 23.66v (RMS).

The same steps are used for other speaker loads.

- Now, calculate the needed primary signal voltage for a given speaker load.
- The primary is 4000 and the secondary is 16 ohms (for our example). Impedance ratio is 4000/16 = 250.
- Voltage is stepped-up/down by the turns ratio, and the impedance ratio is the square of the turns ratio. So we need the square-root of the impedance ratio to see how voltage is changed. The square-root of 250 is 15.81. This is the turns ratio for 4k:16 ohm.
- Calculate the primary signal voltage: 23.66v * 15.81 = 374.06v

We're a little different from the sheet spec, because we rounded in two calculations.

Now, you use this info to select your output tubes (if you're not simply dropping this in to an existing Pro Reverb, Tremolux, etc).

How much current do the tubes need to allow to pass? 35w/374.1v = ~94mA RMS. This is not the needed idle current, but how much RMS current the push-pull pair needs to flow to have 35w at the OT primary. Similarly, the push-pull output pair needs to swing 374.1v RMS when looking at one plate to the other.

Let's cheat:
The Pro Reverb has a plate and screen voltage of 440v, and -51v of bias. Plot this point on a set of curves (the Ec2 = 400v curves on page 7 of the 6L6GC data sheet). Also plot a 1k load line from 440v (0mA) to 0v (440mA).

How far does the plate voltage swing downward until the loadline hits the 0v grid line? 330v. The needed plate swing is 374.1v RMS, which is ~529v peak. But the total voltage swing felt by the primary is the sum of the voltages of the push-pull plates; one is swinging up while the other is swinging down. Our curves only show 1 half of the primary swing. 529v/2 = 264.5v, so the needed swing is well within the capability of our 6L6 half of the output stage.

What about the current? 94mA RMS = ~133mA peak. Since this is a class AB stage, one side will be cut off during most of the time that the other side is one. So each side should be capable of the full peak current. A glance at the plotted curves shows our 6L6 can easily deliver this.

The info tells us a pair of 6L6's at ~440v B+ is a suitable pairing with this transformer to deliver 35w.

[HotBluePlates]

If you repeated all this with, say a pair of 6V6's and assumed 35w output, you'd find that the needed plate voltage swing or plate current swing or both exceed the tubes' capability.

So you'd need to redesign for less power output, or change the B+, or see that a tube's rating is exceeded and select a different tube. Or, you pick a different primary load that suits the tube better. Or you do all these things...

[kagliostro]

Hi HBP

Many Thanks for your good explanations

I'll study it with attention

Kagliostro

[HotBluePlates]

I screwed up regarding the DC and reactive portions of the the data. (not all ohms are equal, just like not all ounces or tons are the same).  I am treading softly because my applications using Ohm's Law whether it is DC or non-DC is limited.

Well, ohms = ohms. However, impedance is the combination of resistance and reactance, both measured in ohms. The combination is done using the pythagorean theorem, if you only care about the magnitude of impedance and not also the phase angle, because the reactive components are +/-90 degrees from the resistive component.

... the impedance of 16 ohms for speaker is determined at a standard 400 Hz, while the 35w power rating is determined at reference 1K-Hz, I know from experience, impedance is frequency related. ...

No, the 35w rating is 70Hz-15kHz, as stated on the data sheet.

You are right, though, that reactance will change with frequency. But... there are a lot of competing reactances and resistances in a transformer. Primary inductance is but one of the many conflicting factors.

For example the frequency the 4000 ohms impedance was not the same frequency that the 90H was determined, I believe the 4000 impedance was determined at 1K-Hz, the 90H was determined at 60 Hz, and since impedance is frequency dependent, I do not believe the 7 Hz calc result is even close to being valid.

You're sorta right, but not exactly for the reason you think.

Assume an ideal transformer. A transformer's job is to "transform". There is a turns ratio, a voltage step-up/step-down ratio (equal to the turns ratio) and an impedance ratio (square of the turns ratio). Working from these, there is also a current ratio, but that is equal and opposite the voltage ratio.

The important fact to recall is the impedance of the primary is determine by the impedance attached to the load, modified by the square of the turns ratio. So the 4k primary is dictated purely by the turns ratio and the load. When the transformer is rated, and the guitar amp designed, we all assume the load behaves like an ideal resistor to simplify matters.

A real speaker does not have a single impedance at all frequencies equal to the nominal (rated) impedance. So yes, that's varying. However, the resulting varied primary impedance is not the "fault" of the transformer, because its impedance ratio is unchanged, but the fault of the speaker's varying impedance.

In general, my gut tells me that when doing calcs when frequencies are involved, the DC Ohm's Law formulas should not be used.  (Anybody for complex number algebra, adjusting for changing frequencies).

The calculations I presented are not frequency-dependant, and no frequencies were used/cited. In fact, many simplifying assumptions had to be made along the way, as is always the case when trying to figure info from pentode/beam output tube curves; you'll find RDH4 suggesting to approach such calculations as though the tube were an ideal resistor. An experienced amp designer then knows that you'll calculate a little too much clean output power with the results, and fudge downward a realistic amount, or do follow-on distortion figuring. Better yet, the designer assumes the calculations are close enough to tell if the plan is viable, then breadboards and measures the results with real parts.

[HotBluePlates]

I will stand by statement, that I would probably not use this OT, on say my Bassman, because Hammond rates this OT between 70Hz, and 15K-Hz.  I definitely would not want this OT on a 5 string bass using a B-0 string, (fundamental Hz of just above 30 Hz).

Well, it depends. In general, Fender Bassman amps do not make great bass amps. This transformer is also not part of Hammond's normal line, which is rated -1dB 30Hz-30kHz. It is part of their line to replicate guitar amp transformers, with reduced power-bandwidth. An interesting side note is that some folks don't like the stock Hammond transformers in replica amps, because the transformers sound "too good" and "too hi-fi". A key element is the small core typical in old guitar amp transformers and the resulting limited power handling at bass frequencies.

A flip-side of that coin is an amp I'm building now. Becuase of the original's hi-fi output transformer, a knowledgeable person suggested I use a Hammond part, rated 30Hz-30kHz, rated at almost 3 times the amp's output capability. Turns out the massive transformer matches the size of the core found in the original amp.

You could spend decades exploring guitar amps while remaining blissfully unaware of all the complexities of a transformer. You could possibly spend as much time studying transformers and nothing else...

[HotBluePlates]

Not all ohms are equal.   In the DC Ohm's Law, the Greek omega symbol is represented by r or R.  In the AC Ohm's law the R is replaced by Z, which includes the reactance which is frequency sensitive.  I believe that the 4000 ohm impedance was determined at the referenced standard of 1K-Hz, and the reference frequency needs to be stated, if not, then the 4000 ohm number is meaningless.

The what do the "AC ohms" equal? What is the relationship between them and "dc ohms"?

I promise I'm not trying to provoke you, except to get you to think about this. You're hung up, in my opinion, on an incomplete view of reactance vs impedance. "Z" is impedance, but reactance is measured in ohms (exact same units as d.c.) and resistance is measured in ohms. There is no difference in the "ohms" except that reactance has to be figured based on frequency. Reactance also has an implied phase angle, but no one mentions that; rather, one resistance and reactance are combined into an impedance (Z), then the magnitude is expressed in ohms and there is a phase angle (but not always specified).

Read about reactance here for a brief overview.

I believe that the 4000 ohm impedance was determined at the referenced standard of 1K-Hz, and the reference frequency needs to be stated, if not, then the 4000 ohm number is meaningless.

Your unstated assumption is that the transformer's primary determines the 4k ohm number. It does not.

If it did, you would be correct, and the inductive reactance of the primary would dominate, and the impedance would be different with differing frequency. However, you are speeding past the fact that a transformer's primary impedance is merely the reflection of the load applied to the secondary, converted by an amount dictated by the ratio of the turns of the primary and secondary windings. If this were not true, would it matter what output tap you attached your speaker to? Why are taps specified as "4 ohm" or "16 ohm"?

For further reference, and an excellent, easy to follow reference, read NEETS Module 2.

The data sheet also states this transformer was rated at approximately 90 H at 240v 60Hz.  The H number by definition will change  when the frequency changes.   

What you're not catching is there's something extra: The measured inductance of a coil or transformer changes based on the current flowing through it. It also changes if the inductance of the primary is measured and the secondary is not left open (is shorted or has a load).

We have deviated on important tangents, and have not answered the question, Kag, wants to know how to be able to compare transformers based on the varied data found on data sheets, and we haven't told him.

Sure we did. PRR explained the production test aspect which explained why "240v" and how the primary inductance number was generated. I explained the derivation of the primary and secondary voltages with an application example.

[PRR]

> many simplifying assumptions had to be made along the way

Right.

One assumption we MUST make, for our sanity, is "resistor load". A speaker is a wonky resistor. We assume it is a perfect resistor just so we can get some clue of performance. Then with a possible design we can spot-check it with typical speaker impedance curves. However it works out that most good speakers (and all guitar speakers) will work acceptably if the design works with a resistor load.

Ohms versus Ohms:

Ideal transformer has infinite inductance. Real transformer has less. However we select it ($) to have "enough" inductance for the job we are doing.

10 Henries is 1.2K Ohms at 20Hz, 2.5K at 40Hz, 5K at 80Hz, 10K at 160Hz, etc.

If we load the transformer with a reflected 4K resistive load, then at 20Hz the sum is maybe 1K, but at 160Hz and up it is very-nearly 4K. At 63Hz we have 4K inductive in parallel with 4K resistive. This is not 2K, but some complex number. It is also, in practive, our -3dB frequency. We usually put the -3dB point outside the band we are interested in. For Hi-Fi down to 50Hz we aim for -3dB near 20Hz. (Guitar amps have further issues.)

So inside the range of "useful response", where the resistive load overwhelmingly shunts the transformer inductance, we "simplifying assumption" that it acts as a pure 4K resistance. 400Hz, 1KHz, whatever! Of course noting that our assumption gets weak and fails at some low frequency outside the pass-band. (Or in guitar amps, near the bottom notes.)

> rated at almost 3 times the amp's output capability.

That amp's transformer was designed for Hi-Fi, and very generously. Not just that it goes to 30Hz (it probably hits 10Hz without NFB), but that the distortion is very-very low far below 50Hz and up near full power. The flip-side (as you know) is that it is very expensive (but at the time there was a fad for these parts) and very heavy. Fender Marshall Ampeg et al opted for "lesser" iron with noticable bass distortion but lower cost and weight. The amp you are recreating is more the Cadillac V-16: the market was/is small.

I'd _try_ the "35W" Hammond 1760J on a "Bassman". It won't burn. It might be "flavorful" in the bass, depending how low and how hard you go. I don't expect Hammond's specs to tell me how it will "sound". They are technicians. They know how to wind. They offer other products that WILL play big and deep and clean. In guitar work, that's not always what we want. Nice price and easy carry are virtues too.

[HotBluePlates]

An extra thought occurred to me; I should have spotted this sooner, but oh well...

Drgonzonm: Your line of thinking is spot-on, if we're talking about a choke rather than a transformer. Without the effect of reflected secondary load impedance, the reactance of a choke has nothing to offset its increasing value with increasing frequency.

Just remember that although inductive reactance will change with frequency, the inductance (90H in the transformer we were discussing) does not change; at least not with changing frequency only.

[HotBluePlates]

So explain to me, how you get 4 ohm impedance over the range 70Hz to 15kHz, on the black-grn secondary circuit when the dc resistance for this is 0.4 ohm.  This implies there is no effect on reactance by frequency.

If you have a battery with an internal resistance of 0.4 ohm, and you add wires from the + and - terminals, then add a 4 ohm resistor between the ends of the wire, what is the total resistance of the resulting circuit? 4.4 ohms. I think you're neglecting the effect of the load. The primary doesn't have any impedance (4k, 8k, 1k or anything else) until an external load is applied to a secondary winding and transformed by the effect of the transformer's turns ratio between the primary and that secondary winding.

The false assumption is that the primary is 4k and/or a secondary is 4 ohm, all by itself. Both are just coils of wire with a lump of iron in/around them.

Did you read the NEETS module I linked? I know it took me a couple days when I was in A-school as an Avionics Tech in the Navy, and I was reading it any time I wasn't in class or asleep.

Forget reactance for the time being. The equations that describe the laws fundamental to describing the operation of a transformer never mention reactance, although they assume and imply that inductance exists.

Crib notes on transformer operation.

Note that the fundamental equations describing the transfer of power/voltage from one winding to another are all based on the number of turns of wire for each winding. After you good a good grasp on the turns ratio concept, look down at the "ideal power equation".

In a transformer with no losses, the power applied to the primary equals the power present at the secondary. Power in = power out.

EDIT: I started to write again another way of saying the same stuff I've already said. You'll simply have to read the NEETS book, which will tell you the same stuff as the Wikipedia article, but in an easy-to-understand format. Once you accept the notion of turns ratio and power in = power out, then when you apply ohm's law with an assumed load and power applied to the secondary, you'll see how it confirms the extension of Faraday's Law that deals with reflected load impedance.

It's like asking me to derive/prove Thevenin's theorem for parallel/series circuits, but you don't buy off that ohm's law is really true...

[HotBluePlates]

The link you provided states Reactive effects are not exhibited under constant direct current, but only when the conditions in the circuit change. Thus, the reactance differs with the rate of change, and is a constant only for circuits under alternating current of constant frequency.   

If the frequency is not constant then the reactance is not constant, and from the formula Z= R +jX, where Z is impedance, R, the DC resistance, X, the reactance, Z has to change as the frequency changes.  Thus, my argument that primary and secondary impedances need to be stated at a given frequency.

I provided the link to hopefully clarify the difference between reactance (ac ohms) and impedance (also ohms) and resistance (also ohms), to challenge that ac ohms are not the same as dc ohms.

Thus, my argument that primary and secondary impedances need to be stated at a given frequency.  Basically, the data presented indicates the nominal 4 ohm impedance is 90% reactance, where in a speaker it is significantly less, if the typical speaker has a dc resistance is 70% of the nominal rating.

When this lightbulb turns on for you, the world will seem like it's gone off axis.

You don't realize that you are assuming the transformer's secondary has 4 ohms of impedance, and the primary has 4k ohms of impedance, all on their own. They do not. If the secondary did, you wouldn't need to worry about having a speaker load, would you? But because you attach a 4 ohm load to the 4 ohm tap, the intended primary impedance of 4k is reflected.

If you attached an 8 ohm load to the 4 ohm tap, the primary would no longer be 4k, it would be 8k.

The primary and secondary impedances are not intrinsic to the transformer, only the turns ratio. The resulting impedance for either primary or secondary is only a byproduct of a load attached to one side being reflected by means of the turns ratio to the other side. You cannot calculate it by knowledge of the d.c. resistance and/or inductance alone.

[PRR]

> the inductance ...does not change; at least not with changing frequency only.

Iron-core inductor's inductance does change: it drops with frequency.

Up to about 200Hz, all the iron works.

Above 400Hz "skin effect" throws the magnetic flux out of the center of each lamination. Inductance drops.

This matters if you use an inductance meter which tests at 1KHz or above.

It is of interest if the intended transformer response extends only to about 200Hz, as in cheap table or pocket-radio applications.

It isn't important in wide-band audio. If there is enough inductance to support say 50Hz, then 200Hz is covered very well. While inductance drops by 400Hz, reactance continues to rise, just not as fast as a pure perfect inductance.

[HotBluePlates]

> the inductance ...does not change; at least not with changing frequency only.

Iron-core inductor's inductance does change: it drops with frequency.

I was not aware of that. But I'm thinking we should keep things simple, and assume an ideal transformer until the basic properties are understood.

[HotBluePlates]

5. Making the assumption, that I know a little about amplifiers.

I've been on this forum for about the last 9 years. I've learned a LOT in that time. But learning is what this place is about!

In this forum is a tome on referred in this thread as RDH4  or the Radio Designer’s Handbook 4th Edition. (1953) (the first edition written in 1923) I have referred to this tome, and with good reason,

Trahsformer Audio Frequency Specifications per RCA
(a)   Operating level, stated in db above or below specified reference level
(b)   Frequency response with permissible variation db from a reference frequency, generally 1000 c/s (1kHz)
a.   Conditions of measurement must be specified – usually normal operating conditions.
(c)   Impedance ratio or turns ratio
(d)   Positions of any taps should be stated.
(e)   Source and source impedance
(f)   Load and load impedance.
(g)   Total r.m.s. harmonic distortion;  This should be measured at max. Output at the lowest frequency of interest.
(h)   Minimum resonant frequency
(i)   Insertion loss in db frequently quoted at 400 c/s
(j)   Permissible phase characteristics at lowest and highest frequencies of interest.
(k)   Direct currents in windings
(l)   Magnetic and electrostatic shielding. 

Where push-pull is intended the maximum out of balance current should be stated. 
Where multiple secondary windings, are employed the power to be delivered to each should be stated.

You might not believe it, but I was reading this same book last night, in hardcopy (thanks again, PRR!).

These things are all important, but they are information you need as a transformer winder to ensure you have information dictated for all the various factors you must consider in the design/winding of a transformer.

You and I rarely know all this stuff, or analyze a given amp circuit to the level of detail needed to answer all these questions. If the answers to all these things were provided to you on a datasheet, you (and I) probably wouldn't know what to do with them.

Anyway, I know I learned some extra stuff about transformers by the reading. Very rough explanation of specs:
- How low can the transformer go, at how loud? Core size (weight) and primary inductance. Bigger is better (lower and cleaner)
       NOTE: For guitar, sometimes, cleaner and wider bandwidth is not better; many old guitar amps used the smallest OT they could get away with, and it results in a certain lo-fi sound that "sounds right" to us now in guitar amps.

- How high can the transformer go? Leakage inductance and winding capacitance; smaller is better (higher)

- What are the *design* primary and secondary impedances? Even though the transformer just has a winding ratio, you know how the transformer designer intended the OT to be used.
     NOTE: Low freq limit of the OT is determined by the proportion of primary inductance to the primary impedance (sometimes in combination with source attached to the primary). If the designer specs a primary impedance, a low frequency limit, and a tube type (Hammond's Hi-Fi line does; it's guitar-amp line implies tube type by stating what amp the OT is for), they've already considered the balance of all the competing design factors.

Think about this stuff in relation to guitars/amps. The manufacturer/salesman might point at some features, but most players up to a mid-level of experience don't obsess over neck width at the nut, neck-to-body angle, headstock angle, etc. Outside of marketing buzzwords, most non-technical amp buyers don't know (or need to know) what's going on in the amp. If it looks cool, sounds good and has the features/flexibility the user needs, the amp sells.

And so it is with amp OTs. In most cases, buyers need to know what OT to buy to drop into an existing amp. More experienced folks simplify the process by stealing/copying known-good plans rather than designing from scratch. The level above that, where a designer might seek to answer all the questions listed above... well, they wind their own transformers, so no spec sheets needed (don't laugh, I've encountered a couple of these folks; you could study your whole life just to learn all the details of transformer designing/winding).

[sluckey]

Amen!

[jjasilli]

Yes, you are right, that is a great handicap I've , I must force myself to memorize the various reports between power, resistance, voltage and current (till now I can remember easily only that I * V = W)

The good news is that you don't have to actually remember any formula in particular.  You just have to remember, in general, that there's a relationship between volts, ohms, current, and watts.  The ohms pie chart "remembers" all the specific formula's for you.  That's the beauty of reference works!

I think those data are usable only by very skilled people, don't think everybody can understand the difference between transformers with specs derived from tests with different values

Yes, we may cross the line into electronic engineering.  "Engineering is the discipline, art, skill, profession, and technology of acquiring and applying scientific, mathematical. . . and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes. . . An academic degree with a major in electronics engineering can be acquired".  Wikipedia articles.  It's no wonder we sometimes feel overwhelmed.  Thanks to Doug once again for a good internet Forum!

[sluckey]

The good news is that you don't have to actually remember any formula in particular.  You just have to remember, in general, that there's a relationship between volts, ohms, current, and watts.  The ohms pie chart "remembers" all the specific formula's for you.  That's the beauty of reference works!

And for those with basic skills with substitution and transposing equations...

All of those formulas in the color wheel are derived from two very basis equations...

          P = EI, and E = IR.

[John]

I remember when I first "got" how easy it is to figure wattage (pdiss). I thought I must be doing something wrong, it was so simple.