Testing Power Transformers

[Dave H]

For my understanding is more simple to conclude: a 300mA bridge power supply application request a 600mA RMS power transformer...for instance. But if rms value is only derivated from sinusoidal currents and not from whatever peaks current operations , things become more complicated to get it

For a bridge rectifier/capacitor I usually assume I DC = 0.5 x I RMS transformer (as you have done above).
Hammond says I DC = 0.62 x I RMS (Hammond Design Guide For Rectifier Use.pdf)
If you post the transformer pri and sec winding resistances I'll run a simulation.

[catalin gramada]

For a bridge rectifier/capacitor I usually assume I DC = 0.5 x I RMS transformer (as you have done above).
Hammond says I DC = 0.62 x I RMS ([ATTACH]53246[/ATTACH])
If you post the transformer pri and sec winding resistances I'll run a simulation.

Thanks Dave
I found 2.64 ohm into 230v primary with 15.52 ohm on HT sec(366v empty / 352.5v/577mA)

[Dave H]

Which primary voltage tap are you using?
And what is the capacitor value?

[catalin gramada]

230v primary tap, 100uF cap

[Helmholtz]

My question for now is if my dmm is realy able to show true rms value no matter by current operation conditions? Or is just a rms when operate only with sinusoidal current ?

For a pure sine current you would not need a true rms meter. In principle a true rms ammeter should give the correct rms current. But even true rms meters have a crest factor limit, the crest factor (CF) being the ratio between peak and rms value. The AC current in this application is pulsed having a high crest factor of maybe 4 or higher. If this exceeds the CF limit of your meter, readings will be wrong.

[J M Fahey]

Thanks Dave

@ catalin: no need to do the Math from scratch (although it´s always possible for the gifted ones), for us mere Mortals PSU Designer *already* does that for us
You just supply it with relevant data, which you already measured, such a winding DCR, capacitors, voltages, etc. and it will spit out all needed results

Side note: the way I described it makes it look like situation might get catastrophic, with transformer *trying* to supply 5x to 10x rated sinusoidal current and die trying, because of typical 10% to 20% duty cycle (50% was just an example to make it more intuitive).
Thanks God, it is not that bad ; when capacitor tries to pull that much current, output voltage simply drops, period, and transformerjust puts out as much as it can.
That´s why scoping +V rails you see a ripple sawtooth, which "tells the truth": it is not a triangle wave by any means , which would mean a 50% duty cycle, half the time rising (charging), half the time dropping (discharging) but a narrower rising part and a much wider dropping one.
There you are "seeing" the duty cycle !!!!!!
Charging is way narrower than discharging.

That said, when charging, transformer does not supply 5x to 10x the needed current to keep voltage steady, it simply supplies "what it can" and just lets +V voltage drop.

That´s why manufacturer´s 5% regulation turns out into 15/20% +V rail voltage drop.

I had heated fights with my transformer suppliers, early on (think early 70´s) until I found out what I write above (no PSU Designer way back then, not even personal computers, just graphs and slide rules ) but as the Japanese and cockroaches say: "what doesn´t kill you makes you stronger"

In a way I am grateful I studied Engineering just *before* computers were everywhere (the University one was an IBM 1620 clunker, with perforated card and paper tape input, "Teletype" (IBM Selectric) or dot matrix printer output, no monitor screens anywhere) so we were forced to do it the hard way.
Best case we could write our own in FORTRAN and run it ... in our 1 hour a week assigned slot that is.

[Dave H]

Green is transformer current, Blue is load current.
A load of 300mA DC requires 610mA RMS from the transformer.

[R.G.]

In a way I am grateful I studied Engineering just *before* computers were everywhere (the University one was an IBM 1620 clunker, with perforated card and paper tape input, "Teletype" (IBM Selectric) or dot matrix printer output, no monitor screens anywhere) so we were forced to do it the hard way.
Best case we could write our own in FORTRAN and run it ... in our 1 hour a week assigned slot that is.

You had a 1620 too?!

I learned Fortran on a 1620. Strange machine. It had 32K characters of storage. Not bytes, "characters".

The memory locations were used and interpreted as ASCII characters. You might think arithmetic was hard on such a machine. It wasn't It did all math by table lookup, character by character, much the way kids (used to) learn to add, subtract, multiply, and divide, two numbers at a time. Slow, but it worked.

We understand that the nickname for the 1620 in the industry was the CADET. That was an acronym, standing for Can't Add, Don't Even Try.

[catalin gramada]

Green is transformer current, Blue is load current.
A load of 300mA DC requires 610mA RMS from the transformer.

[ATTACH=CONFIG]53248[/ATTACH]

Thank you.

[trobbins]

Yes, as you consider deep diving in to measurements, crest factor will be your next hurdle to jump.

Some vintage transformer manufacturers had secondary ratings based on capacitor filtered or choke filtered dc output current, which bypassed the need to make transformer winding measurements, and moved that measurement to the DC output terminals of the application.

PSUD2 can be used to estimate the crest factor you may experience in your test jig, and then it's back to your meter specs to try and clarify whether it is out of its depth/accuracy range. There are meters, such as HP3400A that have a 10:1 rating (it's a voltmeter so needs a sense resistor), but I'd say the majority of dmm's seem to be around 3.

Given the crest factor of the current waveform changes with the level of capacitance filtering, and the winding resistances, it's also easy to see that when a manufacturer states a DC output current rating, then that needs to be taken with a grain of salt unless amazingly they also state the diode and filter set up and transformer temperature for that rating (especially if you were a hi-fi afficiando and wanted to instal a humongous capacitor filter).

Which is all pretty much why you can make educated estimates and tests along the way, and that is good in itself as it makes you think about what margins you may encounter, but its a final application 'snap shot photo' test that gives you the best bench mark to then allow you to make a closer call on margins. But this is all 'amp mass-manufacturing design' deep-diving where warranty and ratings dictate the need for a thorough awareness.

[catalin gramada]

q.e.d. , with almost 300mA it will be able to supply 100W around power amp at 450V with an EL34 quad into 1.7k load, no more.(exactly what Marshall 100 did, except the fact there are a lot of underated 300-400mA power transformers replacements for this model).