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160 Va Toroidal with 15V Secondaries

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tvtech

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Hi Guys

Spec sheet here with attachment. Anyway, this is a project I have resurrected. This whole thing has been dormant for around 9 Years.

Seeing our Eskom is going downhill..and I like stuff to do...I hope someone can refresh my Linear brain.

Simple question: How does one work out continuous Secondary RMS load current again from a Linear Transformer powering a Bridge Rectifier and nice big smoothing Cap and load.

I know 9 Years ago, 8 Amps made the Toroidal too hot after any length of time.

I am now going to go with 0.707 * 10.6 Amps. That equals 7.49 Amps continuous rating.

Sound better?

Regards,
tvtech
 

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And this is not a trick question. Somebody must surely remember this basic stuff before the SMPS brain conditioning?

Regards,
tvtech
 
On an AC-DC rectifier, the current waveform is anything but a sinewave. Therefore the usual values of RMS to average ratios do not apply, as crest factors sometimes approach 3:1 or even higher ratios.

What I would do, is use a true RMS multimeter to measure the secondary output current, while supplying your load and utilizing the actual rectifier components and values.
Nowadays, only the cheapest multimeters are NOT true RMS responding, and the vast majority have a 10 amp scale.
 
True's God

One of the few times I ask a question here..everyone ignores me :wideyed:

I know it has been less than an Hour since my original post...but still. I know you are all here... Watching.

Simple question Guys to refresh my memory. That's all. Basic stuff.

Help me here.

Regards,
tvtech
 
I can take a photo of the 160 Va Toroidal if you want.

Somebody just please say something. Anything is also good enough.

Here it is again: 160Va Toroidal with 15VAC Secondaries...I need to know the SIMPLE calculation for it's continuous rating into a Bridge and Cap and Load.

Like in a Battery Charger scenario.

Like SLA.

And if no regular Member helps me now I swear I will go away forever.

I promise.

tvtech
 
Schmitt trigger posted an answer in Post #3.

One rule-of-thumb is to derate the transformer to about 60% of its VA rating. Thus 160VA would be reduced to 96VA and the maximum total full-wave rectified DC output current from the 15V windings would be 6.4A (I would go no more than 6A).

Note that the more "nice big smoothing" capacitance you add to the output filter, the more heat you dissipate in the transformer since that increases the peak current drawn from the transformer. So the optimum capacitance for minimum transformer dissipation is such that the low voltage point on the ripple output at maximum current is just above the minimum drop-out voltage required by the voltage regulator (I assume) you are using.
 
Schmitt trigger posted an answer in Post #3.

One rule-of-thumb is to derate the transformer to about 60% of its VA rating. Thus 160VA would be reduced to 96VA and the maximum total full-wave rectified DC output current from the 15V windings would be 6.4A (I would go no more than 6A).

Note that the more "nice big smoothing" capacitance you add to the output filter, the more heat you dissipate in the transformer since that increases the peak current drawn from the transformer. So the optimum capacitance for minimum transformer dissipation is such that the low voltage point on the ripple output at maximum current is just above the minimum drop-out voltage required by the voltage regulator (I assume) you are using.

Thanks
crutschow

Much appreciated.

tvtech
 
If you give me the filtering capacitance you want to use, I can simulate the circuit with LTspice to determine the RMS transformer current for a given DC output current.
 
From the data sheet, the no load voltage for the 15 V version is 16.63 V. If the secondaries are in parallel, the full load current is 10.666 A, so the equivalent series resistance is about 0.152 Ω. Some of that will come from the primary and some from the secondary, but for this it really doesn't matter. You are trying not to overheat the transformer.

With that model of 0.152 Ω, the copper losses will be around 17.3 W. The data sheet says 18 W, so it looks as though the model holds.

I've attached a spreadsheet with a calculation of instantaneous and average power. It ignores diode losses. You have to estimate a voltage for the smoothing capacitor at the start of the cycle. It works out the voltage at the end of the cycle and tell you if they are different.

Edit. Playing with a few numbers, it seems that 6 A d.c. is about all you can get from that transformer.
 

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Thanks Guys

This whole mission is part of a UPS project I started years ago. You see, you can take any old UPS, and make it's run time longer by adding a big battery and throwing away the (normal) 7Ah battery that comes with these things.

The problem is to get the big battery charged. One little LM317T that these typical UPS come with ain't going to cut it to charge a 65Ah battery for example.

And I am talking cheap 12V 600VA UPS's here that are good for powering SMPS supplies as in modern TV's, Computers and lighting...no motors or linear transformers though. They will overheat and be damaged/burn out from the modified square wave they are fed with. These cheap ones are not true Sine Wave output.

So, back to the story.

The very same cheap UPS I bought all the Years back and installed on my relations farm in the middle of nowhere and coupled Solar charging together with Generator time at night charging huge truck batteries got these ungrateful people TV time that was not possible before without starting the Generator.

Hey, we can have TV in the day!!!

Okay, fair enough, a 600VA cheap UPS cannot do 600VA continuously on batteries. But it will be OK if used sparingly as far as load goes.

They were running 1 CRT TV, 1 TV decoder and maybe 1 light. Total of around 300VA every day. For around three Hours per day.

And my cheap UPS never blinked an eye.

So. back to the story. Again.

Seeing as ESKOM (our useless Power Utility) is pulling moves on us, I am going to get up to speed at last.

I need very little power here. I have purchased a "Mother" of a battery though to power the UPS. It's 65Ah of sealed lead acid battery with a 10 Year float life....

Running my Computer and the Modem and stuff, I am good for 8 Hours.

Charging....that is the reason why I started this thread. Fine flattening a battery...charging is another thing.

Anyway, 6 Amps DC sounds about right for this Toroidal to be happy with. It's strange how one sometimes forgets the Basics :(

Thanks Guys.

Regards,
tvtech
 
How are you going to regulate the charging current to the battery?

Note that 6A is with the two secondaries in parallel.
 
Before I go on, it's Carl?? right.
I like to be on first name terms with folks who try and help me.

Strange me

Regards,
tvtech
 
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How are you going to regulate the charging current to the battery?

Note that 6A is with the two secondaries in parallel.

My original design was for Cyclons. Small but hectic batteries. Like a 8AH on Steroids.

Never mind that. Very expensive stuff to play with....but an amazing learning curve for me. Teaches you stuff that you never knew about batteries and charging and cycle life and depth of discharge and all that. You see, a Cyclon needs no Current Limiting to stay safe when charging.

A Cyclon will take what it needs to renew itself....if you throw 100Amps at a little 8Ah E Cell battery pack...and it is hungry...it will eat it.

Anyway, I am digressing here.

I have to get real again.

F***. Time to walk the talk. Normally OK with it. However it seems that I am getting stupider by the day.

I used to have answers for things. I used to know stuff. I could answer my own questions.

These days, fixing things for eternally stupid people has made me just as stupid.

I mean Yesterday...an ugly filthy Women walks in to the shop....checking on her TV we are repairing...

She brought the thing here with spares and parts missing...this useless B with an itch expects us to resurrect it and make it like new???

Take it and throw it away. We simply cant help you Mam. Sorry. Better luck next time.

And so it goes on. Sure, there are Liberals that maybe live in England and dislike my comments here...maybe visit family from time to time...

Nice farm and all that...

I am another kind of animal though.....I live the stuff I talk about

Every day of my life. Not just when it suits me.

Thinking back when I got myself into heaps of trouble around this time last Year...jolly season indeed...

You all know what happened......hectic stuff.

Regards,
me
 

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A slightly off topic question TV, does your tv shop have a locked gate so that customers have to be "buzzed in" by some one releasing the lock on the gate?
I found this quite strange, but maybe understandable, during my brief visits to South Africa.

A Cyclon will take what it needs to renew itself....if you throw 100Amps at a little 8Ah E Cell battery pack...and it is hungry...it will eat it.
OK, but if you keep overfeeding it, does it not get to the point where it says "ENOUGH", and vomits all over the carpet?

JimB
 
How are you going to regulate the charging current to the battery?

Note that 6A is with the two secondaries in parallel.

Hi crutshow

Please give me a name I can work Buddy...call yourself Bob or anything you want. Or Dave or Mike or maybe Carl?

So, the charging thing was sorted years ago....in stages.

My original design relies on LM317T beauties....four of them...on separate heatsinks...all in parallel. Here is the thing with a 317 though..

If it is asked to go way beyond it's call of Duty.....2.2 Amps is not unusual...with a good heat sink and all it will allow around that. Yes, a little LM317T is capable of much more than specified....no wonder this little Gem has been a around for so long.

Now I am going all sentimental and stuff again....fact is no Company has managed to design anything as good as a 317...

Copied, faked whatever. All based on the National Semiconductor LM317. A legend.

More next time

Tired now

tvtech
 
A slightly off topic question TV, does your tv shop have a locked gate so that customers have to be "buzzed in" by some one releasing the lock on the gate?
I found this quite strange, but maybe understandable, during my brief visits to South Africa.


OK, but if you keep overfeeding it, does it not get to the point where it says "ENOUGH", and vomits all over the carpet?

JimB

LOL :)

Its a battery Jim. Man you Guys also have a sense of humor.

Now tvtech today was trying to explain stuff on a Technical thread.....I have forgotten most of the stuff I want to talk about but I spoke about it anyway...

I know what I need. And I cannot say it here.

I will get us all in the dwang if I speak now.

So, in the interest of this forum and kids watching, I say bye.

Old buggered tvtech would like to hump something though. Even a humped back whale will do.

That's as clean as they come :)

Edit:(I am a little confused...I thought this string was posted on Off Topic)

Cheers Guys,
tvtech
 
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Note that the more "nice big smoothing" capacitance you add to the output filter, the more heat you dissipate in the transformer since that increases the peak current drawn from the transformer. So the optimum capacitance for minimum transformer dissipation is such that the low voltage point on the ripple output at maximum current is just above the minimum drop-out voltage required by the voltage regulator (I assume) you are using.
That is true, but the effect is not very marked.
I ran some figures for this transformer, and I've attached a graph of the results.
With a 2200 μF cap, the copper losses are just over 10 W, but there is huge ripple, probably too much for a 2200 μF capacitor and only 9.5 V is available.
With a 3300 μF cap, the copper losses are 11.5W, and 13.1 V is available. This is probably usable.
Going all the way up to 22,000 μF, the copper losses are 13.4 W, with 19.4 V available.

What is happening is that even with capacitors that are far too small, there is still a large peak current as the capacitor is charged each cycle.

So yes, the optimum capacitance is the one that gives the drop-out voltage. However, if you go larger, which reduces stress on the capacitors and gives more safety margin, the increase in transformer heating is small.

If you design for a large ripple, there will be larger losses in the regulator and overall efficiency is worse.
 

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But note that the secondary copper loss for the rated RMS current is 8.36W total (both windings) so if you go above that value you may cause overheating and deterioration of the winding insulation.
 
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