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Old 31st August 2007, 11:53 PM   #1
john65b is online now john65b  United States
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Default Total VA Available on Multiple Tap secondary?

I have a 150 VA Transformer that has multiple secondary taps I will not be using.

The tap I will be using - the 48VCT - is rated 2A (96VA), but can I assume 3Amps are available to the 48VCT (all 150VA) if other taps are not used?

There are two other 30VCT (one 1A and the other.5A) and one 18VCT (.5A) on the secondary...all unused...


Sorry for the lame question...tried "googling" the question and got nowhere fast. I know the question can get answered fast here...
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Old 1st September 2007, 01:43 AM   #2
pchw is offline pchw  United States
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The core is capable of delivery this much amp, but the magnet wire of the tap that you plan to use is capable of withstand 3A is questionable. Unless you can find out the size of the wire, it is difficult to say for sure.
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Old 1st September 2007, 05:07 AM   #3
john65b is online now john65b  United States
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Quote:
The core is capable of delivery this much amp, but the magnet wire of the tap that you plan to use is capable of withstand 3A is questionable. Unless you can find out the size of the wire, it is difficult to say for sure.
Reading off the 48VCT Tapped Wire:

E132794 20 AWG AWM STYLE 1015 105 DEGREE C 600 VOLTS

Whoops, your referring to the winding wire, correct?
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Old 1st September 2007, 05:20 AM   #4
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Hi John65b,

If it is rated 2A chances are it will withstand roughly 2A. If you are draw 3A from the winding it may overheat the transformer and cause an insulation breakdown resulting in a possible fire or shock hazard.

You could give it a try, just load that 2A secondary with a load equivalent to 3A and monitor to see if the transformer core is becoming too hot (wait a few hours). An insulation problem usually happens when the transformer becomes way too hot. If yours is equipped with a bi-metal thermal cut-off then it will simply cut the primary power going into the transformer.

My 2 cents

Eric
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Old 1st September 2007, 08:45 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the two lower windings can be connected in series to give 48Vac @ 500mA.
I think each winding should not be run beyond their rating for any sustained period. Yes, they can supply short term overloads.

Now if you rectify the 500mA supply and separately rectify the 2A supply and then parallel them you can draw upto 2.5Aac from them.
This will give you a 67Vdc 1.25A PSU. About 83W continuous.
The smoothing will meet any peqk demand that exceeds the continuous rating.

It would help if you could measure the open circuit voltages of the 48Vac and the 30+18Vac windings.
Then load each to their rated AC currents using a resistor bank.
Now measure the loaded voltages again.
Try to do this with both windings delivering power simultaneously.

Some multi secondary transformers have a total secondary VA rating that exceeds the primary (and core) VA rating. this arrangement allows each secondary to deliver upto it's own maximum output PROVIDED the total output, at any time, does not exceed the transformer rating. This is achieved by using thicker wire for each secondary winding and this results in a lower voltage drop when under light and medium loads (=good regulation). Nice on a ClassAB amplifier with ancilliaries run off separate windings.
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Old 1st September 2007, 11:38 AM   #6
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The core does not really care how much power is drawn, actually the flux is higher when the transformer is unloaded. The core size is chosen so that the windings fit, a bigger core requires less turns and is bigger, so it can be used at a higher power.

Windings are dimensioned by the current they are supposed to supply. 3A on the 2A winding is over double the R*I^2 losses so it probably is not a very good idea.
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Old 1st September 2007, 12:53 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by megajocke
The core does not really care how much power is drawn, actually the flux is higher when the transformer is unloaded. The core size is chosen so that the windings fit, a bigger core requires less turns and is bigger, so it can be used at a higher power.

Windings are dimensioned by the current they are supposed to supply. 3A on the 2A winding is over double the R*I^2 losses so it probably is not a very good idea.
Hi Mega,
your reasoning has lost me.
Quote:
The core does not really care how much power is drawn
this statement seems to be at odds with
Quote:
a bigger core requires less turns and is bigger, so it can be used at a higher power
I thought there was a flux limit beyond which the core became inefficient. More primary current equals more flux due to the extra {current*Turns} product. If the flux limit sets an {I*T} limit then Current I becomes limited as well. That then determines the limiting input VA.
Or am I completely wrong?
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Old 1st September 2007, 02:29 PM   #8
john65b is online now john65b  United States
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I was planning on using two of these trannies to power a stereo set of LM3886, since only 96VA.

But if all 150VA were available on the 48VCT 2A tap with nothing else used, I could get along only using one of these trannies.

150VA is good enough for my use, but 96VA a little short.

If I post rectify parallel the 30 + 18 .5A taps with the 48 2A taps, wouldn't I have different centertaps?
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Old 1st September 2007, 03:12 PM   #9
AndrewT is offline AndrewT  Scotland
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I repeat, you cannot draw 150VA from a 96VA winding.
Mega showed that the heat losses are worse.
Quote:
3A on the 2A winding is over double the R*I^2 losses so it probably is not a very good idea.
If you do combine the 500mA and 2A windings then the centre taps locate on the same power ground.

But first you must measure these two different windings to see how close they are for open circuit voltage and loaded voltage at their rated currents.
If this proves feasible then 120VA can support between 60W and 120W of good audio. That equates to two channels of 30W+30W upto 60W+60W. That seems just fine for 8ohm loads.
The 24Vac (=48Vac CT) is too high for 4 to 8ohm speakers and certainly too high for 4ohm speakers.
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Old 1st September 2007, 04:23 PM   #10
john65b is online now john65b  United States
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So the series connection the 18VCT and 30VCT taps... The connection where these two connect, is the centertap for this .5A secondary, correct? Seems strange as these are not , well, centered...

I can then combine post recified positive (and respectively negative) out .5A to post rectified positive (and respectively negative ) out of the 2A at smoothing caps bank and then connect the two CT..

I know I need to verify loaded and unloaded voltages on all taps first...

Can I connect the 30VCT 1A instead of the 30VCT .5A to the 18VCT to increase the VA Available, or do the Amps have to match??
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