I have a 25V dual secondary transformer and I need the secondaries to be closer to 20V. I am thinking of attempting to unwind a few of the coils. Is there anything I should know before I attempt this? Would you advise for or against it?
(I wasn't sure what forum to post this it. It is here because it is for a
Tripath AMP5 from 41Hz.)
Cheers.
(I wasn't sure what forum to post this it. It is here because it is for a
Tripath AMP5 from 41Hz.)
Cheers.
Stalfo,
I haven't done this myself, but I asked Dave Slagle about it and his reply was that the secondaries are almost always wound on the outside (the primaries are always the same, so they keep primary-wound cores ready to build to demand), and are almost always bifilar wound (for ease of winding) so that it's pretty simple to just unwind a few turns at a time, scrape the wires, connect up, and see what you get.
Only time there's a snag is if the center is potted (like the Amveco ones), it's a booger to get that out of there.
You might want to go a little lower than 20v; I just put an Amp 9 on the end of a 15V 50VA Amveco, and I'm getting about 21Vdc at the chip, so maybe 18-19 volts would be optimum.
Aloha,
Poinz
I haven't done this myself, but I asked Dave Slagle about it and his reply was that the secondaries are almost always wound on the outside (the primaries are always the same, so they keep primary-wound cores ready to build to demand), and are almost always bifilar wound (for ease of winding) so that it's pretty simple to just unwind a few turns at a time, scrape the wires, connect up, and see what you get.
Only time there's a snag is if the center is potted (like the Amveco ones), it's a booger to get that out of there.
You might want to go a little lower than 20v; I just put an Amp 9 on the end of a 15V 50VA Amveco, and I'm getting about 21Vdc at the chip, so maybe 18-19 volts would be optimum.
Aloha,
Poinz
AndrewT said:
Poindexter said:
I believe Andrew is correct in regards only to the secondary, as the guage of wire in secondary winfding is rated for the amperage, this is so independently from the Voltage delivered.
But the primary now is loaded less because you have to consider the VA input as well as the VA on output.
The VA of a transformer will have to consider both the input as well as the output. This is for a design heat rise parameter. THere will be less heat generated at the primary when the secondary Voltage is reduced for a given current output.
So i believe it is not just asimple VA on the output concern, but is a total transformer concern.
Hi,
having thrown away 5/25 (20%) of the VA rating, you now have that same 20% spare VA capacity in the primary.
You can add a third or even a fourth winding to use up that spare capacity.
What you cannot do is increase the output current of the original windings. Modern transformers mostly use 3.1A/sqmm. A thicker wire will improve the regulation and decrease the IsquaredR loss.
If you want the full VA rating in just one pair of secondary windings then you must replace the whole secondary with the correct larger diameter wire to maintain the same heat loss and the same regulation. That is a lot of work.
having thrown away 5/25 (20%) of the VA rating, you now have that same 20% spare VA capacity in the primary.
You can add a third or even a fourth winding to use up that spare capacity.
What you cannot do is increase the output current of the original windings. Modern transformers mostly use 3.1A/sqmm. A thicker wire will improve the regulation and decrease the IsquaredR loss.
If you want the full VA rating in just one pair of secondary windings then you must replace the whole secondary with the correct larger diameter wire to maintain the same heat loss and the same regulation. That is a lot of work.
Another way to reduce output voltage is to add turns to the primary winding. If connected correctly, it will reduce output voltage. Reversing the connection polarity of the additional winding will boost the output voltage (which is how I actually used it).
Doing this will generally require adding more turns (to the primary) than what one would have removed (from the secondary). It also requires making an elongated bobbin, narrow enough to thread through the centre of the transformer.
To make the bobbin, one can use a plastic ruler or similar flat, rigid plastic or wooden object. Cut a V shape into both ends, then wind up a number of turns of enamelled copper wire over the length of the bobbin. This can then be threaded though the transformer....
Doing this will generally require adding more turns (to the primary) than what one would have removed (from the secondary). It also requires making an elongated bobbin, narrow enough to thread through the centre of the transformer.
To make the bobbin, one can use a plastic ruler or similar flat, rigid plastic or wooden object. Cut a V shape into both ends, then wind up a number of turns of enamelled copper wire over the length of the bobbin. This can then be threaded though the transformer....
AndrewT said:
Shaun said:
Good point!, which concept applied to the secondary:
Actually adding bucking turns to the secondary of the same guage wire will in fact maintain the same VA of the transformer
at a lower voltage but should then maintain the same VA rating.
This is a relatively panlless way to lower the voltage but maintain the VA.
There would be a small extra IR drop due to the extra wire length. But this is a second order effect and could probably be ignored. (i.e. 20% 0f 20%= 4%)
I suspect the extra 20% of secondary turns will affect the VA rating.SheldonD said:
The secondary IsquaredR loss will increase by 20% with the result that at the same output current the transformer will run hotter.
You now have a hot running transformer that produces the same current output but at the 80% voltage level.
Now to reduce the IsquaredR loss to match the original (=same heat generation) the I should be reduced to about 90%.
That to me seems like a VA rating that is worse than just removing the turns.
What in the meantime has happened to the regulation?
The only way to retain the original VA rating is to increase the secondary copper area to allow a comensurate increase in secondary current
I believe that the VA rating is more related to core size than copper losses. I^2R losses relates to efficiency. In the scheme I described (adding primary windings) copper losses can be minimised by using oversized wire (the wire I happend to have at hand was about 2x the cross-sectional are of the original primary winding, I estimate). This is not to say that your concerns are nonsense, but I think that the context in which these practises are applied (DIY) would be tolerant of the kinds of performance losses you refer to. (I'm now thinking back to the desperate times of student DIY...)
Hi,
with the high current to low copper area (3.1A/sqmm) used in the cheaper toroids available today. I think you will find that temperature is often the limiting factor for VA rating.
The manufacturers that use <<3.1A/sqmm will find their transformers run cooler (and they will tell you so in the advertising) with the result that the core also becomes a limiting factor. These high copper content toroids will cost more and will have a lower regulation.
Back to a buck winding using large diameter wire.
reduce the voltage to 80% and run the original winding to full output current. Now add on the heat generated in the extra winding (and all the extra insulation you've wrapped around the transformer) you will still have a transformer that runs hotter AND has 80% VA.
To maintain full VA the secondary current would have to be increased to 125%. This increases the heat in the original secondary to 156%. Add on about 16% for the heat in the extra double area buck winding. This is now 172% of the heat in the total secondary winding and the regulation will be terrible.
At full VA the core heat loss and the primary heat loss will remain the same
with the high current to low copper area (3.1A/sqmm) used in the cheaper toroids available today. I think you will find that temperature is often the limiting factor for VA rating.
The manufacturers that use <<3.1A/sqmm will find their transformers run cooler (and they will tell you so in the advertising) with the result that the core also becomes a limiting factor. These high copper content toroids will cost more and will have a lower regulation.
Back to a buck winding using large diameter wire.
reduce the voltage to 80% and run the original winding to full output current. Now add on the heat generated in the extra winding (and all the extra insulation you've wrapped around the transformer) you will still have a transformer that runs hotter AND has 80% VA.
To maintain full VA the secondary current would have to be increased to 125%. This increases the heat in the original secondary to 156%. Add on about 16% for the heat in the extra double area buck winding. This is now 172% of the heat in the total secondary winding and the regulation will be terrible.
At full VA the core heat loss and the primary heat loss will remain the same
For the OP's intended outcome I also don't agree with this advice. If that technique was to be deployed, then half the excess turns (25V down to 22.5V) would be disconnected and wired in reverse with the remaining 22.5V secondary, to buck down to 20V. But you still only operate the secondary at the same current, so the effective output VA is reduced to 20V at that max operating current.
But I also don't reckon that if you remove 5V of turns, that the VA falls pro-rata but rather would be somewhat in between the full derated VA and the full VA. If the temp profile in the windings was a max at the primary to secondary interface then that is one way to appreciate that say operation at 90% VA would lower the heat flow from the primary, which is then offset by the extra heat flow from the secondary, but the secondary winding now has better thermal dissipation to ambient.
And we don't know if the use is actually operating the transformer at its max specified end, or whether there is some operating margin. And then there are the myriad other issues of who built the transformer, and what standards did it pass, and what is the max ambient during operation compared to the spec level.
Shaun,
if you transformer was 95% efficient, then the following applies.
input 100W, output 95W
heat in the transformer 5W.
That 5W of heating causes a temperature rise.
If your transformer's efficiency were reduced by 10% to 85%, then the following applies.
input 111.76W, output 95W
heat in the transformer 16.76W
That 16.76W of heating causes a bigger temperature rise
How can you justify misleading Members with this?
if you transformer was 95% efficient, then the following applies.
input 100W, output 95W
heat in the transformer 5W.
That 5W of heating causes a temperature rise.
If your transformer's efficiency were reduced by 10% to 85%, then the following applies.
input 111.76W, output 95W
heat in the transformer 16.76W
That 16.76W of heating causes a bigger temperature rise
How can you justify misleading Members with this?
Not misleading at all. Pragmatic, yes. Keep in mind that your calculations are for maximum load. I made the point that an amplifier does not operate at maximum load. The loss is a minor concern for most amplifiers, though more of a factor for a class A amplifier.
I would not recommend using the scheme I proposed for a ground-up design. But for a DIYer sitting with the wrong transformer in hand, it is a viable fix. And that is all I am recommending it for.
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