blackboy said:
I don't think it's possible, without two separate transformers.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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that is just the start of more confusion.
Most "grounds" do different jobs and we could champion the cause of clarity if we used different names for each different use.
eg, safety earth,
return,
zero volt reference,
PSU power zero volts.
common,
etc.
Often, these are combined, but that combination should be the result of deliberate thought by the designer and not an accident because everyone called them "ground".
the Carlos' power regulator uses positive chip regs. This requires a pair of isolated inputs to each regulator. The zero volt line after the regulators becomes the power ground. The four inputs must ALL be isolated from the power ground and from each other.
A three input PSU cannot meet this requirement.
Hi,
i have 2 nos Toroidl trafos 35-0-35 , 225 VA rating.
Can i use these trafos with chip LM 3886?
What are the necessary requirements to bring down the out put voltage to 30-0-30 volt after rectification?
Regulated power supply is another option which is also suitable for the same.
Plz guide.
i have 2 nos Toroidl trafos 35-0-35 , 225 VA rating.
Can i use these trafos with chip LM 3886?
What are the necessary requirements to bring down the out put voltage to 30-0-30 volt after rectification?
Regulated power supply is another option which is also suitable for the same.
Plz guide.
OK so it requires some research on the wire gauge used, most I would think are run close to the limit though.
As for the primary, by adding turns you will reduce the efficiency/regulation of the transformer. It will usually also require the whole secondary to be removed in order to get at the primary.
Maybe some people have too much time on their hands or are gluttons for punishment
As for the primary, by adding turns you will reduce the efficiency/regulation of the transformer. It will usually also require the whole secondary to be removed in order to get at the primary.
Maybe some people have too much time on their hands or are gluttons for punishment
let's take your example.jackinnj said:
Reduce the voltage by 15% and increase the current by 17.65% to bring the VA back to exactly where we were before we removed any of the secondary windings.
We all know that Power =I^2 * R.
The internal resistance of the secondary winding has been reduced by 15% due to the shorter winding.
The power loss (=heat) is now 1.176^2 * 0.85 = 1.1765 times higher than it was before we modified this transformer.
The voltage loss at this higher current is also higher. Now, starting from the lowered secondary voltage and with a higher secondary loss, the resulting regulation is worse than the standard transformer. But it gets worse, the higher running temperature increases the resistance of both the primary and secondary windings. This results in more I squared R losses and even poorer regulation and yet higher operating temperature.
In the old days when copper was relatively cheaper and thicker windings were used we could get away with a little removal, but now that the copper content is skimped by the manufacturers, any copper removal will cause deterioration in performance.
Conversely, adding more copper to the windings can improve performance. This can be by increasing the number of turns (more voltage) or increasing the number of windings (multi secondaries), or substituting thicker wire (for lower I squared R losses).
If you have a micrometer you can calculate the gauge or diameter to determine the current carrying capacity of the wire. my experience with Amveco transformers (they use 18ga wire for their 140VA transformers) is that you could easily reduce the Ns by this amount and have sufficient margin of error. The difference in peak currents is about 0.5 amp.
At wholesale, the cost of 18ga vs 20 ga is somewhat less than the differrence in circular area -- but the cost in service of a transformer using 18ga is much lower -- even in a low current application the energy savings is significant...the cost of Cu is a cost factor, but energy efficiency is a selling point.
My suggestion -- measure the guage of the wire -- if it's #20 AWG or lower you're going to be OK.
and one more thing -- don't assume that the impedance of your loudspeakers is actually 8 ohms -- with 6 or 4 ohms the overture chip is going to cook itself to thermal shutoff if you have +/- 40VDC on the rails.
At wholesale, the cost of 18ga vs 20 ga is somewhat less than the differrence in circular area -- but the cost in service of a transformer using 18ga is much lower -- even in a low current application the energy savings is significant...the cost of Cu is a cost factor, but energy efficiency is a selling point.
My suggestion -- measure the guage of the wire -- if it's #20 AWG or lower you're going to be OK.
and one more thing -- don't assume that the impedance of your loudspeakers is actually 8 ohms -- with 6 or 4 ohms the overture chip is going to cook itself to thermal shutoff if you have +/- 40VDC on the rails.
AndrewT said:
Hi Andrew,
I think the VA thing is much less an issue. The transformer may be spec'd at 225VA, but in amp service it will almost never have to supply that power. In normal use the power draw may be just a few 10's of watt average.
One other issue here is the supply capacitor. Extreme capacitances (> 50.000uF for example) will cause extremely sharp and high charging pulses which may heat the wiring of the transformer and cause contraction & expansion noise. But even then, the average power will not raise the average temp.
Jan Didden
I bought up this issue a while a go. It was quite a heated debate from what I remember!
If you're using just one of these PSUs (e.g a total of 2xLM338) to power both channels of an amp, then you can use a single dual secondary transformer. You may or may not run in to the current limit of the LM338s.
If you want two separate PSUs (e.g a total of 4xLM338), one for each channel, then it "works" with one transformer, in that it didn't blow up and it played music, but it was not working correctly, and you will only see this if you have one of the PSU's negative rails adjusted to put out a lower voltage to the other PSU.
A theoretical example of when you might see this happen is trying to run, say, a 2xLM338 PSU for a 15v preamp, and then another 2xLM338 PSU, running both channels of a power amp at 28v on the same transformer. You will find that the preamp supply ends up being +15v as expected, but the negative rail is -28v.
If you want one of these PSUs per channel then, in my experience - and I am still utterly convinced that I was doing nothing wrong - you will need two separate, dual secondary transformers.
If you're using just one of these PSUs (e.g a total of 2xLM338) to power both channels of an amp, then you can use a single dual secondary transformer. You may or may not run in to the current limit of the LM338s.
If you want two separate PSUs (e.g a total of 4xLM338), one for each channel, then it "works" with one transformer, in that it didn't blow up and it played music, but it was not working correctly, and you will only see this if you have one of the PSU's negative rails adjusted to put out a lower voltage to the other PSU.
A theoretical example of when you might see this happen is trying to run, say, a 2xLM338 PSU for a 15v preamp, and then another 2xLM338 PSU, running both channels of a power amp at 28v on the same transformer. You will find that the preamp supply ends up being +15v as expected, but the negative rail is -28v.
If you want one of these PSUs per channel then, in my experience - and I am still utterly convinced that I was doing nothing wrong - you will need two separate, dual secondary transformers.
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