But have very likely been labeled with a good sized safety margin so a transformer rated at 2A AC absolute max will probably be usable in a DC supply supplying 2A absolute max.

No, no, and no. There is no manufacturer that leaves 60% of usable capacity in the garage when they spec their products. They spec what it is and what it does under the conditions they believe are reliable and give stated operating life and performance data. You can run it over spec, and the consequences are known.

i'm interested in your input finding out some good derating figures as the earlier quoted "DC amps = AC amps /1.7" is obviously way too conservative

No, no, and no. That number comes from a fairly complex mathematical analysis of the FWB/cap circuit which relates DC current to the AC RMS current. It is simply the factor to use to follow the transformer spec rating. It's not a "derating" at all, it is the translation of DC load current to AC winding current.

This is getting a bit off-topic so I'm happy to take this to a "transformer max safe ratings" thread if you like

I'll pass, but I can probably copy out the sections on the training material for the published app note I wrote on transformer ratings and post it.

 

No, no, and no. There is no manufacturer that leaves 60% of usable capacity in the garage when they spec their products. They spec what it is and what it does under the conditions they believe are reliable and give stated operating life and performance data. You can run it over spec, and the consequences are known.

I'm not saying they are "leaving 60% capacity". You get that 60% figure based on your choice of "max DC current = max AC current /1.8". I'm not accepting your /1.8 figure as gospel, and prefer to test what the FACTS are on a real transformer.

No, no, and no. That number comes from a fairly complex mathematical analysis of the FWB/cap circuit which relates DC current to the AC RMS current. It is simply the factor to use to follow the transformer spec rating. It's not a "derating" at all, it is the translation of DC load current to AC winding current.

I've done that, if you read my post. The diode conduction angle was 40%, found easily on the scope without needing "complex mathematical analysis". At a diode conduction angle of 40%, the 2A DC output gives a measured 2.5W I2R dissipation in the secondary winding. This is only 1.5W above the AC example, quite a small increase in overall transformer dissipation.

So for the premise that the 15v 2A rated transformer is "not capable" of producing a 15v 2A DC output, I have already disproved that. It was quite capable of providing 16v 2A, which is 1 volt more. So that kills any argument that at the same DC current the voltage will sag too much and not meet the spec voltage.

So now the argument comes to "how much additional heat is caused by this, and can heat that be tolerated WITHIN the manufacturer's deliberate safety margin?".

If you have real industry figures as to what total waste dissipation is allowed in a transformer related to its physical size or mass in grams then we can measure the additional waste heat caused by running close to the AC=DC current, and see how close it is to spec.

I'll pass, but I can probably copy out the sections on the training material for the published app note I wrote on transformer ratings and post it.

Three pages which are completely useless to the discussion, apart from you simply state this line as gospel; "ISEC = 1.8 times max DC output current".

How do you justify that /1.8 figure? The worst case component of this issue in the transformer is the secondary resistance and the I2R loss, which only caused an increase of 1.5W waste heat on a transformer supplying 32W DC. That is not unreasonable on a 30W (VA) rated transformer, which was also backed up by the real world test of temperature rise which was also quite reasonable.

Assuming the big problem is the secondary winding I2R loss, then you should be aware that transformer manufacturers will use secondary wire generally larger than needed (I worked in a transformer winding shop) and they will even pick a secondary wire size above what is needed for the secondary current rating, as they like to fill the former and make the secondary winding have the same copper area as the primary. So if the secondary winding resistance is the big issue, and the manufacturer has chosen extra large secondary wire for a number of reasons, then it's logical that the DC output current can safely be larger than expected.

If you have actual figures on what total heat dissipation a transformer can hendle based on its volume or mass, please post, as I have an AC power meter here and can test the waste dissipation in this transformer.

Or if you can discuss exactly what the main heat issue is with running AC=DC amps and how that /1.8 rule was determined then we can talk about that. From a first glance it looks mainly to be an issue of secondary winding I2R losses. Maybe I'm wrong about that, but like I said we can easily measure the primary and core losses by measuring the input power if you think the primary and core losses will be significant.

 

Transformers are NEVER rated in Watts, they can't be. They are rated in VA, which is Volt-Amps. VA are the product of AC voltage times AC current, Watts are VA multiplied times the cosine of the phase angle (called the power factor) so the Watts it produces is always less than the transformer VA rating which is what is printed on the transformer. Depending on circuit type, it can be much less than the VA value.

Yes, yes, yes. Firstly, thank you very much for enlightening.
The catch is in the power factor. With a higher filter cap, the power factor goes down even further, making the secondary rms current higher.
With about 80ms time constant, 50Hz input, (C=10000uF, R=8, I=2A), the rms secondary current goes above 4A.

Once again, thank you bountyhunter!

I am sorry Salaja, if I have been cluttering your thread!

 

Hello,

Here is the bench top power supply that I am going to get: http://www.mastechpowersupply.com/d...gpc-3030d-triple-outputs-30-v-3a/prod_79.html it is a dual 0-30 volt / 0-3 amp meter with 5 volt 3 amp constant available for $169.95

I was thinking about building a couple of programmable power supplies at http://www.tuxgraphics.org/electronics/200707/bench-power-supply-unit.shtml but by the time I added up the cost of parts, cost of a couple of cases and the labor to put them together and do the machine work on the cases to either build the complete (as 2 units into 1 custom aluminum cabinet) with engraving to make it look good (I am a machinist / CAD/CAM/CNC programmer with sheet metal experience and a water jet machine at my disposal, so I could make it look good.) and the only nice thing about them is that I could control them with a couple of USB ports on my computer (not really nessesary) and I would have a good learning experience and be able to say that I made them myself.
Have a great day!
Rick Linnabary