How to dissipate up to 1kW of heat on a floating ground linear supply?

[Kleinstein]

A transformer with already a DC current rating is unusual. A transformer rated for 24 V 40 A AC would give you something like 30 V and 20 A of DC. If made for operation with a rectifier the transformer might already include intentional output inductance. Anyway, with unclear data on the transformer it would be a good idea to get an idea on the no load voltage and maybe the voltage with a significant DC load and the rectifier / filter caps.

Foldback current limiting might be an option to reduce the worst case power dissipation without reducing the maximum useful power by much.

[Bendba]

You might look into using CPU water cooling blocks. They are efficient and you are extremely unlikely to beat the heat transfer capabilities. The downside to using all these computer water cooling parts is that if you chose a computer water cooling pump they won't work will with high water temps. They are not meant for water temps much above 35C.

I looked into CPU exchangers but they don't really physically fit with the TO-3 package. I think I'll just cast my own.
Pumps won't be a problem, I have some that can handle up to 90°C water.

A transformer with already a DC current rating is unusual. A transformer rated for 24 V 40 A AC would give you something like 30 V and 20 A of DC. If made for operation with a rectifier the transformer might already include intentional output inductance. Anyway, with unclear data on the transformer it would be a good idea to get an idea on the no load voltage and maybe the voltage with a significant DC load and the rectifier / filter caps.

Foldback current limiting might be an option to reduce the worst case power dissipation without reducing the maximum useful power by much.

I know, it is unusual. I'm gathering some 24V light bulbs to do a load test at different loads from 0 to 40A

I didn't know that it called foldback but that is what I had in mind as to do for the variable current limit.

[Kleinstein]

Foldback current limiting is having a lower current limit if the voltage drops. This effectively limits the worst case power dissipation. Many simple regulators like the 7805 / LM317 use this.

CPU coolers are not TO3 compatible, more like suitable for TO247 and similar. However the plastic cases usually have a lower temperature limit and thus might need more de-rating.

[Zero999]

The transformer I have is actually rated for 24V DC 40A, I know, it's an unusual rating but by the size and weight of the transformer (over 25kg, about 45x25x25 cm) that sounds about right.

I'm still trying to get informations from the manufacturer but apparently they are tied up with the medical equipment factory not to divulge information. I do believe that the rating is for the actual rectified output. That would mean I do not have to account for losses in the transformer and bridge rectifier.

That is a strange rating. Does it have the rectifier and smoothing capacitor built-in? Have you done any measurements?

Is there a centre tap? That would be ideal as you could use a relay to switch between half and the full voltage, depending on the output. Can it be easily rewound? That way you could add many taps to adjust the input voltage to the linear regulator.

[Bendba]

Finally got enough light bulbs load it up and make some measurements. 15x 55/60W car light bulbs. That's going to be bright

[Bendba]

I finally got around to load test that big transformer.
Here are the results.
Sorry, I didn't yet take time to format the graph properly, the X axis isn't perfectly linear but that give a good idea.

Here it is at 131% of the rated load



And the test setup, H4 light bulbs tired in pairs (they are 12V bulbs, they didn't have any 24V ones at the wreckers)
The shunt is on the blue write going back to the supply. A bit rough but cheap and it worked.

[MK]

A transformer with already a DC current rating is unusual. A transformer rated for 24 V 40 A AC would give you something like 30 V and 20 A of DC. If made for operation with a rectifier the transformer might already include intentional output inductance. Anyway, with unclear data on the transformer it would be a good idea to get an idea on the no load voltage and maybe the voltage with a significant DC load and the rectifier / filter caps.

Foldback current limiting might be an option to reduce the worst case power dissipation without reducing the maximum useful power by much.

The opening lines say this is a consant Voltage (ferroresonant) transformer, they have an S shaped input VI curve, and require a minimum load, and the output self collapses when you draw too much power too.