In your last diagram altering the Variac output won't change the linear regulator output, in a good way anyway.
Is that because I forgot to draw the neutral wire from the variac to the power transformer properly?
Or am I misunderstanding something else?
My understanding is:
Variac > 2x 42vac power transformer > rectifiers > cap bank = 0- 60vdc +/- linear supply(thereabouts)?
You tell me. I had a 1kv beautiful vintage unit smooth like a Bentley. I built a nice box with multiple outlets few direct and few variable. Somehow it didn't occur to me to put a fuse on it and one shorted amp took care of it. Live and learn .
I'm wondering if both a fast blow fuse for the variac and a breaker for the device being tested might be a good way to go.
So does that mean that any pair of boost buck converters should work as long as they are fed from isolated supplies.
Essentially the same operation as making a bipolar supply out of a pair of lm338 positive regulators: as long as they run off separate secondaries, rectifiers etc and the connection between the 2 is only at the output.
So if I used the 2 secondaries on the above 42vac transformer to make 2 separate 60vdc linear supplies to feed a pair if boost buck converters. Then the boost buck converters should be happy having their outputs connected in series.
If you don't have regulators then sure, it will work. But Vout will vary with mains voltage.
Regarding your last question as long as they are not referenced in any way you should be able to connect GND of one to Vout of the other to have +/- rails. At least that's how I understand it.
perfect suitable, as only a few volts needed to actually start an amp up.
There is something to be said for keeping the variac just a variac and making a separate adjustable lab supply using some dc-dc converters.
I have avoided dc-dc converters and thus know little about them.
From what I can understand, the advantage of a boost buck converter is that it allows stepping up voltage as well as down.
Lets say I had 2x 65vdc linear supplies, is there a good reason to not use a pair buck converters.
The reason I ask is that I don't like the look of the voltage/current selection on the DPH5005 units.
Running a pair as a bipolar supply would involve setting 2 different meters.
https://www.aliexpress.com/item/100...D4x86Z4k&utparam-url=scene:search|query_from:
Something like these would allow replacing the trimmers with dual gang potentiometers for dialing current/voltage.
Thus allowing analog control of both channels at once.
There is also this one which is cheaper but has no current control:
https://www.aliexpress.com/item/100...cene:pcDetailBottomMoreThisSeller|query_from:
Simpler to operate: 1 dual gang pot for voltage on both channels.
Current limiting would have to be done separately but perhaps a simple switchable 2 transistor current limiter at the output might be a good option.
Eg: 100mA seems reasonable for testing power amps.
Am I missing anything on the disadvantages of using a cheaper analog buck converter?
A fast blow fuse might increase the life expectancy some, but the winding can take a lot of abuse as long as you dont start arcing at the wiper electrode. These might suffer some , so make sure you have access to spares (generally difficult with older varieties)
A slow blow makes it possible to start heavier loads such as transformers.
An automatic fast fuse is probably a good choise.
I have gym-load of older variacs 1kVA upwards that surpasses me in age, all working fine but the wiper are close to EOL. New ones can be found on some of the american brands , for a hefty price but my older european ones are unobtanium, but maybe some handicraft can be useful. Dont worry about the anisotopy , they work fine using carbon electrodes from batteries etc.
