Some thought about variable bench dc power supplies

[exodia]

Hello there

I want to buy a variable power supply for my projects and i need your help-share some thoughts. I live in Greece and unfortunately many ebay sellers don't ship their heavy stuff international. I have found some power supplies here in Greece that are 0-30V and 0-5A, i would be happier if i could find something about 0-24 and 0-8 but... The other think that bothers me is that if the power supply is able to provide 30V and 5A = 150 watt why not be able to provide 15V and 10A also? Maybe a mod could do the trick

[alm]

Some power supplies do have that feature (eg. Agilent E364xA). You're right about the dissipation in the power transistors. You would need a transformer with two identical secondaries that you can switch from series to parallel for high voltage / low current or lower voltage / higher current.

[Kiriakos-GR]

Start looking at the German ebay ... you will find amazing tools over there, at acceptable shipping prices. 

[exodia]

@ ALM don't quite understand that about secondary. If the transformer is say 200VA what will be his problem if i ask for more current that volt as long as i keep the relevance of VA? Is there something i miss?

@kyriakos

I had a look at ebay.de but they have the same problem. Nice prices but high shipping cost

[alm]

You need to lower the voltage to the pass transistor, so you reduce its dissipation at low voltages (a linear power supply converts all of the excess voltage to heat). If you don't change the input voltage, the pass transistor will have to dissipate more, and will probably overheat. For the pass transistor, lower voltages require more dissipation than higher dissipation.

To lower the voltage input voltage to the pass transistor, you want to use only part of the secondary windings. In the case of a transformer with two secondaries for a total of say 30V, you would only use one winding (or both in parallel) for 15V output. The total rating is the total for all secondaries. If you only use half of them, you can only use half the power. You can draw somewhat more power when not loading one of the secondaries, but I wouldn't count on twice the power. The wire gauge in the secondary is designed for a certain current (eg. 200VA/30V), it will get hotter at twice the current).

When modifying an existing design, you may also have to replace the bridge rectifier and smoothing capacitors. You'll also have to tweak the current limiting circuit.

For switching power supplies, it's a completely different story, I wouldn't recommend doubling the current without a thorough review of the design.

[Simon]

with a switching supply you will get a more constant amount of per available but it won't be a direct relationship because switchers peak in efficiency at a certain point and generally at lower voltages are less efficient