Power Supplies

[melrel]

The time has come for me to purchase (or build) a power supply.  After listening to Dave's recommendations I am still a little confused over what he calls 'dual rail.'  Believe me opinion does vary!Does this mean the same as dual output?  I am considering the Thurlby Thander models similar to the PL320, but need your assistance. If I build one which I guess would be a good learning project where the heck do I start looking for a suitable schematic/kit amoungst the wilderness out there.

[ThievingSix]

I've been trying to build a PSU for the past 6 months, its a slippery slope considering i have no background engineering experience. Often for certain regulators you can find PSU circuits on the datasheets themselves, other times you'll have to be a bit creative and work off the example circuits and datasheets and theory you've picked up elsewhere.

I've always understood dual output to mean that there are two separate outputs, i.e. one at 0-30V and the other at 0-30V separate from each other, so one can run at 12V while the other runs at 5V. I'm not entirely sure about dual rail but i believe it generally refers to two separate outputs running at the same voltage . i.e. the two rails spread the load across each other preventing one from being overloaded if your drawing a large amount of power. So in a computer PSU there might be two +12V rails, one powering the CPU, the other powering your graphics card. Alternatively i've heard the term used to describe two outputs, +5v and -5v(negative five volts).

[Psi]

Dual rail almost always means a negative and positive output to ground.
So -20  0  +20 for example. They're linked and controlled by a single voltage pot. (0-40V)
The word "rail" means "supply rail" and supply rails are with respect to a common ground.

Some supplies have separate outputs with controls for each and can be connected together to make a dual rail system by the user if needed. I wouldn't call those "dual rail" but rather "dual supply"

[ejeffrey]

What psi said, although dual rail could also mean two different positive voltages like +5 V and +3.3 V for use in powering digital logic.  That is less common, and if I saw a dual rail supply with no elaboration I would expect it was +/- voltage typically used for powering opamps.

[melrel]

Hey thanks guys.  I guess from your replies one doesn't just dive into this subject.  I take the point about powering op amps and this was one of my areas of interest. 

[Mechatrommer]

i believe the proper term for +/- rail is bipolar rail, but "dual" is used interchangeably. confusing world, just as the word "differential" it can be +signal and -signal perfect inverted mirror to one another as in usb protocol. but it can also means A-B where B is not necessarily inverted of A. lets dont talk about whats saturation in fet and bjt, i guess ee or engineers are not very good at linguistics.

[PA4TIM]

They also call them simetrical powerssupplys. If you buy a double supply they often add the possibility to couple the two rails. In Series to get a symetrical psu, and you control the voltage with one knob. That is called tracking. Or paralleled for more current.

Building a variable voltage psu is easy, just use a LM317 and 337 but building a good one with protection, constant current mode and current limitation, digital reading ect is a whole different story and if you want to make it nice, in a good cabinet, 4 difpgital meters, quality connectors ect, it is probably more expensive as buying one. The TTI is a very nice example, still regret I decided to save the difference between that TTI and a 50 euro cheaper one from an other brand.