OK, what *is* it with these power supplies?
I know that I can't know whether a PSU is linear or switched, buffered or regulated or both or neither without opening up the darn things or just depending on the salesman...

But at least I'd like to know what's the difference between them all!

As technically as you can manage, if you please... I can take it I hope

The job of a power supply is to provide regulated steady power.

Let's assume you have a 24 Volt DC input and you want a 12 volt out. You insert a regulator between the input and output. A linear PSU will just absorb (very simplistic description) all the execess power. So there is a 12 volt drop between the input and output. The PSU therefore has a efficiency of 50%. This operation is really voltage regulation.

A switching PSU will turn DC into AC and then DC again by switching. So it is possible to have a 24 volt input and a 48 volt output. The switch also will switch in more power (again in very simplistic term), when needed. So let's assume 100% efficeicency, a 24 volt in and 12 volt out with a 1 amp load, the input current requirement will only be 0.5 A. The advantage of switching PSU is that it is more effecient so a smaller heat sink is needed and it can also boost DC voltage (DC to DC converter). The disadvantage is that it is noisy.

I don't know what's a buffered PSU. Some regulators (lab) do provide a sense input to control regulation. And some PSU buffer the ground (see DIYs). Maybe that's buffered PSU.

Quote:



What about when the input is AC? That's the situation most of the time really

Generally when you have an AC input, you convert it to DC via a transformer, diode, and large capacitor. The diode is in series with the transformer and the capacitor in parallel. How big of a capacitor you need depends on how much current your load pulls. There will still be some ripple from the AC - the ripple goes as I/(Cf).

You can also get more complicated with a diode bridge: This device lets you use both the positive and negative sides of the AC, and thus lets you get away with a smaller capacitor. Plus it helps prevent ripple.

I'd recommend borrowing a copy of Horowitz and Hill's The Art of Electronics from your local library. It has a lot of discussion about power supplies, amongst a multitude of other electronics topics.

That's in my high school physics classes, ah you've jogged my memory but yes, I still know all that

But what you posted in the other post about big caps not responding to HF... now, that's some freaky stuff

In the meantime, I just want a little buying guide in terms of what terms to look for. Right now I know that I should

1. Avoid 'switched'
2. Look for 'regulated'
3. Look for the biggest, heaviest block for the same rated output current (well, at least not one so small that you would suspect it of being switched)


So what else can I look for? I saw one that says 'double noise filter'... whatever that means?

Also it seems really hard to find a regulated PSU that outputs 24V. The only regulated PSU that output to 24V that I've found is switched

What does regulation (the IC kind of regulation, not switched) do and how does it work? Hmm is it something like this?

1. Capacitor limits the range of voltage oscillation so that the voltage is above X at all times
2. IC acts as a variable resistor such that all voltage above X is dissipated--output becomes constant at X

Am I even anywhere near half clued in?

A linear voltage regulator has three components; a drive transitor (also know as a pass transitor), a correction amplifier and a voltage reference.

The pass transitor provides the current driving capability. The correction amplifier senses the output voltage, and drives the pass transitor to provide for the correct voltage and the voltage reference provides a reference for the amplifier to act on.

The capacitor is used as a filter to filter out the ripple.

If you are considering an exteral PSU, switched or linear should make no difference. A "well designed" PSU should have very difference. The noise generated by the switcher should be isolated in the unit. Any RFI can be eliminated with a ferrite ring.

If size is not a concern, you can get a cheap lab PSU that will do the job. A PC power supply ($10 from the junk yard) will do the job.

You should be able to get a PSU easily and very cheap in Hong Kong. Nearly 90% PSU are made in Asia. I know you can get Wall PSU in Taiwan on the street for $4 to $6.

In US here, a PC PSU from the junk yard can be bought foe $10.