Basic Switch Mode 230-20v PSU

PC power supplies are switch mode and available with 5 volt outputs in fairly high amps. Just be aware that many of them need at least a small load on the 5 volts and/or 12 volt outputs to operate. And they include the cooling fan ;-)

Or check the sizes of the supplies offered here:

John

In message , Luke writes

Far, far easier to get hold of a 24 volt unit and adjust/modify it to suit than design something from the bottom up.

If you haven't done it before and think it's easy............ Forget IT !

Graham

What about if I get a PSU book? Are worked examples usually easy? I know the theory is quite involved but I thought there must be some easy standard circuits for commonly used voltage outputs or at least ICs available where you just add a few specified components.

Is the problem that there are too many variables involved as in current/voltage/frequency/noise vs cost etc?

Luke

Hi, Luke. You're not getting a lot of responses because you're kind of way off base on many of your assumptions.

Non-isolated DC-to-DC switchers can be done by a hobbyist (the National Semi Simple Switcher series comes to mind). But line voltage AC-to-DC switchers are a lot more complex. Typically, you will rectify the line voltage to produce 170VDC or 340VDC, to charge up HV caps, then use high voltage transistors or MOSFETs to push the power through a specially made transformer. Feedback from the voltage output at the other side of the transformer is done through an optocoupler. The ICs which accomplish this are typically on the line side instead of the isolated side.

All of this means a lot of peril on many fronts for a newbie. As you mentioned in the OP, there are scads of papers and theoretical information on how to do this, mostly because it's tough to do well without a fairly extensive background in electronics engineering. Nobody writes papers on using a LM7805 regulator, because it's easy. Not only that, but doing this well requires tools (such as a digital storage oscilloscope) which just aren't available to hobbyists.

Let's see -- what else. Oh, yes. Switching power supplies are a commodity item, with vicious competition between manufacturers, on price but also on size. These guys have dozens of electronics and manufacturing engineers trying desperately 50 or 60 hours a week to squeeze every tenth of a cent out of their product, as well as every cubic millimeter. Assuming as a newbie that you'll get a smaller size (not to mention a lower cost) than these large manufacturers on a one-off is, well, kind of earnest, but not realistic.

And the biggest point. Playing with offline switchers is a bit of a hazardous activity. Any troubleshooting will have to be done with potentially lethal DC and AC voltages present. In short, it's not really a job for a newbie. And this is supposed to be an adjunct to your real project, which I guess is model trains, right?

Do yourself a favor. If you want, shop around and get a 150 or 200 watt switching power supply that will do the job. If you want a 20VDC

Good luck Chris

Sorry:

If you want a 20VDC 5A AC-to-DC switching power supply for your trains, just buy one.

Good luck with your train set Chris