Getting 5V from a 18V SMPS

[idpromnut]

As the title suggests, I have a project that requires ~18V@1.5A. Since this project was going to be powered from the mains and I didn't want to get into designing a SMPS or a transformer-based supply.  At my local surplus store I found an HP brick that outputs 6A @ 18V. My project also requires a 5V rail; I tried seeing if I could get away with simply powering a standard 7805 with a heatsink but that just barely works, and really, that's just pissing a huge amount of power away. My next approach is to throw together a small DC-DC converter using the MC34063.

My question is this: is there another way to get my 5V rail from the 18V input other than what I've already tried? I supposed I could drop a bunch of 2-3A rated diodes in series to drop 18V to something near 7V and then throw a 7805 on that, but again, it seems awkward. Should I reconsider the whole thing and just find a transformer with 18V and 5V (or 8-12V) taps?

[Lurch]

What's the current usage on the 5V rail?

[idpromnut]

I've measured about 110mA; with a bit of buffer I wanted to aim for 150mA.

[peter.mitchell]

Do you need the 18v at all? You could probably just change the feedback resistors in the brick...

[SeanB]

You can simply cascade regulators, using a 7812 as a first stage then a 7805 as second stage. Both on a common heatsink capable of dissipating the 3W that it will have without getting too hot. Any heatsink with a rating under 20C/W will work for this.

[megajocke]

Whether you cascade or not, the regulators will still have to dissipate the same power. The required 2 W would not be a problem for a single regulator on a 20 K/W heatsink.

The suggestion of using diodes to drop the voltage would not change the amount of power wasted either. A switching regulator doesn't sound like a bad idea.

[idpromnut]

Do you need the 18v at all? You could probably just change the feedback resistors in the brick...

Yup, this is why I decided to choose a brick with the highest voltage I would need for the project and then figure a way of winding it down for the 5V rail.

[AG6QR]

A quick, cheap, and dirty solution for a prototype would be just to use one of those little USB supplies intended for phone chargers to convert mains power to 5V.  Do this separately from the 18V supply for the other part of the project.

But for 150mA, doing it via a properly heatsinked linear regulator isn't out of the question.  The alternative is to design or use a switching DC-DC converter. 

I'd say throwing together that switching DC-DC converter is the most elegant solution, but if you want to concentrate your effort on the other parts of your project instead of the power supply, I might forgo some elegance.

[SeanB]

A lot of inkjet printers have a power supply in a module that is easy to remove, and some are fully encased as well, AC input on one side and a 3-5 pin lead on the other. they typically give 30V at 1A or so and a second supply of 5V at 1A for the logic side. If it has a single output then the printer board often has a section that you can just cut off which has the buck converter and inductors to make 5V out of the high voltage main rail.

[Chris Wilson]

A lot of inkjet printers have a power supply in a module that is easy to remove, and some are fully encased as well, AC input on one side and a 3-5 pin lead on the other. they typically give 30V at 1A or so and a second supply of 5V at 1A for the logic side. If it has a single output then the printer board often has a section that you can just cut off which has the buck converter and inductors to make 5V out of the high voltage main rail.

Damn, not going back to the tip now, but I flung three today. They sound useful things to cannibalize from them, thanks Sean.

[SeanB]

One powers my PC speakers now. 36V for the TDA2009 amplifier board, and 5V for the USB sound unit. Still have some issues with noise, but it works.

[bingo600]

Get a few of these
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=171193232080

Should be ok for your need

/Bingo

[Rigby]

Those say 3A but they burn up very quickly at 3A, and they get very hot before that; over 100C.

[idpromnut]

Those say 3A but they burn up very quickly at 3A, and they get very hot before that; over 100C.

I will only need about 150mA, but thanks for the heads up!

[Kjelt]

Replace the pot with fixed resistors if you only want 5V output. And place some ceramic caps over the elco's for lower ESR.