A transformer question

[Benedict]

Hi there! I'm trying to make a small psu for the o2 amp i bought recently. It has to be 14-20v ac @ 500ma output, I live in 220 volt area. Can I use just a transformer rated to such specs? Won't it screw up the phase angle, be incredibly low efficiency, and even draw power when nothing is connected to it? Any alternatives? Perhaps some feedback circuit and/or phase correcting caps? Is that neccesary? Any idea about perhaps a switching psu with ac output? I'm not afraid to learn. Thx for your time!

[Paul Price]

You should be able to find a wallwort rated at 12-Volt and able to deliver >500mA to either salvage the transformer that you need from within, or if the wallwart is a high-efficient switcher, just connect it  to your amp if it will work with DC.

You should also easily be able to find a 12V 1-Amp (aka 12.6VAC RMS) transformer that will work just fine. Stop worrying about upsetting phase angle and all that other stuff, a small transformer just doesn't cause these problems.  If you worry about power wasted, just add a power switch to turn it off, but a new, good quality transformer will not be a waster of any significant power when nothing is connected to it. You can use your digital wattmeter (your fingers) to determine if power is being wasted. It should be cool without any load.

[Benedict]

Awesome, thank you. Unfortunately it *has* to be 14-20volt *AC*. Wont even power on with dc.
So one last question, how much sense is there to add a 20 volt overvoltage protection (those clipping diode-like expensibe thingies, forgot their names) to it, as anything over 20 volts will prolly kill it?

[Zero999]

It's very likely the amplifier has a voltage doubler circuit to get a dual rail power supply so it needs to be powered from AC not DC.

Unfortunately, small transformers are inefficient and draw lots of power, even when not in use.

[mariush]

Just buy a small toroidal (or regular) transformer and the proper connector. Like Hero999 says, most likely the device requires AC input because internally it derives negative and positive voltages, so cannibalizing a wallwart may not work.

You don't say what country you're in.. have a look at tme.eu  or farnell.com  ... here's some suggestions below from Farnell (uk version of the site, there may be a version for your country) :

Classic transformers :  http://goo.gl/ZM7PUm   (farnell link, it's miles long so i shortened it)

Toroidal transformers : http://goo.gl/tII6vN

Links above also have transformers that have two secondary windings, for example 2 x 7v or 2x9v .. you can connect those in series to get one 14v AC or 18v AC ... job done.

ps. I wouldn't worry much about waste of power, at most you're probably looking at 5-10 watts of power being wasted, which is basically nothing. A couple of kWh a month, less than a dollar on the electricity bill if you keep it pluged in 24/7... but you could add a power switch to disconnect it completely from mains.

[Benedict]

Thx guys. I was wondering wether to get toroidal or classical myself infact. However, would you suggest to add overvoltage protection, TVS diodes http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode just to be sure, or is that unnecessary crap?
Edit: I'm in Croatia, but I have no problems picking a proper part, that's why I didn't ask, no need to bother you guys with that, but thx for the exra effort

[Benedict]

http://imgur.com/a/eqfml/

Here is the finished product! Thanx for your insights once more!

[Dave Turner]

I suggest that you add a fuse on the mains input.

[Benedict]

That's a good idea, thank you. Will do.

[Benedict]

It's rated as follows (transformer)
U1 : 220V
U2: 17V
I2 : 1.17 A

I'm thinking, 20W, so a 0.1 Amp fuse on the primary?

[Dave Turner]

I'll bow to others' experience but I'd work out what the maximum power draw you expect from the secondary is, reconvert that for the primary, allow a margin; then decide whether a slow or fast blow fuse is appropriate.

[mariush]

17v * 1.17A = ~ 19.9VA

19.9 VA / 220V = 0.09 A

I would probably go with a 0.25 A fuse, maybe slow blow (time delay), because you don't know what's going to be at the end of the connector (large capacitors can draw a lot of current for a short time).

[Benedict]

I think I'll go with the 0.2amp slow blow and integrate it somewhere in the primary wires.

What would you guys say, would it make sense to integrate one more bidirectional transient voltge suppresion diode, 1.5k type, either in the primary or secondary side, for overvolt protection? thx

[Benedict]

Okay, so if anyone is looking for this info,

basically the best protection is usin a transil (tsv) and a circuit breaker / fuse combo.

For transils, read the first page here : http://www.microsemi.com/document-portal/doc_download/14650-how-to-select-a-transient-voltage-suppressor

And then it's just the case of choosing a proper fuse that can kill the circuit before your transil blows up.