What is the real output current of this power supply?

[sspj9]

I have this AC adapter which states that the input is of 230-240V AC, 50Hz, 23mA and the output is 12V AC, 250mA and max 3VA. But as I peeled the label there was another one beneath it which states that the input is of 230-240V, 50Hz, 38mA and the output is 12V AC, 500mA and max 6VA. Therefore I would like to know whether it is safe to convert it to DC via a full bridge rectifier and 100 micro-farad capacitor and a 12V voltage regulator to power a 12V DC 0.21A fan and a 12V DC 0.25A fan connected in parallel.
Tks.

[mariush]

12v AC @ 250mA (= 0.25A  )  = 12vx0.25a = 3VA

When rectified, Vdc PEAK = 12x1.414 = 16.91v , minus the voltage drop on the diodes so about 15-16v 

Current will be about 0.65 x I ac = 0.16A .. so about 150mA would be a safe bet.

If it's 6VA, just double the current.

Most fans are 0.12-0.25 A, you should be able to power a fan easily. If you really want the voltage to regulated to 12v, the linear regulator will need 1-2v over 12v to output 12v properly (the value varies from regulator to regulator). In this case, 100uF won't be enough, I'd look for at least 470uF.
Fans don't really care about exact voltage, in fact you might want to run it at 9-10v just to get less noise, in which case the uF value won't be that important.
Two fans might be a bit of a stretch, they'll work but you'll probably overheat the transformer and/or get lower voltage

[sspj9]

So is it safe to power these two fans in parallel which would need a current of 460mA? How should I know if the power supply produces enough current if one says 250mA and the other says 500mA?

Also, I wasn't talking about the capacitor which I should connect to the voltage regulator, but the smoothing capacitor which I should connect to the rectification. Is 100 micro farads enough for the rectification?
Tks once again.

[mariush]

Open it up and check the transformer.... it may say on it what it's capable of.

Or build a constant current load ( http://www.sleepyrobot.com/?p=136 ) and slowly raise the current while checking the temperature of the transformer and the output voltage.

Yes, I was talking about capacitor after rectification.

If you use a 7812, if you want 12v out, you need to always give the regulator at least 13.5-14v. At low loads (0.15-0.5a) 1.5v over 12v would probably be enough.

So assuming you have a peak voltage of 15.5v and you want a minimum of 13.5v you have a ripple voltage of 2v  (the difference, 15.5 - 13.5). 

At 0.5A output, to have a minimum of 13.5v you'd need about  C = current x duty / [ 2 x mains frequency x Vripple ]  =  0.5a x 0.7 / ( 2 x 50hz x 2v )  = 0.35 / 200 =  0.00175F  = 1750uF 

So ideally, you'd use a 1800-2200uF.

But like I said, fans will run at various voltages, as low as 5-6v, they just won't spin as fast as at 12v.  If the linear regulator won't have 1.5v or so over the 12v value, it would probably simply output less than 12v and your fan will still work but the speed will be lower (natural, less voltage is less speed)

[sspj9]

There was nothing written to the transformer. However I took a look at the fuse which turned to be a 600mA one!! Yeyy . I took out the 100 micro farad capacitor and soldered a 1000 micro farad one - which I only had and attached the voltage regulator. It worked pretty gd.

Tks

[mariush]

The fuse is usually on the primary side, and it's much bigger than the current going through the primary side. 

The current in the primary is 23mA for the 3VA version, or 38mA for the 6VA version, so in theory you could use a 100mA fuse BUT ... when you plug the adapter in and the transformer "charges" up with energy it uses more than that average current.

Also, when you have a capacitor after the bridge rectifier, the capacitor also acts as a short until it fills up, so the current will be much higher for a few milliseconds. Hence, that's why fuses are often time delay ones and with bigger current rating. The value of the fuse is not a true indicative of the transformer capability.

Make sure the capacitors are of proper voltage rating. Use 25v or higher rated capacitors, unless you want the capacitor to leak or blow up due to getting more than 16v through it.