Strange problem with DC converter

Hello,

I have a simple DC adapter that you plug straight into a wall socket. It has a transformer that brings the voltage from 220VAC to 12VAC, then 4 diodes for the full bridge, and a smoothing capacitor. The problem is that it outputs ~16.8VDC. I have checked the transformer, and it outputs 12VAC as it it is supposed to, I have checked the diodes, and have replaced all 4 of them, still the same thing, i also have replaced the capacitor, but nothing changed. It is a really strage problem for me since the transformer outputs 12VAC without any problem.

Am i missing something here?

Thanks.

AC voltages are usually characterized by rms, or "root mean square" voltage. When AC voltage is rectified and filtered, the peak voltage (no load) is about 1.4 times the rms voltage. 12 x 1.4 = 16.8

Hi, The Amplitude of your mains voltage is not 220V but 220V*sqrt(2) which is approx. 311V. Your multimeter however measures the rms value and therefore 220V. Same thing is true for the voltage on the secodary side of the transformer which doesn't have an amplitude of 12V but 12*sqrt(2) = 16,97V. On the other side of your full-bridge you then have a DC voltage of roughly that value which is in your case 16,8V. Guess you will need some additional components to get your 12V.

Cheers, Dominik

Wow, did not think about that. Its crazy that its actually acomercial product that you can buy and on the back of it it states it outputs 12VDC 1000mA.

Is this a common thing for theese kind of cheapo wall adapters? Also, is there any easy way to get my 12VDC from it?

If you draw 1000 mA from it, the output probably will be about 12 V. This is perfectly normal behavior.

You need a voltage regulator to provide 12 VDC, regardless of the current draw (up to the allowed maximum). There are many to choose from.

onix45: Wow, did not think about that. Its crazy that its actually acomercial product that you can buy and on the back of it it states it outputs 12VDC 1000mA.

Well, it does, sort of. It is after all, unregulated.

There are two factors here. One is that when not loaded, the capacitor will rise to the peak voltage of the waveform. And when fully loaded at that one Amp, it will have a number of volts ripple at a frequency twice the mains, so the average will be around 12V, but deviating a few volts either side.

The second is that the windings have substantial resistance, and this is more significant in relatively lightweight transformers, so it is designed to be more than the necessary voltage, but deliver about the rated voltage when fully loaded. I am surprised the transformer read only 12V AC.

onix45: Is this a common thing for these kind of cheapo wall adapters? Also, is there any easy way to get my 12VDC from it?

Dead normal. If you want a smooth, regulated 12VDC, you can build a regulator but frankly nowadays, you are better off to go and buy a (much more efficient, cooler) switchmode supply. Won't cost an awful lot more.