I have a transformer scavenge from a VCD player, written 25W on VCD player, so that should be the transformer's VA rating ?I also measured the transformer, it has 2 secondary. It have 11.1VAC on 1st output winding and 13.1VAC on 2nd output winding.So if I were to use into DC (rectification) and calculate for other maths, should I use the value of 11.1VAC / 13.1VAC, or their average which is roughly 12VAC ?
nicely the voltage could desire to be smash up around the resistors (as resistors in sequence artwork as a voltage divider) with a million/11 going around the 100ohm and 10/11 going around the 1kohm, so i could anticipate look on the 0.4W resistor. With R of one thousand and P (max vigour) of 0.4 you need to use the equation ( P VI or greater, PV^2/R) 0.4 V^2 / one thousand 4 hundred V^2 V 20V
A transformer rating is specified as a number of volt-amps (VA) where A is the RMS current. In cases where there are rectifiers, the RMS current may be far greater than the average current. This means that the power that can be delivered will be far less than the VA rating.
yes, 25VA would be the sum of the two outputs. IF they are balanced, that would be about 1 amp current output. That assumes any motors operate off the transformer. you can use either secondary separately to get two voltages (about 14v and 17v DC), or you can put then in series to get 24.2 VAC (32 VDC). You CANNOT put them in parallel without the transformer smoking. The DC voltages listed above assume a FW bridge and large capacitor filter. You can probably get about 0.7 amps DC out of the supplies. Again, that assumes the two windings have about the same current rating, ie, they use the same size wire.
Well, 25 watts is the amount of power consumed by the VCD player. I would assume that the power supply can provide at least that much power. One thing you need to be careful of is that newer electronic devices use switching power supplies, which operate at high frequencies, and therefore use much smaller transformers. These transformers will probably blow up or get very hot if you try to put 50/60Hz on them. The measurement you got is a little odd; I would bet that if you put some kind of load on the secondaries when measuring the voltages you would have found that both secondaries are operating at about 12VAC. It's very unusual for a transformer to have 2 different voltages that are so close together. Put a 1K resistor or something like it across the secondaries, and re-measure them. Also, there's no guarantee that the two secondaries are designed for the same power; for example, one secondary might be rated at 25VA and the other at 1VA. Look at the wires going to the terminals and see if they are the same size or if one is a thinner gauge than the other. (This is actually very common; a unit might need a lot of power at +12Volts, but only a tiny amount at -12, for example).
