sony_apm_fan said:
Thanks Super...great explanation.
So there is basically nothing to protect the primary winding from this sort of failure? Or mistake?
I was sure I have seen fuses before windings in some boxes...but could be mistaken.
Either way, your explanation of how a fuse actually works is great and very helpful.
I looked at the PS schematic for the FA-C5 (next model up) which doesn't have DC and neither of the fuses (there are 2) protects the transformer primary winding. You just can't make a single blanket generalization for where a fuse might be located but I will say that in almost all cases, the primary is not protected. You need to keep in mind that prior to eBay, regionally distributed electronics, especially larger ones (unlike walkmans or cameras) are hardly ever imported. So an electronic device distributed in North America generally does not end up in Europe. With that, the protection components such as fuses are designed to protect based on the calculated max anticipated current based on the proper voltage applied.
In general, current consumption is based upon the boombox load so if the boombox draws more current from the secondary, it should draw more current from the primaries too. Therefore, protecting the secondary by extension should protect the primary windings too if the proper voltage is applied. Protecting the primary windings with fuses is a little more tricky and may require multiple fuses on multivoltage units. Consider this: On the secondary side, the voltage should be standard for that model regardless of input voltage. Therefore, a 5-amp fuse (on the FA-C5) in series with the secondary windings is correctly sized to protect the internal boombox electronics regardless of what input voltage is supplied if the voltage selector is properly chosen. In other words, if operated properly, the expected secondary voltage might be 18v regardless of whether the boombox is operated at 115v or 230v. However, if a fuse is sized for the primary, say 1-amp @115v, then that fuse would need to be 1/2 amp when used @230v and other different values depending upon what other voltage choices are present. As you can see... protecting on the primary side will increase the costs of manufacturing and complicates the circuit.
Going back to
talking fuses, you will likely note that even though your boombox secondary is someplace between 12 and 20 volts, those glass fuses probably are marked somewhere between 2.5 to 5amps at 220 volts (or some other high voltage). As long as current remains below the rated value, the actual voltage makes no difference to the fuse at all. 5amps at 10 volts = 50watts. However 5amps at 220 volts = 1100watts!!!!! In either case, the fuse is within it's limits and should not blow unless the current exceeds the 5 amps. If the fuse is theoretically 100% accurate, it will blow when 51 watts are consumed with 10volts applied to it. But if 220 volts is applied, it will not blow until 1101 watts is consumed.
Now, in actual practice, applying more voltage generally causes more power to be consumed if the load is strictly resistive. Using Ohm's Law, we can conclude that doubling the voltage through the same resistive load will result in double the current draw and therefore, blow the fuse, right? In reality, it is far more complicated than that. When operated at low volume, a boombox can be very thrifty (.2 amps) and when blasting at the max, it might consume a whole lot more, maybe 3.5 amps or more (but less than the fuse rating). If you pump double the voltage to the boombox, it would consume twice the amount of current presuming the internal resistance remains the same. However, when operated at low to moderate listening levels like before, even at double the input voltage, a boombox does not consume nearly enough to blow the fuse (2 x .2a is only .4 amps). But the whole time, the boombox will be subjected to double the normal system voltage which could rise to 36-40 volts (or even more). Most transistors, resistors, diodes can handle the increased voltages well. Electrolytic capacitors on the other hand has limited voltage ranges (based on intended system voltage), and IC's usually have far lower max input voltages than 36 volts.
In any event, the transformer primary, if not open circuit, could be shorted. It is important to get that resolved, even if you never want to use AC again and intend to use only DC. The reason is that the AC jack remains available and sometime, someplace, maybe years later, somebody else could potentially plug it in, and if the primary failed short, then next time, it could end up burning down the house. When a transformer winding is overheated, the insulative qualities of the wire used in the winding has been compromised and if short or partially short circuited, readjusting the voltage selector to the proper setting will not fix that and still likely result in more fire