A 2-pole fuse protection of DC connection cables is required for DC low-voltage systems in motor vehicles and on watercraft without a fixed earth or ground reference for safety reasons. For example, there are still some older, English (vintage) car models which, in contrast to the worldwide standard, have the positive pole of the battery connected to earth. In these cases, a short-circuit current could flow completely unprotected, mainly via the negative cable. In this case, positive and negative cables are always fused. The additional fuse in the minus cable prevents the radio from being destroyed, the cable from burning and the battery from blowing up.
The results of a cable calculator, using three common cable cross-sections as examples, give an idea of the voltage drop on the connecting cable of a short-wave transceiver.
300 W DC input power at 13.8 V, corresponding to a current consumption of 21.7 A, was taken as a basis.
|Conductor cross-section||Output voltage||Voltage drop|
|6.0 mm²||13.67 V||0.13 V = 0.94 %|
|4.0 mm²||13.61 V||0.19 V = 1.41 %|
|1,5 mm²||13,28 V||0,52 V = 3,75 %|
Many station accessories are small consumers with only a few 10 or 100 mA. Cables with a core cross-section of 0.75 mm² are sufficient for this. In the current range of several amperes, it should be 1.5 mm². For supplying the transceiver with 20 to 30 A, at least 4 mm² are required.
This problem occurs with transformer power supplies when the transformer core is badly packed and there is an air gap between the individual transformer laminations. In this case, tightening the core screw connection can provide a remedy. It is also possible that the strong magnetic field of the transformer causes the housing cover to vibrate. Especially if it is a sheet steel housing and the screw connections have loosened. Furthermore, the hum with the 50 Hz mains frequency also occurs when the load limits of the transformer are exceeded. If mains hum is heard during reception, the sieve chain of the power supply unit is no longer working properly; the charging and sieve capacitors could be incorrectly dimensioned or defective.
Yes, something can be done in DIY without interfering with the unit: For this purpose, the mains cable and the 12 V cable on the output side must be choked directly at the input and output into the unit with high-permeability ferrite ring or ferrite folding cores. This measure improves both the radiation resistance and the transmission of reception interference to the transceiver.