I have designed an elaborate DC Protection and Distribution unit for the 70MHz SSB transceiver project, the purpose of this module is to provide two stage reverse polarity protection, power on/off switching, filter and a switched DC bus for transmit and receive.
I have designed the 70MHz linear power amplifier (yet to be finished) to provide no more than 25W PEP with a 10W fixed output to drive an external linear amplifier, so the DC unit has been designed to cope with a maximum of 10 amps, the relays are Omron G5LE SPDT 10 amp which are not too expensive and can be found on Ebay, RS Components, Farnell and other electronics suppliers around the UK.
The DC unit comprises of two PCB modules, the first is the protection and control unit, the second is the distribution unit, under normal operating conditions +12V DC will be applied to X17 through the first relay K1, through the filter and finally to K2 which is controlled from a front panel switch, the presence of the relay K2 allows for a low current, low profile front panel switch to be used.
Under a fault condition where the careless application of revered polarity power supply is applied to the DC input connector X17, D3 will conduct and blow the fuse followed by K1 closing and sounding the mechanical buzzer to alert you to the fault condition, the reason K1 does not close before the fuse is blown is two fold;
- We want the fuse to blow and make a conscious effort to open the lid of the transceiver and change it once the power supply has been corrected.
- If D3 fails to conduct or goes open circuit, relay K1 will close sounding the buzzer and disconnecting the power from the rest of the transceiver.
The reason K1 does not close at first is due to the forward biasing voltage of diodes D1 and D2, these require around 0.6V to turn on and under load with the relay coil more realistically around 1V with 35mA, the protection diode D3 is a fast switching Schottky barrier diode rated at 60 amps and has a much lower turn on voltage than D1 and D2 so always conducts first blowing thus the fuse and taking itself out of circuit allowing D1 and D2 to turn on causing the relay to close and the buzzer to sound.
You may ask why the complex, elaborate design – why not just use the relay (K1 and D1, D2) at the first stage or a single diode and fuse on its own? Well firstly, if K1 malfunctions under a reverse polarity condition and either fails to close or the contacts have welded, without the additional function of D3 and the fuse then you can kiss goodbye to everything else that follows and watch all your hard work go up in smoke, yes each stage could have its own local protection diode but this does not guarantee protection without a fuse, you need to allow for sods law and anticipate the worst case, however remotely unlikely scenario possible and design for it.
Here is the schematic for both the DC protection and the DC distribution modules, they are identical in size and are designed to mounted above and below each other respectively using hex spacers to save space inside the radio chassis.
DC Protection Unit:
DC Distribution Unit:
Some images of the constructed modules: