If you connect your MST and DDS VFO to a power supply with the wrong polarity you are sure to blow something.
To protect against accidental reverse polarity two possible prevention methods are described here.
The first method uses a fuse and a power diode and the circuit is shown below.
When the power supply polarity is correct the diode is reverse biased and does not conduct. However if the polarity is reversed, the diode conducts and limits the reverse voltage across the MST to about -0.8V. If the power supply is capable of supplying more than 2 Amps the fuse will blow and disconnect the power supply completely. It's a simple but effective system and the only downside is that you will need a replacement fuse if you happen to blow one. The 1N5408 diode is quite rugged and will withstand the short circuit current until the fuse blows.
1. Install a fuse holder on the rear panel close to the power supply input connectors. A M205 holder is small enough to be unobtrusive.
2. Wire the positive power supply connector to the supply side of the fuse holder.
3. Wire the load side of the fuse holder to the MST PCB.
4. Solder a 1N5408 diode between the load side of the fuse holder and the negative power supply input connector. The banded or Cathode end goes to the positive connector.
A photo of a completed installation can be seen in the GALLERY page.
This idea is kindly supplied by Bruce VK2ZZM.
It's a more modern approach but requires a bit more effort to implement than method 1. You could wire it in across the power supply connectors spaghetti style, but a more elegant soluiton would be to make up a small PCB to hold the components.
As shown below a P channel power MOSFET (IRF4905) is used and it is not obvious at first glance how it works, so hopefully this explanation will help.
Firstly ignore the 15V Zener, we will get to that later. Secondly remember that for a P channel MOSFET to be turned on the Gate must be at least 4V negative with respect to the Source. In other words Vgs must be less than -4V. We will assume a 12 volt DC power supply is used.
As part of the internal structure of a MOSFET a diode is connected between the Drain and Source terminals and is normally reverse biased.
However in this circuit if power is applied with the polarity shown, the internal diode will be biased on and the Source voltage will be a volt or so below the Drain voltage.
The Gate however is effectively at ground potential because of the 33K resistor and so the Gate voltage is 11V negative with respect to the Source. In other words Vgs is -11V and this turns on the MOSFET.
Once turned on the Drain to Source becomes a very low resistance, bypasses the diode, and supplies power to the MST.
If the power supply is connected with reverse polarity the internal MOSFET diode will not conduct. As a result the Gate to Source voltage never exceeds -4V and the MOSFET remains off protecting the MST.
Let's look at some practical considerations. The Zener diode is included to protect the Gate from excessive voltages. The IRF4905 is rated for +- 20v Vgs and while this seems adequate in this application the Zener is cheap insurance against unwanted spikes.
If you choose another MOSFET, ensure it is a P channel type, has at least a 60V Vds rating, can handle the required current and has a very low On resistance. The IRF4905 data sheet shows a maximum On resistance of 0.02 Ohms.
The prototype device measured around 20mV drop from Drain to Source at 2A load curent.