Driftwood morse key

I brought the FT-817 & microphone on holiday with me, but I forgot the morse key.

I searched the beach for driftwood today & found a useful bit. A clothes peg some drawing pins (thumb tacks) and a canabalised set of headphones did the rest.

The movement adjustment features a Coca Cola bottle top, so I’m expecting offers of sponsorship to flood in any day now.


Reverse-polarity protection using MOSFETs Infineon IPD90P03P4-04 and Fairchild FDMS6681Z

Applying the DC supply polarity the wrong way round can happen in a number of ways and Sodde’s Law says it will happen eventually. Building in reverse-polarity protection can help frustrate Sodde’s Law and save you a lot of time fixing pieces of equipment.

In the past I have tended to use relays for reverse-polarity protection, where a diode ensures the relay only turns on if the polarity is the right way round. I wanted to try a MOSFET for doing the same job.

I had a search through Radiospares online for suitable P-channel MOSFETs for reverse-polarity protection. I found two which I might order in small numbers:

Infineon IPD 90P03P4-04 at about £2 each,

Fairchild FDMS 6681Z at about £1 each.

These have max Vds of 20V, max Ids of 90A or more and an Rds when switched on of less than 5mΩ. This seems to be an astonishing combination of parameter values to me!

I set about adding reverse-polarity protection to an ICOM DC RF filter.

The first picture shows the board with a channel cut through the copper on the upper side, (I also had to make a corresponding cut on the under side), and a mark made where I would make a copper pad for the Gate connection. I then stripped copper to make the Gate connection pad, scraped lacquer off the copper & tinned the contacts for the Source, Gate & Drain connections.

I selected the Infineon device for this project. The second picture shows the MOSFET fitted. I included a 10K resistor & 10V zener diode in order to give some protection to the Gate against voltage spikes. I didn’t bother connecting pin 2; I just used the tab connection instead.

Pin 1, Gate (on right)

Pin2 & tab, Drain (centre & bottom respectively)

Pin3, Source (on left)

I tested with reverse polarity & got about -8mV on the output with -14V on the input, no load. The correct polarity fed straight through. It seems to work.



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