Yaesu FT-817ND Power Supply

I misplaced the wall-wart battery charger for my Yaesu FT-817ND and realized that even if I found it, I'd have to modify it to use it now, as I have added an Anderson Power Pole adapter to the rig.

That got me thinking. I didn't have a small power supply capable of running the rig during transmit (in addition to performing the battery charging function). So I checked the specs. Looked like the ideal supply would be 13.8VDC at more than 2.5A.

The 817 tolerates a wide operating voltage of 8.0 - 16.0 V, so I began my search for a suitable supply among my surplus.  I narrowed it down to 2 candidates, a 15V 3A supply and a 12V 5A supply.  After asking a few hams, I decided to go with the 15V 3A supply and drop the voltage a little to get to 13.8V.  Ideally, this would provide max output.

I measured a no-load voltage of 15.27V.

I procured a bunch of 10A rectifier diodes - like 20 for $5 shipped.  I measured each to have a forward voltage drop of around 0.35V.  I was expecting more like 0.6V.  But these were marketed as "low voltage drop", so I thought perhaps they were a little different.

Sure enough, 4 of them in series dropped the voltage ~ 1.4V (about half as much as expected), and this meant 13.8V.  Perfect, I thought. But once I put a load on the circuit, merely turning the radio on (receive mode is ~450mA), the display noted 11.6V!

Well, I should know better than to expect a circuit with no load to behave like one under load. So, I recalculated and realized just 2 diodes would do the job.

I couldn't get it to exactly 13.8V with diodes, but 13.5V is very close.  And since I modified the circuit somewhat, it was important to label the adapter as such. It has a nice Anderson PP connector and connects directly to anything expecting ~13.8VDC.  At 3A, it's enough to power almost any rig in receive and smaller rigs in transmit.  Seems not to put out much RF noise to speak of.  It is small and best of all, mostly free.


Popular Posts