On Thu, 9 May 2019 21:31:39 +0100, tony sayer
wrote:


Heres one I'd like to run past the leccy whizzes hereon for comment.

I have a need to twiddle a large FM radio aerial to three distant
transmitters that are 110 deg apart.

Managed to find a very nice aerial rotator on e-bay, brand new made by
RCA in America at a very reasonable price.

Unit has turned up and as expected its 110 volt AC operation now i was
intent on using a small autotransformer to power this but it might be
better if i could run the aerial cable to more than the one location
thats just a 3 core mains type flex but its a PITA lugging the
autotransformer around.

Now i could change the transformer in the unit that on a VARIAC at 110
volts in has two secondaries of 20 volts power for the aerial motor
and 9 volts for the microprocessor and display etc, but thats two
transformers and i can get them from RS and a few others but its going
the be a very tight fit so hence this idea of putting a series connected
cap inside the unit, there is space for that so that drops the volts and
doesn't get warm they used to this trick in some Valve telly's years
ago.

They did!

It was quite effective but the caps were prone to short.

Reliability aside, valve heaters are resistive. Transformers have
inductive reactance. Guess where the "Wattless dropper" used to go.

Feeding heaters means very few hard sums. Everything is predictable so
its one over a pair of pies or fruitcakes or Brexiters- Take yer pick

Personally I wouldn't bother if you are driving with semiconductors.
It may get a trifle expensive.

Even when the reactance of the circuit is taken into consideration I
seem to recollect that the arcs and noise used to be pretty impressive
even with small caps when switching on and off.

Buy a couple of SMPS's off Ebay and ditch the tranny altogether.

Three sepeate aerials plus installation wouldn't be much more than an
RS low power transformer :-)


AB