On Sat, 11 May 2019 11:38:17 +0100, Fredxx wrote:

On 09/05/2019 21:31, 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.

So unit says it is rated at 25 watts so thats about .227 amps on 110 at
full load so thats a resistance of 484 ohms on the supply

So assuming that we bung in a cap of around 6.6 micro farad thats equal
to 480 odd ohms then it should all work.

However it's a transformer load I'm feeding and it's also at 60 Hz
anyone see any snags in doing this at all that I've missed and a prolly
have with the bloody headache I've got at the moment!

As others have said, using a capacitor dropper for a variable load
simply doesn't work.

Do the secondaries need to be isolated from each other? If not then a
10-0-10 would be close enough to 9V and in series give 20V. I'm not sure
if there are many transformers with this output. 9-0-9 used to be more
common.

Maybe the cheapest auto-transformer is a standard one with 2 x 110V
primaries. I expect a true autotransformer with the same rating will be
smaller.

If you opt for 2 transformers remember they don't have to be so large.
The rotator power is also likely to be intermittent and that can be
reflected in the transformer rating.

Whoever reads the posts to you obviously failed to point out that the
person seeking advice is now fully furnished with a solution.

I do note that your posts indicate that you are being fed by a
constant stream of disinformation. Maybe it's time to sack your carer?

AB