On Thursday, 9 May 2019 21:34:06 UTC+1, 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!


Cheers..

The issues...
1. Neither the motors, transformer nor the dropper cap are pf 1, nor is any BRed electronic load. So it's not as simple as treating them like resistors. However you could use that value as a starting point & tweak it until correct.
2. When load i is lower, V is higher. This may be a problem for the transformer and for the loads.
3. Dropper caps are vulnerable to shorting, so suitable fusing & thermal protection are wise, and in some cases overvoltage protection.

So in short, maybe, if you can address the load regulation issue.


NT