On 10/05/2019 11:06, The Natural Philosopher 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
voltsĀ* in has two secondaries of 20 voltsĀ* 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!
Problem witha cap as a dropper is that the power consumption of te unit
will vary and if the motors are NOT running you might end up with a far
far higher votakge than you anticipate.
+1
The answer is either a small autotransformer or simply get the mains
transformer rewound.,
The other thing to watch is that some US stuff is really tight on the
power rating for the transformer and it may be close to saturating the
core when run at 50Hz instead of the designed 60Hz. Those crap mains
electric razors they used to sell at Xmas relied on a 60Hz mechanical
resonance to work. Sold in the UK they only lasted a couple of years.
Oddly enough one of te best ways to get a sm,all auto transformetr ois
to find say an old RS mains transformer that has two 110V promraies -
the type that could be uses in series or parallel to get 230/115v
capability
+1
Wire the primaries in series ignore the secondaries and take 115V from
the center tap and neutral. It wont be perfect, but it will be cheaper
than a custom auto
Or one of these ?
https://www.ebay.co.uk/itm/Step-Down...W/232415947981
You can get neat compact low power step downs intended for 50W loads.
Or DIY as above.
--
Regards,
Martin Brown