Using a capacitor as a "wattless" dropper on 110 volt equipment?

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

I'd agree when these caps go the area tends to fill with very nasty smelling smoke as well Also as this is for a radio receiver and one hads to think weak signals, whatever power supply you use needs to be clean. Switch modes can be clean, but most el cheapo ones are a disaster for rfi. Some most definitely do not like inductive loads either, I'd still think an auto transformer hidden away is a good solution myself. Brian

Especially since an autotransformer can be considerably smaller than an isolating transformer with the same VA rating.

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.

The answer is either a small autotransformer or simply get the mains transformer rewound.,

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

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 ?

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

Bloody simple innit? Anyone spot the slight problem with the sums?

Not so much a headache as a total incomprehesion of ac theory methinks.

That's bloody deep! Where on earth did you find that useful little snippet?

For all the Brexit merchants out there!

AB

In article snipped-for-privacy@bancom.co.uk>, tony sayer snipped-for-privacy@bancom.co.uk> scribeth thus

Thanks for all the replies! In no real order, the aerial concerned is a FUBA UK Stereo 8 this is a bit of a beastie its an old design but its been "renovated" It has a very narrow forward acceptance angle if it were as simple as using a couple of aerials I'd have done that!. But there is a situation where one Transmitter is only a 100 kHz away from another so as best possible discrimination is needed.

I think we'll give up with the cap dropper idea, they did back in the day go short which was fun not! for the series connected valves but there do look to be much better plastic poly whatever around today.

The problem using the Switch mode units is I believe the motors are AC driven, there does not look to be any rectifier or switch mode built in to the unit furthermore the transformer looks a bit fiddley to rebuild and the extra wire needed for the UK mains may not fit so the idea of using an existing tranny and tapping that at half way in autotransformer mode seems OK and I do have a Toroid that would fit in there, but TNP's reference to a simple 230 to 110 volts plug type reducer looks the job so i think we'll go for that!

RS have one but its on flying leads and three times the price!

Cheers..

Class X2 caps would get you better reliability in that respect. You'd probably need a bunch in parallel.

some ac motors are ok on dc, some aren't

yep

it does. You use thinner wire.

The only time a saleswoman made me laugh was when she told me they were cheaper than RS.

NT

Not quite that simple.. if its a motor the moving magnetic field changes things too. It depends on what type of motor it is.

There is quite a lot of stuff cheaper on RS than ebay. But cpc are usually cheaper still.

The hall effect switches I use on my train layout are 69p at CPC or £3 on ebay and amazon.

I thought about buying a few hundred and listing them myself but it not going to make me a fortune.

This the sort of thing needed and its more likely to actually be safe than one off some dodgy ebay trader from some unspecified place in china.

I agree totally.

It's just a brief introduction for those that assume that a 50 Ohm resistor has the same effect as a 50 Ohm impedance.

AB

????

By God RS have changed then.

I don't use RS now. I went off them when I found I could no longer buy

I did wonder when RS went AWG, whether they knew something was afoot Brexit wise, and they went AWG around 5- 6 years back.

Had hassle with CPC, I always used my personal card for company expenses, but CPC would not deliver to site on a personal card. Farnell were a lot better, but oddly enough they seemed to have a lot of stock problems and CPC usually had what they listed.

I use Ebay now. Some of the finished goods can be total junk, but most of the cheapo Chinese components and basic electronic control assemblies are amazing.

PID controller, N type T/C, SSR and a heating element all for less than £20-00

Do that with a Eurotherm based system from CPC or RS :-)

AB

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

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.

You could use the primary of any 0-120 0-120 or 120-0-120 transformer as an autotransformer. Just keep an eye on the power rating but as the rotator will have such a low duty cycle you will probably get away with using something seriously under-rated.

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

Correct. Autotransformers are wound with no secondary and lighter gauge wire on the HV side of the primary.

But Tony only needs a few watts anyway. So I am thinking that a POS low voltage split primary mains transforner will do the job

I've used that arrangement without problems enough times. It's just more issues to address, less situations where it's ok.

NT

It makes the term variable somewhat interesting.

A bank of capacitors to dim a light bulb perchance?

Even high st discounters flog regulators to do that for a few quid.

I cannot think of an instance where you would want to supply a varying load through an impedance, or even a resistance, with the intention of dropping the supply Voltage to a useable value. There is just no point. A Triac or SCR can select the chunk of the sine wave you want and the scource impedence is so low that it can be ignored.

I seem to rcollect that train sets and model car race tracks used resistors to drop the supply from 12V, but I would guess that they are fully electronic now. The lack of back EMF was painfully obvious when using the early ones.

'Wouldn't like to see the controllers replaced with LW tuning cap's, it would make for a slow race.

AB

The Mistral digital clock kit I made in 1974 (maybe 1975) used a capacitor dropper to generate the assorted voltages for the digital side (GI AY-5-1220 clock) and a the heater and anode voltages for the VFD display. Not a varying load though.