Capacitive voltage dropper for 220V->110V operation - UK Vintage Radio Repair and Restoration Discussion Forum

ricard · 28th Aug 2015, 3:51 pm

Ok, so I'm not saying I'm going to do this, but the thought crossed my mind:

I have a piece of equipment - let's say its a radio, for simplicity sake, that is designed to operate on 110V, with a power rating of 12W .

Now, if I had a lamp of that type, I could connect a suitable capacitor in series in order to get 110V across the lamp without the power loss one would get from a dropper resistor.

But would that work with a transformer? A transformer in itself is inductive, but when loaded the secondary impedence is transformed to the primary so it appears less inductive and more resistive.

In order to limit the voltage across the transformer should the secondary load disappear (for instance due to a blown fuse), one could have an additional capacitor to neutral (same value as the dropper capacitor to live). The output voltage across the capacitive divider would never be more than half the input voltage then.

Ok, so much for a bit of theorizing. I haven't done any math, but searching the Internet I haven't seen this done anywhere. There are capacitive dividers, but the only circuit I came across with the divider driving a transformer was in a high voltage probe (measurement) circuit. So what's the culprit? Would the transformer+capacitor combo result in resonance at the mains frequency? Is the transformer too inductive for this to work?

For the moment, I'm leaving safety issues aside, such as it being inappropriate to connect equipment in any way designed for 110V to a 220V mains. Mostly a thought experiment.

ms660 · 28th Aug 2015, 4:25 pm

The possibility of a series resonant circuit across the mains would create some serious voltages, not something I'd favour.

Lawrence.

David G4EBT · 28th Aug 2015, 5:12 pm

Hmmm, sounds a bit like a complicated solution to a simple problem!

12W is a modest load, and 85W 220V/110V step down adaptors are very cheap. EG, in the UK on e-bay £3.50 GBP or from Amazon, at almost seven times that price.

This 85 Watt one from Amazon for example, plugs into a UK standard 3-pin 13A mains socket and has a USA outlet.

http://www.amazon.co.uk/Step-Down-Vo.../dp/B000XROE3Y

Typically, on e-bay a 100W one costs less than £7.00 GBP 'buy it now', post-free.

Presumably similar adaptors are widely available for use in Sweden?

(I appreciate of course that you are theorising rather than wanting to fulfil an immediate need Ricard).

TonyDuell · 28th Aug 2015, 6:30 pm

A word of warning.

Make sure your stepdown converter is a transformer (probably an autotransformer). Some years back there were a load of them designed for 'heating appliances' which were essentially fixed triac lamp dimmers -- in other words a triac with a fixed trigger point so that the RMS output voltage was 110V (or whatever). Of course the output was far from sinusoidal and the peak output voltage was way more than sqrt(2)*110V.

Having seen what one of these did to a switch-mode power supply (and having had to replace just about every semiconductor on the primary side of that supply as a result) I have an intense loathing of them.

JacKam_ · 28th Aug 2015, 6:51 pm

Ricard,

Good idea and if you have enough time - keep experimenting. But experiment is to verify a theory, so the first thing is to create the theory. Probably a LTSpice would work fine.

I know that the output voltage will strongly depend on load and frequency, so the best solution to get 110v would be a transformer.

Many inventions have been made, just because someone else said "That is impossible"

Good luck

Jacek

28th Aug 2015, 3:51 pm	#1
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Capacitive voltage dropper for 220V->110V operation Ok, so I'm not saying I'm going to do this, but the thought crossed my mind: I have a piece of equipment - let's say its a radio, for simplicity sake, that is designed to operate on 110V, with a power rating of 12W . Now, if I had a lamp of that type, I could connect a suitable capacitor in series in order to get 110V across the lamp without the power loss one would get from a dropper resistor. But would that work with a transformer? A transformer in itself is inductive, but when loaded the secondary impedence is transformed to the primary so it appears less inductive and more resistive. In order to limit the voltage across the transformer should the secondary load disappear (for instance due to a blown fuse), one could have an additional capacitor to neutral (same value as the dropper capacitor to live). The output voltage across the capacitive divider would never be more than half the input voltage then. Ok, so much for a bit of theorizing. I haven't done any math, but searching the Internet I haven't seen this done anywhere. There are capacitive dividers, but the only circuit I came across with the divider driving a transformer was in a high voltage probe (measurement) circuit. So what's the culprit? Would the transformer+capacitor combo result in resonance at the mains frequency? Is the transformer too inductive for this to work? For the moment, I'm leaving safety issues aside, such as it being inappropriate to connect equipment in any way designed for 110V to a 220V mains. Mostly a thought experiment.

28th Aug 2015, 4:25 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Capacitive voltage dropper for 220V->110V operation The possibility of a series resonant circuit across the mains would create some serious voltages, not something I'd favour. Lawrence.

28th Aug 2015, 5:12 pm	#3
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,765	Re: Capacitive voltage dropper for 220V->110V operation Hmmm, sounds a bit like a complicated solution to a simple problem! 12W is a modest load, and 85W 220V/110V step down adaptors are very cheap. EG, in the UK on e-bay £3.50 GBP or from Amazon, at almost seven times that price. This 85 Watt one from Amazon for example, plugs into a UK standard 3-pin 13A mains socket and has a USA outlet. http://www.amazon.co.uk/Step-Down-Vo.../dp/B000XROE3Y Typically, on e-bay a 100W one costs less than £7.00 GBP 'buy it now', post-free. Presumably similar adaptors are widely available for use in Sweden? (I appreciate of course that you are theorising rather than wanting to fulfil an immediate need Ricard). __________________ David. BVWS Member. G-QRP Club member 1339.

28th Aug 2015, 6:30 pm	#4
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,215	Re: Capacitive voltage dropper for 220V->110V operation A word of warning. Make sure your stepdown converter is a transformer (probably an autotransformer). Some years back there were a load of them designed for 'heating appliances' which were essentially fixed triac lamp dimmers -- in other words a triac with a fixed trigger point so that the RMS output voltage was 110V (or whatever). Of course the output was far from sinusoidal and the peak output voltage was way more than sqrt(2)*110V. Having seen what one of these did to a switch-mode power supply (and having had to replace just about every semiconductor on the primary side of that supply as a result) I have an intense loathing of them.

28th Aug 2015, 6:51 pm	#5
JacKam_ Pentode Join Date: Jul 2015 Location: Newbury, Berkshire, UK. Posts: 243	Re: Capacitive voltage dropper for 220V->110V operation Ricard, Good idea and if you have enough time - keep experimenting. But experiment is to verify a theory, so the first thing is to create the theory. Probably a LTSpice would work fine. I know that the output voltage will strongly depend on load and frequency, so the best solution to get 110v would be a transformer. Many inventions have been made, just because someone else said "That is impossible" Good luck Jacek