Voltage divider

[Dick Glennon]

Hi again. Is it possible to incorporate a voltage divider to power an American radio ie 240 volts to 110 volts. The radio measures 210 milliamperes on load with an industrial transformer. Any thoughts?

Dick.

[Boater Sam]

A resistor voltage divider has terrible regulation, the voltage will change with varying current drawn such as the audio output of a radio. And get very hot.
A straight resistor dropper, like the old line cord dropper, will work but the heat dissipation will be huge.
Step down transformer is the best way.
You could consider a capacitor voltage dropper.

[Skywave]

You need a step-down transformer: 240-v. primary, 115-v. secondary. 210 mA @ 115-v. equals 24 V-A. A small auto-transformer will be suitable. I might have something in my transformer collection.

Al.

[Herald1360]

Originally Posted by Boater Sam

A resistor voltage divider has terrible regulation, the voltage will change with varying current drawn such as the audio output of a radio. And get very hot.
A straight resistor dropper, like the old line cord dropper, will work but the heat dissipation will be huge.
Step down transformer is the best way.
You could consider a capacitor voltage dropper.

Yes, regulation is poor with a resistive divider but it's even worse with a straight series dropper such as a line cord. In any case it's not relevant for a typical AA5 set- the class A output stage current is constant average dc and the smoothing caps in the set will stop any a.c. audio component getting on to the mains input side.

Other options like capacitor or diode-resistor are non trivial to get just right for a 110V set so yes, a small auto transformer is the way to go. A cased one with an American outlet socket would be ideal since the set could keep its original plug making it at least difficult to plug into 240V by mistake! I'd go for something a bit beefier than 25VA though, since there's a significant dc component in the set's mains input current.

[Mike. Watterson]

Note that an isolated transformer is needed for sets with no internal isolation transformer as spacings and capacitors may not meet safety limits for 240V, even though the voltage is apparently dropped to 110V to 120V via resistor, capacitor or transformer.
I use battery valve mains sets (USA or European/UK) on boxed shaver transformers adapted so a 5A 3 pin plug can plug into 230V outlet and the US 2 pin plug can only plug into 115V outlet.
These use a 4KV isolated 20W transformer, usually with a thermistor. The outlets are typically fed from a centre tapped 230V secondary, with centre tap being one of the 115V outlet connections. I usually add a 10nF 2KV XY rated ceramic capacitor between mains earth and the centre tap. This greatly reduces interference buzz. The hole drilled in the panel near the edge for the 5A 3 pin plug need not connect to earth as the outlet is isolated.
Such transformers can also be used for replacement 120V (two rectifiers) or 240V (bridge) DC supplies up to about 90mA. A separate transformer would be used for LT.
Some of the transformers also have the laminations designed to current limit as well as having the thermistor (which I remove).

[Dick Glennon]

Hi again, Would a 45 watt step-down transformer be powerful enough to power the radio. It has to accommodate a power surge while heating up. Dick

[Herald1360]

That should be adequate. Short term overloads like switch on surges from cold heaters won't worry a traditional heavy copper and iron transformer but they might be problematic for a lightweight solid state type.

[Skywave]

Re: post #6. Just one small point. Transformers are rated in terms of V-A, not watts.

Al.

[Al (astral highway)]

True 'dat.

However, I see that confused simplifications often appear because the Volt-Ampere (VA) measures power in a direct current ( DC ) electrical circuit, not an AC circuit.

I see quite a few folk thinking that one VA in a transformer has equivalency with watt (1 W). It does of course in DC circuits, when power is expressed by the product of volts and amps.

But as some but not everyone using this terms knows, in AC circuits, reactance complicates the relationship between real and apparent power. People rarely know the reactive component of their power supplies, so it's easiest to offer a rule of thumb...

So to be safe, the the true wattage rating should be taken to be only 50 to 65% of the VA rating.

I don't know what hooked you into making the helpful if slightly restrained comment you made, but I guess it's something along these lines?

One of these https://www.amazon.co.uk/Stepdown-vo...s=110v+adaptor will do nicely.

[Skywave]

Originally Posted by astral highway

However, I see that confused simplifications often appear because the Volt-Ampere (VA) measures power in a direct current ( DC ) electrical circuit, not an AC circuit.

No. I think you got a bit confused when you wrote that - or probably a poor choice of phrase.

V-A does not measure 'power', per se: it describes V-A . . . and 'V-A' only has any relevance to A.C. circuits where there is typically a phase difference between the voltage and the current. A transformer manufacturer has no prior knowledge of what type of load that will be connected to his transformer, so he states its 'V-A'. However, in D.C. circuits, watts: the V and the A are in phase.

I'm sure you know all that, but I just felt I needed to correct that minor slip.

With due respect,
Al.

[TrevorG3VLF]

How do you get a D.C. circuit to be driven from a transformer?
Surely you mean a resistive load to equate V-A to VA (W)

[Mike. Watterson]

Originally Posted by merlinmaxwell

One of these https://www.amazon.co.uk/Stepdown-vo...s=110v+adaptor will do nicely.

Zero indication (even in manual) if it's an autotransformer or isolated transformer. Really I'd only run something I knew had spacings, insulation, capacitors etc suitable for 240V to metalwork / earth / terminals or was double insulated and/or had 110V isolating transformer at least 1K proof.
A lot of 110V "Converters" are plain nasty.

[Mike. Watterson]

Transformers SHOULD be rated in VA, though in past few years I've bought some that make no mention of VA, but Watts. Including about four brands of isolated "shaver outlets".

Zero indication (even in manual) if it's an autotransformer or isolated transformer.

I have got a similar one, it is an autotransformer (hardly surprising considering the size) with UK neutral on one of the output pins, I did pop the lid and it isn't badly made and even has different size wires for the halves of the transformer, just cheap and unrefined. It is used to check that US targeted products work on 110V at 50Hz it is a good test for 60Hz stuff.

I consider this good enough for a US set, most of them had an isolated chassis anyway.

[Skywave]

Originally Posted by TrevorG3VLF

How do you get a D.C. circuit to be driven from a transformer?

Err, I'm not sure why you have asked that Q.
If that "d.c. circuit" has active devices in it - e.g. transistors - then you can't (if you want it to work!) - at least, not without rectification.

Originally Posted by TrevorG3VLF

Surely you mean a resistive load to equate V-A to VA (W)?

Err, V-A and VA are the same thing, expressed in two different but similar ways. The both mean 'volt-amps'.
But V-As and VAs are not watts: two different things. 'V-As' refer to wholly or partially reactive loads; 'watts' refer to pure resistive loads. In a partially reactive load, for the power source, the V-A will not equal the watts, because of the phase difference (< 90°) established by that load.

With the greatest of respect to you, I'm sure you know all that. I'm simply trying to provide a comprehensive (but not an exhaustive) reply to your Q.

Al.

[Herald1360]

For a fully comprehensive answer VA does include watts- they are simply a particular case of VA when a pure resistive load exists. VA are just that, volts * amps, whether resistive or reactive. Watts themselves are a unit for power with no exclusive electrical connotations. Watt himself was a steam engine pioneer with no electrical connections

For transformers, the significance of the VA load rating is that their windings have current ratings since it's the current in the winding that determines its resistive power losses.

Then you get all the fun of "utilisation factor" as soon as you stick a rectifier on the transformer output..... the guy who published the seminal work on that always reminds me of schadenfreude- kind of appropriate in the circumstances

[Alvin]

Originally Posted by merlinmaxwell

One of these https://www.amazon.co.uk/Stepdown-vo...s=110v+adaptor will do nicely.

Did you read the reviews? Some negative comments would be expected but this many?? https://www.amazon.co.uk/Stepdown-vo...l&pageNumber=1

[TrevorG3VLF]

Originally Posted by astral highway

True 'dat.

However, I see that confused simplifications often appear because the Volt-Ampere (VA) measures power in a direct current ( DC ) electrical circuit, not an AC circuit.

The point is that VAPhi is applicable to a DC or sine wave AC circuit. If driving a DC circuit with rectifiers, then the current will not be a sine wave and all bets are off. I estimated that the losses in a resistor feeding a rectifier and reservoir capacitor can be up to five times what would be calculated from the mean current. Such losses will also be apparent in the transformer windings.

"A good big un will beat a good little un" - George Jessop?

[Skywave]

Trevor: What is VAPhi, please?

Al.