Running 115v device on 230v

I have a purely resistive (heater) load of 1.5Kw intended for 115v AC that I want to run off 230v mains. Ruling out a conventional transformer from the bulk and price my next thought was to use a simple power diode in series to block one half of the cycle so running off effectivly the designed power ratings. Any down sides to this approach ?
AWEM

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I think it is a common method and is even used in a commercial adapter. Only good for resistive loads such as heaters and incandescent lamps, of course. Just be sure the diode is hefty enough and adequately cooled. If it shorts then your heater may light up pretty good.
Don Young

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115v AC
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Well Jim, that is really what the diode is doing - as I see it, only the (say) positive excursions will pass through so that the average voltage will be 230/2 = 115 so the power will be the same as designed, but that vision has been querried by Barry in his posting above - I would welcome others to comment !
AWEM

On Wed, 20 Dec 2006 13:35:36 -0000, "Andrew Mawson"
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Is it a question of whetehr the 'area under the graph' of half-wave 240V is equivalent to full wave 110V? It's all too long ago for me, & I was never all that good on the maths, I thinkt you are much more up to speed than me on these things Andrew?
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

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The area under the graph of half a 240V sine wave is roughly (110/(240/2)) the same as the 110V, but power as Barry pointed out is given by P = V^2 / R, so with R being unchanged, the power dissipated in half the 240V cycle is still twice that in the full 110V cycle.
The cheapest way to do this is with a thyristor, which is like a diode, but needs a voltage applied to make it open. Using one of these you can set a bias voltage so that instead of taking one complete half of the 240V cycle, you can have it switch on and off at the voltage you choose.
240V AC actually has a peak voltage around 340V (depending if your 240 is really 240), so that the average power supplied to a resistive load is equivalent to a 240V DC supply. If you bias the thermistor to around 320V you will get about the power you need as obviously the bit at the top of the wave gives a lot more power per second than the lower voltage parts of the cycle.
Your heater is around 8-9 ohms, so the peak current the thermistor would need to support would be 42 Amps and I have seen surplus stock in the past with 50 Amp+ ratings going for a couple of quid each.

Cliff Ray wrote:
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There's a slightly important typo here - a regular thyristor latches on when a voltage is applied to the gate and switches off when the load current reduces below the holding current. It's effectively a diode that can be switched-on. To complete the picture ... a Triac is effectively two thyristors wired back-to-back and therefore works in both directions; a GTO Thyristor can be switched-off by the gate but are slightly specialist devices.
Now back to the original question. As you spotted, the simplest solution for a purely resistive load is to use a diode, but it will obviously cause an asymmetric load on the supply. To overcome this you could buy a dimmer (generally Triac-based) and adjust the conduction phase angle (otherwise known as turning the knob!) until you get the power you want. They're pretty simple to build if you feel like it, but you should add some EM filtering.
Dave

Andrew Mawson wrote:
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That would work. But, as someone else said, what does your electricity-supplier say? I guess, they don't like it. OTOH, who cares? Maybe by having a look inside, you find out that you could rewire it for 230V. Or add a second heater in series. This is gonna be a cold winter! ;-)
Nick

On Wed, 20 Dec 2006 13:35:36 -0000, "Andrew Mawson"
SNIP
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Power is V squared over R so your 1.5KW resistor would dissipate 6KW if supplied with 230V. The 50% duty cycle provided by a diode would only reduce this to 3KW average which is still 100% overload.
As Baz points out, a phase controlled triac dimmer is the way to go.
Jim

On Wed, 20 Dec 2006 13:35:36 -0000, "Andrew Mawson"
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<BULSHIT> Comment is that you need the square root of the average of the square or the voltage. Not the average of the voltage.
This being because the instantaneous power is proportional to the square of the instantaneous voltage.
This is because power is Volts times Amps and the Amps are Volts divided by ohms
Thus instantaneous power is V^2/R </BULSHIT>
Hay grandma, I can show you how to suck eggs <G>
Mark Rand RTFM

On Tue, 19 Dec 2006 23:49:51 -0000, "Andrew Mawson"
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Looking at all the options and answers, I'd go for a 240/115 auto-transformer, as long as you don't need isolation. That would be much smaller than a full isolation tranny and you could possibly use the primary side of an existing transformer if it had the right values/taps.
Pretty sure we haven't anything that size at the factory, but I'll have a look tomorrow.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: http://www.oldengine.org/members/diesel

Peter A Forbes wrote:
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ISTR that in the garage I have a couple of 1500VA 240/115V auto transformers. they are relativly small toroidal ones, Ill check when I go out later, but I can certainly spare one if its of use.
Dave

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115v AC
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....ooooh ! Christmas comes early ?
AWEM

Andrew Mawson wrote:
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Its Bl**dy freezing out there, Im going to puzzle some Weff. anyway the details are: in 230V out 120V (perhaps a bit much?) 2000VA 6 1/2" diameter 2 1/2" tall, quite heavy. 3 leads. made by Ulveco USA txfmr number 23339
Dave

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Dave, I reckon I could mount that nicely on the inside roof of the cabinet if you could spare it? Where are you?
Regards,
AWEM

Andrew Mawson wrote:
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Its spare, I have 4 of them... Im a couple of mins from J23 M1
Dave

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Ah - only 134 miles from me then .... sound of thinking . . . . whirr ... whirr . . . .
Anyone coming South from thereabouts in the next little while and would like a mince pie and a glass of sherry ???
AWEM

Andrew Mawson wrote:
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We are heading to cambridge boxing day for a few days if that makes it any easier for anyone.
Dave

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We've been to Nottingham and Burton three nights on the trot since Sunday, but nothing else planned, but if Dave was to meet up with us somewhere on Boxing Day I could hold it for Charles Ping who 'may' be down Kent way??
We are just off the A45 at Rushden/Higham Ferrers.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: http://www.oldengine.org/members/diesel