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 ?
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.
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 !
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! ;-)
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.
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 & Rita Forbes Email: firstname.lastname@example.org Web:
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.
The special bit is that it screws into the sump of that rather large genny engine that you saw the other month and turns the treacle in the sump back into oil when the frost is on the brussel sprouts ! The control cabinet has run out of wall space to put anything bigger than a small pack of kippers sideways. I do actually have two elements that I can wire in series but that means tapping another hole in the sump which I'd like to avoid, also 3KW of oomph is rather a lot even in the
30 pints of oil in the beast.(It's supposed to drive the furnace not be the furnace
Aha! Now we see the problem. If it's any consolation, the old man's Landrover had a 3kW sump heater. Downside of that was that most Scandinavian hotels only have 6A supplies for block heaters in their car parks, so he tended to be unpopular with them.
How about a 555 timer and a triac to give the heater .5 seconds on, two seconds off? There should be enough thermal inertia in the element that it would cope with that easily. That would give you just a bit under 1.5kW and could be adjusted to suit.
You could just leave the engine ticking over from now until next April