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
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 !
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
Traditional & Modern canal craft repairs
Vintage diesel engine service
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
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
Peter & Rita Forbes
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%
As Baz points out, a phase controlled triac dimmer is the way
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.
1) How limited are you for space?
2) What is special about the heater (240 1kW open or quartz elements are less
than a tenner)
As Barry and co have said. A rectifier will still give double the output and
will increase the tendency of the substation transformer to saturation.
Saturation may not be a major problem, since you would only be drawing about
13A from a substation transformer that is probably rated at 2400A (for a 1MVA
tranny). But It is still going to be too hot :-(
I don't suppose you've got two elements you could wire in series, do you?
I _do_ have a 5kVA 240/110 isolating transformer that I'm not using at the
moment, and have access to another, similar sized one but they are a bit
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
Thus instantaneous power is V^2/R
Hay grandma, I can show you how to suck eggs
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
Its Bl**dy freezing out there, Im going to puzzle some Weff.
anyway the details are:
in 230V out 120V (perhaps a bit much?)
6 1/2" diameter 2 1/2" tall, quite heavy.
3 leads. made by Ulveco USA txfmr number 23339
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