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Steppin’ down da volts . . .
Do you have excessive mains voltage at your place? In some
parts of Australia the mains voltage can be above 250VAC
and that can spell early death for imported appliances rated
to work on only 220VAC. In this article we show you how to
reduce the mains voltage to a safe level for your equipment.
by Leo Simpson
66 Silicon Chip
siliconchip.com.au
D
espite most people still believing Australia’s mains voltage is
240VAC (we get emails and letters telling us we’re wrong!) according
to Australian Standard AS:60038 the
“official” mains voltage is 230VAC –
and has been for more than a decade.
However, in many parts of Australia
the supply is still nominally 240VAC;
indeed in parts of Western Australia
and many mining towns it is even
higher at around 250VAC – and sometimes even more.
There are several reasons for this but
primarily it’s because a higher voltage
is fed into the network at the substation
so that at the end of (often very long)
supply lines, with their inevitable I2R
losses, there is sufficient voltage to do
the work required.
That’s fine if you are at the far end
of the line but if you’re at the near
end, you cop that excess voltage. It’s
long been the bane of people with
incandescent light globes (the globe
life is dramatically reduced) but now
that the government has banned those
globes, that problem obviously would
have gone away. (Oh yeah? Believe
that and I have this big steel bridge
over Sydney harbour that I can sell
you real cheap!).
220V equipment
Problems caused by excess voltage
have been the case for many years
but with so much imported electronic
equipment, rated for only 220VAC,
there is a big problem of reduced reliability at those elevated voltages.
The higher voltage can be a problem
for two reasons: higher power dissipation and greater risk of catastrophic
breakdown.
For example, if your equipment is
rated for an input voltage of 220VAC
and it is actually being fed 250VAC,
that amounts to a 13.6% increase in
voltage. But because power, for typical loads, is proportional to the square
of the voltage, the increase in power
dissipation over the rated voltage is
up to 29%.
That’s a very large increase and if
you are in an area where the ambient
temperature regularly tops 40°C or
more, that can mean that your equipment could easily expire.
Even if your mains voltage is around
240VAC, that still represents an increase in power dissipation of 19%
(compared to running at 220VAC)
which is still pretty significant, essiliconchip.com.au
pecially in equipment which has a
tendency to run hot even at its rated
input voltage.
circuit, is dangerous unless it is unequivocally proven otherwise!
Reducing dissipation
So for the purpose of this article,
we decided to take a readily available
step-down transformer with a range
of secondary voltage taps and show
how to connect it to reduce 250VAC to
220VAC (or pretty close to that figure).
Also for the purpose of this article,
we decided to limit the power of our
notional load to no more than 450W.
Typical examples of electronic
equipment which would be under
this power limit would include most
large screen plasma and LCD TVs and
video monitors, most PCs and most
audio amplifiers. If in doubt about
how much power an appliance uses,
check the nameplate or failing that,
the owner’s handbook.
The reason for the limit of 450W is
that we wanted to use a small transformer rated at 60VA, with a multitapped secondary.
This type of transformer is commonly available (eg, from Altronics [Cat
M-6674L] or Jaycar [Cat MM-2005]).
They have a number of secondary voltage taps between 9V and 30V and the
rated secondary current is 2A.
That last figure is significant because
it sets the maximum power of the appliance we want to drive. To explain
this point, if you have a 450W appliance rated for a 220V input, its nominal current drain will be 2.045A; close
enough to 2A as to be an insignificant
difference.
The transformer is wired as shown
in the diagram of Fig.1. In this case,
Maybe you don’t have a problem
with the level of the mains voltage
itself. But there is another good reason
to run a key (expensive!) appliance
such as a large screen TV or such
like from a lower voltage: it reduces
the internal power dissipation and
therefore causes it less stress – which
in turn should prolong its life.
Either way, the method we are
describing for reducing the mains
voltage is the same; using a step-down
transformer.
More specifically, we are going to
show how to connect a step-down
transformer as an “auto-transformer”.
While this allows the input voltage
to be reduced, it does not provide
any voltage isolation between the
input and output as does a normal
transformer.
However, first we need to emphasise
the point about electrical isolation.
If you connect an appliance via a
conventional step-down transformer
with completely separate primary and
secondary windings, there will be electrical isolation between the 230VAC
mains supply and the circuity of the
appliance, making it “safer” to work
on if you are doing a repair.
But no such isolation is provided
when you connect an appliance to the
230VAC mains supply via an autotransformer.
The answer to this problem is
simple: always assume a circuit, any
Our Mains Moderator
FUSE
S1
T1
0V
BRN
POWER OUTLET
A
30V*
E
BRN
S2
E
N
BLU
FUSED IEC
INPUT
CONNECTOR
NEON
ILLUMINATED
SWITCH
TRANSFORMER
CORE & FRAME
SC
2011
N
BRN A
240V
BLU
BLU
* OR OTHER TAP AS REQUIRED
--- SEE TEXT
MAINS moderatoR
Fig.1: the circuit is pretty simple but is capable of taming a 250V AC supply down
to a much more healthy 220V AC. Because it is so simple, don’t be complacent or
sloppy when it comes to wiring. Even 220V can ruin your whole day!
March 2011 67
we have shown the full 30V secondary
winding connected in series with the
240V primary winding of the transformer. In effect, this gives a 270V
primary winding and we “tap off”
the output across the 240V winding,
giving a step-down effect.
Now, if 250VAC is applied to the
input, via the IEC socket, the output
will be 250/270 x 240 = 222V. This
would be ideal if you have a 220VAC
appliance and your mains voltage typically hovers around 250VAC.
Alternatively, if your mains voltage is typically around 240VAC or a
little more and you want to reduce it
to around 220VAC , you would use
the 20 or 21V secondary tap of the
transformer, giving a ~261V total primary winding, with the same output
connections as before. In this case, the
output will be 240/261 x 240 = 220.7V.
Transformer current
We mentioned previously that we
wanted to use a standard transformer
with a 2A multi-tapped 30V second-
ary. This would set the limit on the
maximum load. And while the current
of a 450W 220V appliance will nominally be 2.045A, the current drawn
from a 250VAC mains supply will be
somewhat less. To be precise, it will
be 240/270 x 2.045 = 1.82A. (This assumes an “ideal” transformer with no
quiescent magnetising current).
This current will flow in the transformer’s 30V secondary and while it
is less than the 2A rating, it will be
somewhat more than 1.82A because
we are using a “real” transformer with
its associated losses. Hence, to run
a 450W 220VAC load, the suggested
transformer will actually be running
fairly close to its maximum ratings.
In theory, you could apply the same
method to drive a much bigger load.
So if you had a 250VAC mains supply
and wanted to drive a 1000W 220VAC
load, with a nominal current of 4.5A,
you would choose a transformer with
a 30A 5A rating (ie, 150VA or more)
and connect it the same way as shown
in our diagrams.
By the way, we mentioned incandescent globes earlier; this project would
be ideal for them (up to the 450W rating, of course).
Unsuitable loads
While we are suggesting this
auto-transformer connection to drive
electronic appliances, we are not
recommending it for any heating
or motor-driven appliances such as
power tools, fridges, freezers and so
on. The main reason for this caution is
that motor driven appliances typically
have very high starting surge currents
which will overload the transformer.
Wiring it up
We installed our demonstration unit
in a sealed plastic IP65 case measuring 171 x 121 x 80mm (Jaycar Cat
HB-6254).
With the exception of the transformer (which is earthed) everything
is mounted via Nylon 3mm screws and
nuts, for safety.
Ensure that the earth lug under the
T1
HEATSHRINK
SLEEVES ON
ALL SECONDARY
LUGS (THESE ARE
AT MAINS
POTENTIAL)
0V
COVER WITH
SILICONE
SEALANT, ETC
21V
24V
30V
15V
9V
12V
E
WARNING:
LIVE WIRING
A'
All sections of this circuit,
including transformer
secondary, operate at
mains voltage.
N
18V
HEATSHRINK
SLEEVES
HEATSHRINK
SLEEVES
S1
OVERALL
HEATSHRINK
SLEEVE
E
A
N
Fig.2: here’s the “works” of the
(POWER OUTLET)
Mains Moderator we built. Use
3mm Nylon screws and nuts to mount
everything except the transformer (which is earthed). If you wish to use a different transformer tap to achieve a different
output voltage, move the wire connected to the 30V tap ONLY (ie, leave the 0V tap where it is!).
68 Silicon Chip
siliconchip.com.au
And here’s the internal photo While the wiring in this photo is correct, you should find it a little easier to follow that in
the diagram at left. Don’t take any shortcuts with insulation and wiring dress – it is still a mains device after all.
mains transformer screw makes good
contact with the transformer body by
scraping away any passivation which
may have been used on the body.
We used a fused IEC male chassis
connector (screw mounting, not snapin) mounted on the end and a standard
single 240V switched GPO (power
point) mounted on the side of the case.
Also included was a 250VAC 10A
SPST switch with inbuilt neon illumination and a short length of insulated
terminal strip to terminate some of
the wires.
A length of 3-way terminal strip is
used simply to allow two mounting
holes – one position is unused.
You will need to make cut-outs in
the case for the IEC socket and the
switched GPO (general purpose outlet) as well as drill holes for the SPST
switch, transformer mounting feet,
rubber feet for the case, two holes for
the insulated terminal block etc. We
siliconchip.com.au
mounted the switched GPO so that
its two securing screws are attached
to the bottom section of the case – see
the photos. Note that you will need
to make irregular cut-outs in the top
and bottom sections of the case to
clear the rear terminal section of the
switched GPO.
The SPST switch requires a 20mm
mounting hole with a small notch
on the right-hand side. It is a snapin mount switch and you may need
to slightly chamfer the inside of the
20mm hole to allow the snap-in lugs
to lock properly into place.
All the wiring details are shown in
Fig.2 and note that all wiring is run in
250VAC-rated wire.
Let’s just briefly summarise the main
points of the wiring, assuming that all
the hardware items have been installed
in the case.
(1) A brown wire is run from the Active terminal of the IEC connector to
the top zinc-plated terminal of the
SPST switch. Then a brown wire is
run from the centre terminal of the
switch to a connector on the insulated
terminal strip. This same terminal
also connects to the 0V secondary
tap on the transformer.
(2) Another brown wire runs from the
30V terminal on the transformer
to the Active (A) connection on
the switched GPO. This same connection also terminates the brown
wire from the 240V primary of the
transformer.
(3) A blue wire is run from the Neutral
terminal of the IEC connector to
another terminal on the insulated
terminal strip (leave one terminal between Active and Neutral unused).
(4) This Neutral terminal is also used
to terminate the blue wire from the
240V secondary of the transformer
and a blue wire to the Neutral connection on the switched GPO.
March 2011 69
(5) As well, you need run a blue wire
to the third (brass-plated) terminal
of the SPST switch. This is necessary to power the inbuilt neon lamp.
(6) Once the wiring to the SPST switch
is complete, it should be fitted with
a suitable length of heatshrink sleeving to shroud the whole assembly.
(7) On the same theme, as each wire
is soldered, it should have a short
length of 5mm heatshrink sleeving
applied. The exceptions to this are
the two green/yellow wires which
terminate at the Earth terminal on
the IEC connector.
(8) One of those green/yellow wires
earths the frame of the transformer
while the other runs to the Earth
connection on the switched GPO.
(9) All the unused terminals on the
transformer should be fitted with
short lengths of heatshrink sleeving. Use short cable ties to anchor
the wiring, as shown in the photo
and diagram.
(10) Finally, there is an exposed mains
metal strip on the back of the IEC
input socket (it connects the Active pin to the fuse). For maximum
Parts List –
Mains Moderator
1 IP65 case measuring 171 x 121
x 80mm (Jaycar HB-6254).
1 9-30V, 60VA 2A multi-tapped
transformer (Jaycar MM-2005)
1 male chassis IEC connector
with integral fuseholder
1 M205 2A fuse
1 IEC mains lead with 3-pin plug
1 single switched mains outlet
1 red neon illuminated SPST
switch (Jaycar SK-0962)
1m 3-core mains flex
1 3-way 15A mains-rated insulated terminal block
1 5mm crimp-type eye lug (earth
connection to transformer)
4 rubber feet
Heatshrink sleeving
3 100mm cable ties
1 transformer warning label
Screw and nuts
10 Nylon M3 x 15mm screws, nuts
and shakeproof washers (to
mount IEC socket, insulated
terminal block, GPO and rubber
feet)
2 M4 screws, nuts & shakeproof
washers (to mount transformer)
70 Silicon Chip
You’re going to have to perform a bit of minor surgery on the side of the case
(both top and bottom) to accommodate the power outlet, along with the holes in
the end for the IEC mains input connector and the neon switch. Note how the
power outlet mounts low down on the side of the case (not in the centre!) to give
its mounting screws something to hold onto.
safety, cover this with insulation
– preferably, a thin bead of silicone
sealant.
When all your wiring is complete,
check it very carefully against the photos and the diagram. Then fit a 2A fuse
in the IEC connector and insert an IEC
power lead. Do not connect any load
to the GPO. Apply power and switch
on. The neon should illuminate. If not,
disconnect the IEC power lead and find
your mistake.
If the neon did light up, you can
then use your multimeter to check that
the transformer is wired correctly to
reduce the mains voltage.
It is possible that you may find that
the transformer is actually boosting the
voltage rather than reducing. This is
not necessarily an error on your part
but can be because there is no convention as to how mains transformer
primary and secondary windings are
phased.
So if your mains voltage is around
250VAC, for example and you have
wired the transformer as shown in this
article, then the output voltage at the
switched GPO should be close to 220V.
If, on the other hand, the output
voltage is closer to 270VAC, then you
should switch off, unplug the IEC
power lead and swap over the 0V and
30V wires from the secondary of the
transformer. Now try it with a load
connected and everything should be
working correctly. As we mentioned
earlier, an incandescent light globe,
mains rated of course (say about 60W
or so), is ideal as a load for checking.
No misteak misstake error!
Finally, we should make a couple of
points. You may wonder why we refer
to the “240V” primary of the transformer and to “250VAC” ratings when
we made such a point of the official
mains voltage in Australia being 230V.
It is not a mistake, nor a contradiction – it’s simply because that is how
transformers are labelled (manufacturers have been a bit slow to catch up!).
Also, throughout this article we
have referred to a switched GPO.
GPO is “electrician talk” for General
Purpose Outlet. Mere mortals know
them as “power points”.
We deliberately used a single outlet,
rather than the doubles more commonly available (and usually cheaper!), to
minimise the risk of someone – who
didn’t realise the significance of, or
reason for, the circuit – plugging in
something “extra” and exceeding the
450W rating.
If you had a specific application you
could, of course, use a double outlet
SC
but bear our reasoning in mind.
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