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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. siliconchip.com.au