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Although this may appear the most basic of aids, why it should be in the PQ bag is not always that clear. The reason behind this page is nothing more than to prompt you to build such a device (sadly we've not seen any such devices on the market!). There are many situations that exist where voltages from many points need to be fed into one instrument fitted with a common input (e.g. Neutral), but each voltage may not be coupled to the other for fear of high currents. There are, effectively, two 'flavours' of such a device being the fully isolated 1:1 transformer and the common mode choke. The former is simply a transformer with two separate windings, each capable of the input voltage. If it is possible, obtain a transformer of higher than required voltage - 440VAC is not a bad voltage to consider. This will function fairly well with all voltages from 110-440VACrms.
The drawback of isolation transformers is that they are prone to failure if taken above their working voltage. They can also mask, or at least reduce, any transients that appear on the monitored network. If the wrong type is chosen, harmonics too are reduced making it, all-in-all, a rather second rate device. The second type of isolation transformer, the common mode choke (those who have been involved with RF will know this as a "current balun"), has many benefits. The first is they can work at any voltage as long as the common mode voltage does not exceed the winding voltage, and the differential does not exceed the breakdown voltage between the windings. My design criteria is 1:1 10VACrms 50-2500Hz split-bobbin with at least 600VAC isolation capability. Manufacturers who understand about winding audio isolation transformers are better at this than those who simply wind 'mains' types. If it is suspected that the common mode voltage can exceed the winding voltage (such as a loose neutral) then please ensure adequate fusing is used. I usually opt for no more than 100mA quick-blow, and sand-filled ceramic for safety. In both cases the only restriction is that the voltage on the windings must not be exceeded. As the output is only feeding the high impedance input of a recorder, the power handling capability is not an issue. However, having said this, if the VA is too low, the 1:1 version tends to run hot. In both cases, a large VA starts masking or upsetting the harmonic levels. The main problem with building such a device is finding a transformer to suit. In both cases the idea is isolation between the windings. You may have to approach a transformer manufacturer to wind a suitable device. If you do, limit the power capability to between 5 and 10VA. This has proven to be a good match between running cool but allowing most of the harmonics through.
© 04.09.06 |
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