DSEi Cover Story


The alternative to obsolete passives


Debbie Rowland, director, Charcroft Electronics, talks about obsolete components within the defence industry


board. Now, the increased emphasis on using Commercial-Off-The-Shelf (COTS) products for non-critical systems is reducing demand for the highest specification devices. This not only means that many high-spec parts are being made obsolete, it is also changing the way in which replacement devices are being identified. The easiest scenario for replacing an obsolete passive component is when the alternative can be sourced simply by relaxing the specification. Does the system really need a part with a one per cent tolerance or has it simply been specified at the tightest tolerance when this was considered standard practice? Could it be replaced by a device with five per cent tolerance and still meet the real-world performance criteria? If the original device was specified in a metal case can it this be replaced by a plastic- cased alternative, possibly with the addition of an epoxy end-fill to enhance reliability? The distributor needs to ask the same questions for the capacitance, temperature, voltage, and stability in order to find the real- world criteria in each application.


Debbie Rowland T


he defence industry is changing. The days of every component being specified to the highest performance and quality levels have been replaced by a more system-specific approach and by the introduction of new technologies. The change is, perhaps, most evident when


alternatives are needed for obsolete capacitors and resistors. This more flexible approach also increases the chance that a distributor can identify a direct alternative, or even an upgraded alternative, if they take the time to ask the right questions. For many years, the


trend in engineering departments throughout the defence sector was to use the highest electrical and quality specifications for every component. Quality and reliability were paramount and, for passive components, that meant specifying the most robust packages and the highest performance parameters across the


Balancing performance parameters A more difficult scenario is when the technology has been made obsolete. An example of this is the polycarbonate capacitors which were effectively made obsolete when the dielectric film was withdrawn around ten years ago. Although a limited range polycarbonate is now being manufactured again it can be useful to consider other dielectrics when replacing legacy devices. The choice of the substitute dielectric will depend on the reason why polycarbonate was specified in the original design. Bearing in mind that these devices tended to be over-specified, identifying the capabilities which are important to each application provides the flexibility to identify the best alternative.


If the key parameter of the circuit is its capacitance value, then a film, silver mica or ceramic dielectrics can be considered. The combination of low- to mid-range capacitance


Image 1: A 63V film capacitor could replace a 400V polycarbonate capacitor


and stability could indicate that a polyester capacitor with a five per cent tolerance may be the most suitable replacement. Where the need for a relatively low capacitance is combined with high stability, then a PolyPhenylene Sulphide (PPS) capacitor with a one per cent tolerance would be the best option. At higher capacitance values, a ceramic or silver mica capacitor with a one per cent tolerance would be the obvious choices. When using a film capacitor as a


replacement for a polycarbonate, the voltage of the film capacitor can often be reduced. The comparatively low capacitance value of polycarbonate capacitors means that they were often specified with a high voltage simply to achieve the required capacitance. This could mean that a 63V polyester capacitor may be a viable replacement for a 400V polycarbonate device. The added benefit here is that changing to a readily available polyester capacitor would also lower the unit cost and the Minimum Order Quantity. To match the need for high voltage, a polycarbonate could be replaced with a silver mica capacitor which can achieve a voltage of up to 1kV. A circuit which is sensitive to noise, however, would use a silver mica or ceramic if the capacitance value in under 22nF or, for higher capacitance values, a polypropylene film capacitor. For high-temperature applications the first consideration when replacing a metal- cased polycarbonate capacitor would be a ceramic device which can handle temperatures up to 125°C.


Replacing for improved performance Sometimes an alternative can improve on the performance of the original device. Advances in wet tantalum capacitors, for example, mean that manufacturers have gradually made older technology versions obsolete whilst continuing to manufacture improved alternatives. The original silver case and elastomer seals which were featured in earlier parts were replaced by silver hermetic cases and then by an all-tantalum hermetic seal. The latest developments also mean that polymer capacitors are becoming a viable alternative to tantalum devices, particularly in applications where minimising PCB size is important. There are also sound commercial reasons for


replacing these older devices with new alternatives because the modern devices are manufactured in higher volumes and often provide a lower unit cost.


Asking the right questions


Perhaps the most important factor in identifying alternatives for obsolete passives is dialogue between the distributor and engineer. For their part, the distributor must know which questions to ask and use every bit of product knowledge and industry expertise to identify the alternative which is right for each application.


Image 2: Modern alternatives can improve tantalum performance 8 CIE DSEi Supplement July/August 2017 www.charcroft.com


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92