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SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $70 12 issues (1 year): $130 24 issues (2 years): $245 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Editorial office: Unit 1 (up ramp), 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: Editorial Viewpoint The hydraulic analogy is valuable for beginners Recently, I came across someone who was new to electronics, explaining that they were having a lot of trouble understanding how even simple circuits work. It reminded me of how helpful I found the hydraulic analogy when I was first learning electronics. Many readers will be familiar with this, and some will also recognise how all sorts of other physical systems (involving heat transfer, mechanical energy, spring oscillation and more) can be modelled similarly to electronic circuits. This analogy involves thinking about an electronic circuit like a series of water pipes instead of wires. The flow of water is equivalent to the flow of electrons, with the volume of water that flows being equivalent to current and the pressure of water at a given point (or, more accurately, pressure difference between two points) being similar to the voltage in an electronic circuit. The equivalent for resistors are skinny pipes; the smaller the diameter of a pipe, the more it resists the flow of water, the greater the pressure (voltage) drop through that pipe, and the more restrictive it is to current flow. Just like with electrical conductors, the smaller the cross-sectional area of a pipe, the higher its ‘resistance’. A power supply can be considered like a pump, or alternatively, water being delivered by a reservoir at a higher level. In either case, the source provides both water pressure and flow. Capacitors are modelled as rubber bladders. As the pressure (‘voltage’) increases, the bladder expands and stores more water (‘charge’). When the pressure drops, the bladder shrinks and pushes water out, briefly sustaining the pressure as it does so. Inductors are equivalent to a turbine in the water flow, with a higher inductance being equivalent to a turbine with more mass (inertia). As water (‘current’) flows through the turbine, it spins up at a rate determined by the pressure differential across it. If the source pressure (‘voltage’) drops, the turbine continues to spin and force water (‘current’) through the outlet. Diodes are easy to model: they are simply one-way valves. The equivalents to transistors are valves that can open or close partially to restrict (or not) the flow of water. A Mosfet equivalent would be controlled by the pressure in a second pipe; you could imagine this second pipe joining the main one, except that there is a rubber diaphragm between them. As the pressure in this second pipe varies relative to the first, the diaphragm flexes and actuates the valve to control the flow of water. A bipolar transistor would be modelled similarly, except that the second pipe would actually have a one-way valve opening into the main one, allowing a small water current to flow. That current flow would impinge upon a flap that controls the opening of the valve, opening it more as the flow through that small valve increases. There are real hydraulic devices that operate like that, called ‘hydraulic servos’, although they are actually closer in behaviour to op amps (another useful analogy!). Other components can be modelled too (zener diodes, Triacs, logic gates etc). These are not necessarily perfect analogies, although I think a hydraulic system could be built that operated pretty similarly to an electronic circuit. The point, though, is that this analogy makes it a lot easier to visualise what the electrons are doing in a circuit, at least until you have more experience with electronics and the understanding comes more naturally. by Nicholas Vinen 24-26 Lilian Fowler Pl, Marrickville 2204 2 Silicon Chip Australia's electronics magazine siliconchip.com.au