17/01/12 Design Process – Prototypes
Find out what goes on behind the scenes...
Virtual meets reality
The prototype of any circuit is the first time that all the theory and calculation meets the real world. You hope that the circuit works first time but this is an optimistic stance and fraught with supposition. There is a myriad of small details that make up a circuit; connections and component values being only a part of the the whole.
Parasitic components are an example. A parasitic component is not a physical component but one that can exist due to placement of parts, proximity to another circuit or simply due to the length of wire. Leakage (high value resistance), capacitance and inductance can all exist. In most cases these are predicted but the final outcome requires careful determination – it is not just function that is tested but also how well it performs.
With some circuits you do not just “throw the switch” and hope for the best. It is quite common to power up the circuit in stages. It may appear to take longer but it saves time and money to proceed in a cautious manner. If there is any kind of fault (miscalculated part, faulty device) there is a very real possibility that large chunks of the entire circuit could be destroyed.
Thankfully errors are rare and the above approach avoids compounding the original problem. The most usual reason for staged power up is actually to save time. If the entire circuit is forced into life you could find circuit chaos on your bench. How do you know what is causing the issue? Staged power up allows the various blocks of circuitry to be assessed in more detail without the distraction of the other stages. Bit by bit the entire realm of blocks is allowed to function taking the circuit to the desired level of function.
Testing the prototype means rather more than the odd meter reading or visual inspection with an oscilloscope. These bench tests are important and should be completed. However the testing should also include any integration with case work and its function with any other devices in a system configuration.
There is another side to testing which is often underestimated. In application the circuit will be required to function whenever the user needs it. This might mean 24/7 functionality or it might be just hours at a time. Testing should reflect that. There is another article already on the site regarding the importance of “play”.
In summary it is vital to make as much use of the prototype as possible in a manner similar to an end user. That way you can be sure that you have done the testing and not left your end customers to be unwilling guinea pigs.
Posted by: Peter Hawkins on 17/01/12.