Posts Tagged ‘television’

Systems Engineering In Medical Device Design – Production, Part 4

Sunday, March 17th, 2013

      Did you know that from the early days of the Industrial Revolution until well into the 20th Century it was common practice for all aspects of a product to be built entirely under one roof? For example, a wheelchair manufacturer in the 1890s would buy the various raw materials needed to construct component parts, everything from bars of steel and wooden boards to rattan stalks and gum rubber, then produce every part of the wheelchair in one facility. Items as diverse as chair frames, footrests, wicker seat cushions, springs, wheel rims and spokes, and tires would all be constructed from the raw materials purchased, then assembled into the finished product.

      Doesn’t sound like an efficient process to you? Henry Ford didn’t think so either. In fact, he is credited with pioneering mass production in manufacturing when he observed during the production process of his line of automobiles that inefficiencies abounded.

      Inefficiencies in manufacturing are common, as they are in everyday life. Last time we saw how robots, i.e., the introduction of industrial automation, can be used during the Production stage of our systems engineering approach to medical device design to increase efficiency and reduce manufacturing costs. Today we’ll take a look at another inefficient practice, along with its solution.

      Returning to our wheelchair manufacturer, the problems associated with manufacturing and assembling all aspects of a product are many. At the top of the list is the substantial cash outlay that’s required to buy and maintain a huge factory complex and all the specialized equipment required to make each and every part. In addition, there’s the ongoing expense of employing and training employees needed to fabricate each component. In other words, the wheelchair factory has a lot of fixed overhead expense to carry, and the more overhead there is, the more expensive the end product. Expenses such as these are almost always passed on to the buyer.

      The solution? Outsourcing. That is, using outside manufacturers to produce many, perhaps even all, of the component parts. Then our wheelchair manufacturer would simply assemble the purchased parts into the finished product, resulting in lower manufacturing costs and higher profits. The benefits of outsourcing were widely recognized in the decades following World War II, when the post-war economy was booming and demand for consumer goods increased dramatically.

      That ends our look at the Production stage. Next time we’ll move on to the Utilization stage to see how the systems engineering approach is put into play once the medical device has been introduced into the marketplace.


medical device manufacturing

Transistors – Voltage Regulation Part XV

Sunday, October 28th, 2012

     We’ve been discussing the advantages of using limiting resistors within Zener diode regulating circuits to lessen the probability of circuit failure. Today we’ll continue our discussion, focusing on the Zener diode’s advantages and disadvantages.  See Figure 1.

Zener diode voltage regulator circuit

Figure 1


     Figure 1 discloses the simplicity of a voltage regulator employing a Zener diode.  There are only two components, a limiting resistor, RLimiting, and the Zener diode itself, which also makes the entire assembly cost effective to manufacture. 

     Despite the obvious advantages, there is one major disadvantage to the Zener diode voltage regulator.  Ironically, the limitation imposed by RLimiting on the current IPS itself creates an operational dilemma.  When electronic devices are connected to the output terminals of the regulator, RLimiting  and its current-limiting action becomes a disadvantage.  See Figure 2.

electronic voltage regulator

Figure 2


     The amount of current I available to flow through to electronic devices is limited, sometimes too much, and the net result is that the Zener diode voltage regulator can only be used to power electronic devices drawing small amounts of current.  It is unacceptable for many applications, such as powering kitchen appliances or flat screen TVs.

     Next time we’ll see how to improve upon the Zener diode voltage regulator circuit by adding a transistor.  This will eliminate the road block imposed by RLimiting, thus allowing higher, but still regulated, current to flow through to the output terminals.


Transistors – Voltage Regulation Part VIII

Sunday, September 9th, 2012
     Back in the early 1970s my dad, a notorious tightwad, coughed up several hundred dollars to buy his first portable color television.  That was a small fortune back then.  The TV was massive, standing at 24 inches wide, 18 inches high, and 24 inches deep, and weighing in at about 50 pounds.  I think the only thing that made this behemoth “portable” was the fact that it had a carrying handle on top.

     A major reason for our old TV being so big and clunky was of course due to limitations in technology of the time.  Many large, heavy, and expensive electronic components were needed to make it work, requiring a lot of space for the circuitry.  By comparison, modern flat screen televisions and other electronic devices are small and compact because advances in technology enable them to work with far fewer electronic components.  These components are also smaller, lighter, and cheaper.

     Last time we looked at the components of a simple unregulated power supply to see how it converts 120 volts alternating current (VAC) to 12 volts direct current (VDC).  We discovered that the output voltage of the supply is totally dependent on the design of the transformer, because the transformer in our example can only produce one voltage, 12 VDC.  This of course limits the supply’s usefulness in that it is unable to power multiple electronic devices requiring two or more voltages, such as we’ll be discussing a bit further down.

     Now let’s illustrate this power supply limitation by revisiting our microprocessor control circuit example which we introduced in a previous article in this series on transistors. 

microprocessor control

Figure 1


     In Figure 1 we have to decide what kind of power to supply to the circuit, but we have a problem.  Sure, the unregulated power supply that we just discussed is up to the task of providing the 12 VDC needed to supply power for the buzzer, light, and electric relay.  But let’s not forget about powering the microprocessor chip.  It needs only 5 VDC to operate and will get damaged and malfunction on the higher 12 VDC the current power supply provides.  Our power supply just isn’t equipped to provide the two voltages required by the circuit.

     We could try and get around this problem by adding a second unregulated power supply with a transformer designed to convert 120 VAC to 5 VAC.  But, reminiscent of the circuitry in my dad’s clunky old portable color TV, the second power supply would require substantially more space in order to accommodate an additional transformer, diode bridge, and capacitor.  Another thing to consider is that transformers aren’t cheap, and they tend to have some heft to them due to their iron cores, so more cost and weight would be added to the circuit as well.  For these reasons the use of a second power supply is a poor option.

     Next time we’ll look at how adding a transistor voltage regulator circuit to the supply results in cost, size, and weight savings.  It also results in a more flexible and dependable output voltage.


Transistors – Voltage Regulation Part IV

Sunday, August 12th, 2012
     We’ve all popped a circuit breaker sometime in our lives, often the result of making too heavy of an electrical demand in a single area of the house to which that circuit is dedicated.  Like when you’re making dinner and operating the microwave, toaster, mixer, blender, food processor, and television simultaneously.  The demand for current on a single circuit can be taxed to the max, causing it to pop the circuit breaker and requiring that trip to the electrical box to flip the switch back on.

      Last time we began our discussion on unregulated power supplies and how they’re affected by power demands within their circuits.  Our schematic shows there are two basic aspects to the circuit, namely, its direct current source, or VDC,  and its internal resistance, RInternal.  Now let’s connect the power supply output terminals to an external supply circuit through which electrical current will be provided to peripheral devices, much like all the kitchen gadgets mentioned above.

Figure 1


     The external supply circuit shown in Figure 1 contains various electronic components, including electric relays, lights, and buzzers, and each of these has its own internal resistance.  Combined, their total resistance is RTotal, as shown in our schematic.

     Current, notated as I, circulates through the power supply, through the external supply circuit, and then returns back to the power supply.  The current circulates because the voltage, VDC, pushes it through the circuit like pressure from a pump causes water to flow through a pipe.

     RTotal and I can change, that is, increase or decrease, depending on how many components the microprocessor has turned on or off within the external supply circuit at any given time.  When RTotal increases, electrical current, I, decreases.  When RTotal decreases, electrical current I increases. 

     Next time we’ll continue our discussion on Ohm’s Law, introduced last week, to show how the static effect of RInternal  interacts with the changing resistance present in RTotal to adversely affect an unregulated power supply’s output voltage.


Transistors – Digital Control Interface, Part II

Sunday, June 24th, 2012
     Not too long ago I was retained as an engineering expert to testify on behalf of a plaintiff who owned a sports bar.  The place was filled with flat screen televisions that were plugged into 120 volt alternating current (VAC) wall outlets.  To make a long story short, the electric utility wires that fed power to the bar were hit by a passing vehicle, causing the voltage in the outlets to increase well beyond what the electronics in the televisions could handle.  The delicate electronics were not suited to be connected with the high voltage that suddenly surged through them as a result of the hit, and they overloaded and failed.

     Similarly, lower voltage microprocessor and digital logic chips are also not suited to directly connect with higher voltage devices like motors, electrical relays, and light bulbs.  An interface between the two is needed to keep the delicate electronic circuits in the chips from overloading and failing like the ill fated televisions in my client’s sports bar.  Let’s look now at how a field effect transistor (FET) acts as the interface between low and high voltages when put into operation within an industrial product.

     I was once asked to design an industrial product, a machine which developed medical x-ray films, utilizing a microprocessor chip to automate its operation.  The design requirements stated that the product be powered by a 120 VAC, such as that available through the nearest wall outlet.  In terms of functionality, upon startup the microprocessor chip was to be programmed to first perform a 40-minute warmup of the machine, then activate a 12 volt direct current (VDC) buzzer for two seconds, signaling that it was ready for use.  This sequence was to be initiated by a human operator depressing an activation button.

     The problem presented by this scenario was that the microprocessor chip manufacturer designed it to operate on a mere 5 VDC.  In additional, it was equipped with a digital output lead that was limited in functionality to either “on” or “off” and capable of only supplying either extreme of 0 VDC or 5 VDC, not the 12 VDC required by the buzzer.

     Figure 1 illustrates my solution to this voltage problem, although the diagram shown presents a highly simplified version of the end solution.

microprocessor control

Figure 1

     The illustration shows the initial power supplied at the upper left to be 120 VAC.  This then is converted down to 5 VDC and 12 VDC respectively by a power supply circuit. The 5 VDC powers the microprocessor chip and the 12 VDC powers the buzzer.  The conversion from high 120 VAC voltage to low 5 and 12 VDC voltage is accomplished through the use of a transformer, a diode bridge, and special transistors that regulate voltage.  Since this article is about FETs, we’ll discuss transistor power supplies in more depth in a future article.

     To make things a little easier to follow, the diagram in Figure 1 shows the microprocessor chip with only one input lead and one output lead.  In actuality a microprocessor chip can have dozens of input and output leads, as was the case in my solution.  The input leads collect information from sensors, switches, and other electrical components for processing and decision making by the computer program contained within the chip.  Output leads then send out commands in the form of digital signals that are either 0 VDC or 5 VDC.  In other words, off or on.  The net result is that these signals are turned off or on by the program’s decision making process.

     Figure 1 shows the input lead is connected to a pushbutton activated by a human.  The output lead is connected to the gate (G) of the FET.  The FET is shown in symbolic form in green. The FET drain (D) lead is connected to the buzzer and its source (S) lead terminates in connection to electrical ground to complete the electrical circuit.  Remember, electric current naturally likes to flow from the supply source to electrical ground within circuits, and our scenario is no exception.

     Next time we’ll see what happens when someone presses the button to put everything into action.