Posts Tagged ‘electronics’

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

Systems Engineering In Medical Device Design – Introduction

Monday, November 26th, 2012
     I once worked with a medical device design engineer who, although talented enough, was not adept in the subtle yet indispensable skill of verbal communication.  He lacked a concise, organized approach to his projects, and his problem solving skills were unilateral and obtuse, that is to say, his only aim was to satisfy his personal requirements, what he felt was important.  Creative problem solving and brainstorming with customers as to what they desired did not fall within his repertoire.  As a result customer complaints and a long string of product failures eventually led to him losing his position.

     Where specifically had he failed?  The net result of his approach was that he designed devices that did not deliver the desired customer results.  They also had a varying tendency to be either unnecessarily expensive to produce, unreliable to operate, or difficult to service.  All concerned with the product were often dissatisfied, from customers to service technicians.  This caused the company we worked for to incur considerable expense to rectify his design errors.  The company also lost some of their customer base to competitors.  Sadly, none of this would have happened if my coworker had used a systems engineering approach in designing his projects.

     Before we get any further into a discussion on systems engineering, let’s get a handle on what is meant by a system.  In a nutshell, a system is a combination of interacting components that are organized to achieve one or more specific purposes.  The components can be tools, machine parts, electronics, people, or any combination thereof.  For example, hundreds of parts can be combined by a manufacturer into a system to form a medical device such as an x-ray film developing machine, the end result of which is to produce a film of diagnostic quality. 

      The system part of Systems Engineering stays true to this definition. It is an interdisciplinary approach to complex engineering projects which guides all activities during the course of a product’s life cycle, from conception to production. While doing so it will integrate and monitor work processes between all departments involved, with a constant eye towards optimization of processes and reduction of costs in order to satisfy stakeholder requirements.

     A key objective of systems engineering is to produce systems that satisfy stakeholder needs by producing reliable, cost effective, and safe products capable of performing tasks as designated by the customer.  Within the medical device arena stakeholders include patients, nurses, doctors, the US Food and Drug Administration (FDA), device service technicians, device dealers, as well as the device manufacturer.

     Next time we’ll begin our exploration of how systems engineering addresses the medical device design process with a discussion on the first of its five stages, known as Concept.


 Systems Engineering In Medical Device Design

Transistors – Voltage Regulation, Final Chapter

Monday, November 19th, 2012

     Last time we learned how the transistor opens a path for electric current to flow from the collector to the emitter in our example circuit.  It does so by making use of an unregulated power supply.  Now let’s see how the Zener diode fits into the mix.

transistor series voltage regulator

Figure 1


     It just so happens that bipolar transistors, like the one in our example circuit in Figure 1, are designed so that voltage at its emitter is dependent upon the voltage applied at its base.  This makes them ideal for use in voltage regulator circuits where this kind of predictability is required.

     For example, in our transistor series voltage regulator, the Zener diode is connected to the transistor’s base, B.  When the branch current flows from RLimiting down through the diode, a Zener voltage, VZener, is established.  Since the diode is connected to the transistor, VZener voltage is also applied to the transistor’s base.  Thus the transistor’s emitter voltage will be regulated according to the Zener voltage.

     Bipolar transistors are designed by manufacturers to typically operate with a standardized voltage difference of 0.6 volts between the base and emitter.  This is represented in Figure 1 as VBE, where BE stands for base-emitter.  VBE is standardized at a known quantity of 0.6 volts to simplify things within the industry and aid engineers in their calculations to design transistor circuits, as we’ll now see.

     With the Zener diode connected to the transistor base in our example circuit, the voltage difference is denoted as:

VBE = VZener   VE

where VE is the emitter voltage.  Rearranging terms to solve for VE, we get:

VE = VZener – VBE

Inserting VBE,  which we know is standardized at 0.6 volts:

VE = VZener – 0.6 volts

Since the emitter is physically connected to the output terminal of the transistor series voltage regulator, the emitter voltage is going to be equal to the output voltage, VOut.

     We learned earlier in this series of articles that VZener is a reliable source of consistent voltage.  Because it is present in our transistor series voltage regulator, our example circuit will produce a nice, constant regulated output voltage of VZener – 0.6 volts, a voltage that is useful for many of today’s applications.  However the transistor series voltage regulator provides us with a major advantage over the Zener diode voltage regulator circuit.

     The advantage of a transistor series voltage regulator lies in the fact that  RLimiting is on a separate branch all to its own within the regulator circuit, and because of this it no longer acts as a roadblock to limit the main path of current flow, as happens within the Zener diode voltage regulator circuit discussed previously.  Refer to the red path shown in Figure 1.  With RLimiting in this position the transistor series voltage regulator is able to feed more current to the external supply circuit than is possible through the Zener diode voltage regulator alone.  This means it can be used in more power hungry applications like energizing today’s TVs and modern kitchen appliances.

     That wraps up our discussion on transistors.  Next time we’ll begin a new topic, how medical devices can be designed using systems engineering, a systematic approach that ensures that designed devices satisfy both user and regulatory requirements.



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 XIII

Monday, October 15th, 2012

     Last time we learned how the Zener diode, an excellent negotiator of current, is involved in a constant trade off, exchanging current for voltage so as to maintain a constant voltage.  It draws as much current through it as is required to maintain a consistent voltage value across its leads, essentially acting as voltage regulator in order to protect sensitive electronic components from power fluctuations. 

     Now let’s revisit our example power supply circuit and see how Ohm’s Law is used to determine the amount of electric current, IPS, that flows from the unregulated power supply and why this is important to the function of the Zener diode.  See Figure 1.

power supply

Figure 1


     If you’ll recall, Ohm’s Law states that current flowing through a resistor is equal to the voltage across the resistor divided by its electrical resistance.  In our example that would be IPS flowing through to RLimiting.  In fact, the voltage across RLimiting is the difference between the voltages at each of its ends.

     Applying this knowledge to our circuit, the voltage on one end is VUnregulated, while the voltage at the other is VZener.  According to Ohm’s Law the equation which allows us to solve for IPS is written as:

IPS = (VUnregulatedVZener) ÷ RLimiting

     And if we have a situation where VUnregulated equals VZener , such as when the voltage of an unregulated power supply like a battery equals the Zener voltage of a Zener diode, then the equation becomes:

(VUnregulatedVZener ) = 0

And if this is true, then the following is also true:

IPS = 0 ÷ RLimiting = 0

     In other words, this equation tells us that if VUnregulated is equal to VZener, then the current IPS will cease to flow from the unregulated portion of the circuit towards the Zener diode and the external supply circuit.  Put another way, in order for IPS to flow and the circuit to work, VUnregulated must be greater than VZener.

     Next week we’ll continue our discussion and see why the resistor RLimiting is necessary in order to prevent the circuit from self destructing.


Transistors – Voltage Regulation Part XI

Monday, October 1st, 2012
     Without limits on our roadways things would get quickly out of hand.  Imagine speeding down an unfamiliar highway and suddenly coming upon a sharp curve.  With no speed limit sign to warn you to reduce speed, you could lose control of your car.  Limits are useful in many situations, including within electronic circuits to keep them from getting damaged, as we’ll see in a moment.

     Last time we introduced the Zener diode and the fact that it performs as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage.  Let’s see how it works.

Unregulated Power Supply

Figure 1


     In Figure 1 we have an unregulated power supply circuit introduced in a previous article in this series.  We learned that this power supply’s major shortcoming is that its output voltage, VOutput, is unregulated, in other words, it’s not constant.  It varies with changes in the direct current supply voltage, VDC.

     It also varies with changes in, RTotal, which is the total internal resistance of components connected to it.  RTotal changes when components are turned on and off by microprocessor and digital logic chips. When VOutput is not constant, those chips can malfunction, causing the device to operate erratically or not at all.

     But we can easily address this problem by adding a Zener diode voltage regulator between the unregulated power supply and the external supply circuit.  See the green portion of Figure 2.

Zener Diode Voltage Regulator

Figure 2


     Our power supply now consists of a Zener diode and a limiting resistor, RLimiting.  The limiting resistor does as its name implies, it limits the amount of electric current, IZ, flowing through the Zener diode.  Without this limiting resistor, IZ could get high enough to damage the diode, resulting in system failure.

     Next time we’ll see how the Zener diode works in tandem with the limiting resistor to control current flow and hold the output voltage at a constant level.


Transistors – Voltage Regulation Part X

Monday, September 24th, 2012
     Through the ages it’s been common practice to name important discoveries after those who discovered them.  For example, James Watt was a mechanical engineer who improved the steam engine by finding a solution to the problem of steam condensing into water inside the engine, a phenomenon which resulted in the engine cooling and reducing its efficiency.  Thus it was fitting that a metric unit of power, the watt, was named in his honor.  Today we’ll become acquainted with the man behind the naming of the Zener diode, Clarence Zener, and take a look at his contributions with regard to the function of this electrical component.

     Last time we began our discussion on electrical components known as diodes and saw how they’re used on circuit paths to govern the flow of current.  The Zener diode is a particular type of diode and a key component in transistorized voltage regulator circuits, as we’ll see later.  For now, let’s see how it works.     The symbol for the Zener diode is almost identical to that of a standard diode, introduced in my previous blog, but the Zener version has a bent line going through it resembling a distorted letter “z.”  See Figure 1.

Zener diode voltage regulator

Figure 1       


      Electric current flows through the Zener Diode just as it does through a standard diode.  But when the current flows in reverse, that’s where the similarity ends.  See Figure 2.

  Zener diode

Figure 2    


     When current tries to flow in the reverse direction, the Zener diode acts as an electrical conductor and allows current to pass through it.  In other words, it doesn’t block current flow as standard diodes do.

      At this point, you may be asking, “What’s so special about that?”  Perhaps you’ve made the connection that it behaves no differently than a metal wire.  But that isn’t entirely correct.

     You see, when current passes in the reverse direction through the Zener diode, it maintains a constant voltage.  This is called the Zener Voltage and is denoted as VZener.  The significance here is that within the circuit, any electronic component connected across the leads of a Zener diode will be supplied with a constant, unchanging voltage.  Thus the Zener diode works as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage.  It should be noted that this phenomenon only happens when the current flowing through the Zener diode is flowing in reverse.

     Next time we’ll look at a basic regulated power supply circuit to see how a Zener diode is incorporated in order to maintain a consistent output voltage.


Transistors – Voltage Regulation Part IX

Sunday, September 16th, 2012
     One way streets frustrate me, and I usually end up wasting a lot of time and gas driving in circles to get to my destination.  Generally speaking, I prefer a two way street.  Electric current flowing through electronic circuits is somewhat analogous to traffic flow.  There are circuit paths that act like one way streets and others that act like two-way.

     An electrical component called a diode can be used on circuit paths to govern the flow of current.  They are a key component in basic transistorized voltage regulator circuits, as we’ll see later.  For now, let’s get a basic understanding of how they work.

     Diodes are typically made of a semiconductor material, such as the element germanium.  These materials behave in a complex way that fall along the lines of quantum physics.  Esoteric phrases such as electron-hole theory, crystalline atomic lattice theory, and impurity doping are some of the concepts involved and would require a book onto themselves to explain.  For the purposes of this article all we have to know is that semiconductors have two properties.  The first property is that of an electrical conductor, that is, a material which allows electric current to pass through it.  Copper wire is a good example of this.  The second property is that of an electrical insulator, which blocks the flow of electric current.  Materials such as glass, wood, and rubber fall into the insulator category.

     A photo of a diode is shown in Figure 1, along with its symbol used in electrical schematics.

A diode and a diode schematic symbol.

Figure 1


     When electric current flows through a diode in one direction, as shown in Figure 2, the semiconductor material inside of it acts as a conductor, ushering it along a single path.

diode one way current flow

Figure 2


     When current tries to flow through the diode in the reverse direction, the semiconductor material acts as an insulator.  That is, it blocks the flow of current as shown in Figure 3.

diode stops reverse flow of electric current

Figure 3


     So we see that diodes can act like one way streets, restricting current flow.  But, not all diodes work this way.  Next week we’ll introduce a special kind of diode called the Zener diode, which allows current to flow in two different directions, and we’ll see how this functionality is put to work in regulated power supplies.


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 VII

Monday, September 3rd, 2012
     Back when television had barely escaped the confines of black and white transmission there was a men’s clothing store commercial whose slogan still sticks in my mind, “Large and small, we fit them all.”  It’s a nice concept, but unfortunately the same doesn’t always apply to electronic power supplies.

     Last time we learned that when the electrical resistance changes on an unregulated power supply its output voltage changes proportionately.  This makes it unsuitable for powering devices like microprocessor chips, which require an unchanging voltage to operate properly.  Now let’s look at another shortcoming of unregulated power supplies, that being how one supply can’t fit both large and small voltage requirements.

     Figure 1 shows the components of a simple unregulated power supply. 

unregulated power supply electronics

Figure 1


     The diagram illustrates the voltage changes taking place as electric current passes through the supply’s four components, which ultimately results in the conversion of 120 volts alternating current (VAC) into 12 volts direct current (VDC).

     First the transformer converts the 120 VAC from the wall outlet to the 12 volts required by most electronic devices.  These voltages are shown at Points A and B.  The voltage being put out by the transformer results in waves of energy which alternate between a positive maximum value, then to zero, and finally to a maximum negative value.

     But we want our power supply to produce 12 VDC.  By VDC, I mean voltage that never falls to zero and stays at a positive 12 volts direct current consistently.  This is when the diode bridge and capacitor come into play.  The diode bridge consists of four electronic components, the diodes, which are connected together to form a bridge and uses semiconductor technology to transform negative voltage from the transformer into positive.  The result is a series of 12 volt peaks as shown at Point C.

     But we still have the problem of zero voltage gaps between each peak.  You see, over time the voltage at Point C of Figure 1 keeps fluctuating between 0 volts and positive 12 volts, and this is not suitable to power most electronics, which require a steady VDC current.

     We can get around this problem by feeding voltage from the diode bridge into the capacitor.  When we do that, we eliminate the zero voltage gaps between the peaks.  This happens when the capacitor charges up with electrical energy as the voltage from the diode bridge nears the top of a peak.  Then, as voltage begins its dive back to zero the capacitor discharges its electrical energy to fill in the gaps between peaks.  In other words it acts as a kind of reserve battery.  The result is the rippled voltage pattern observed at Point D.  With the current gaps filled in, the voltage is now a steady VDC.

     The output voltage of the unregulated power supply is totally dependant on the design of the transformer, which in this case is designed to convert 120 volts into 12 volts.  This limits the power supply’s usefulness because it can only supply one output voltage, that being 12 VDC.  This voltage may be insufficient for some electronics, like those often found in microprocessor controlled devices where voltages can range between 1.5 and 24 volts.

     Next time we’ll illustrate this limitation by revisiting our microprocessor control circuit example and trying to fit this unregulated power supply into it.