Posts Tagged ‘output terminals’

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 XII

Sunday, October 7th, 2012

     Let’s continue our discussion with regard to the example circuit discussed last time and 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.

Zener diode voltage regulator

Figure 1


     To recap our discussion from last week, the unregulated power supply portion of the circuit in Figure 1 generates an unregulated voltage, VUnregulated.  Then the Zener diode, which acts as a voltage regulator, takes in VUnregulated and converts it into a steady output voltage, VOutput.  Because these output terminals are connected to the ends of the Zener diode, VOutput  is equal to the voltage put out by it, denoted as VZener.

     The Zener diode, an excellent negotiator of current, is essentially involved in a constant trade off, substituting electric current that originates in the unregulated power supply portion of the circuit for voltage, VOutput, that will serve to power the external supply circuit.   In other words, the Zener diode draws as much current, IZ, through it as it needs, its objective being to keep VOutput at a constant level, and it will continue to provide this constant output, despite the fact that VUnregulated varies considerably.

     So, where does the current IZ come from?  From IPS, that is, the current flowing from the unregulated power supply area, as shown in Figure 1.  

     IPS flows through the limiting resistor to a junction within the circuit.  At this junction, IZ splits off from IPS and continues on to the Zener diode, while current I splits off from IPS on its way to the total internal resistance, RTotal, in the external supply circuit.  

     What this means is that when you add IZ and I together, you get IPS.  Mathematically speaking this is represented as:

IPS = IZ + I

     Why solve for IPS?  We’ll see why this is important when we revisit Ohm’s Law next week and gain a fuller understanding of how IPS, VUnregulated, VZener, and RLimiting relate to each other with regard to the Zener diode.  


Transistors – Voltage Regulation Part VI

Sunday, August 26th, 2012
     Believe it or not as a kid in grade school I used to hate math, particularly algebra.  None of my teachers were able to decipher its complexities and render it comprehensible to me or the majority of my classmates.  Then in high school everything changed.  I had Mr. Coleman for freshman algebra, and he had a way of making it both understandable and fun, in a challenging kind of way.  With 40 years of teaching under his belt, Mr. Coleman knew exactly how to convey the required information in an understandable manner, and to this day I find his insights useful in solving engineering calculations.

      Last time we began our discussion on Ohm’s Law and how it may be applied to our example circuit to solve for the electrical current flowing through it.  Let’s continue our discussion to see how the Law applies to only one part of the circuit.  Then, we’ll use a little algebra to show how the output voltage of an unregulated power supply is affected by changes in RTotal.

electronic power supply

Figure 1


     To help us see things more clearly, in Figure 1 we’ll cover up the inside workings of the unregulated power supply side of the circuit and concentrate on the external supply part of the circuit alone.  Since RTotal is connected to the terminals of the power supply, the voltage applied to RTotal is the same as the power supply output voltage, VOutput.

     In my previous article, we learned that according to Ohm’s Law, the current flowing through a resistance is equal to the voltage applied to it, divided by the resistance.  The fact that RTotal is connected to the two output terminals like we see in Figure 1, allows us to use Ohm’s law to solve for the electrical current, I, flowing through  RTotal:

I = VOutput ÷ RTotal

     Now let’s pull the cover off of the unregulated power supply again to see what’s going on within the circuit as a whole.

electronic circuit

Figure 2


    In Figure 2 we can see that the current, I, flowing through RTotal is the same current flowing through the balance of the circuit.  In the preceding blog we found that value to be:

I = VDC ÷ (RInternal + RTotal)

     We can combine the above two equations for I to develop an algebraic relationship between VOutput and RInternal, RTotal, and VDC:

VOutput ÷ RTotal   =  VDC ÷ (RInternal + RTotal)

     Then, by rearranging terms and applying the cross multiplication principle of algebra we can solve for VOutput.  This involves multiplying both sides of the equation by RTotal:

VOutput =  RTotal × (VDC ÷ (RInternal +RTotal))

     This equation tells us that although RInternal doesn’t fluctuate, VOutput will fluctuate when RTotal does.  This fact is demonstrated in our equation when we make use of algebra.  That is to say, when a term changes on one side of the equation, it causes the other side of the equation to change as well.  In this case, when RTotal  changes, it causes VOutput to change in proportion to the fixed values of VDC and RInternal.

     Next time we’ll look at another shortcoming of unregulated power supplies, more specifically, how one supply can’t power multiple electrical circuits comprised of different voltages. 


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.