Posts Tagged ‘limit current flow’

Transistors – Voltage Regulation Part XIV

Monday, October 22nd, 2012

     As we’ve come to know through this series of blogs, all electronic components pose some degree of internal resistance to the electric current flowing through them.  This resistance results in electrical energy being converted into heat energy, heat which poses potential problems to sensitive components like electronic circuit boards.  If things get hot enough, components fail and fires may ignite. 

     To address these issues engineers design circuits with resistors whose job it is to limit the current flowing to electrical components.  In this article we’ll see how a limiting resistor protects a Zener diode from this fate, allowing it to continue doing its job of regulating voltage.    

     In our last blog we applied Ohm’s Law to our regulated power supply circuit, which makes use of a Zener diode.  See Figure 1.power supply

Figure 1


     Ohm’s Law gave us the following equation to determine the amount of current, IPS, flowing from the unregulated power supply portion, through the current limiting resistor RLimiting, and making its way into the rest of the circuit:

IPS = (VUnregulatedVZener) ÷ RLimiting

     We learned last week that for the circuit to work, the voltage of the unregulated power supply portion of the circuit, VUnregulated, must be greater than the Zener voltage, VZener.

     Looking at the equation above, we see that the voltage difference is divided by RLimiting, the value of the limiting resistor in the circuit.  This limiting resistor is there to constrain the current flowing to the Zener diode, allowing the diode to keep things under control within the circuit. 

     Basic mathematical principles hold that if a smaller number is divided by a bigger number, the resulting answer is an even smaller number.  Applying this principle to the equation above, if RLimiting is a big number, then IPS must be a smaller number.  On the other hand the smaller RLimiting gets, the bigger IPS becomes. 

     So what does it take for our circuit to fail?  Remove the limiting resistor as shown in Figure 2 and the value for RLimiting disappears.  In other words, RLimiting becomes zero.

zener diode with no limiting resistor

Figure 2


     In this case our Ohm’s Law equation becomes:

IPS = (VUnregulatedVZener) ÷ 0 =

     The resulting answer is said to go to infinity, or , as it is represented mathematically.  In other words, without a limiting resistor being employed within our circuit, IPS will become so large it will overwhelm the diode’s current handling capacity and lead to circuit failure. 

     Next time we’ll go over some advantages and disadvantages of this Zener diode voltage regulating circuit, and why the disadvantages outweigh the advantages for many applications.