Overload protections

This page describes commonly used protection circruits against overloading by too high currents.

Current limiting


                   T1            Rs        I
 o-----*---------.     .---*---/\/\/\---*-->--o
       |         C\   /E   |            |
       |          -----    '-----.      |
       |            |            |      |
Uin    |            |          -----    |    Uout
       |     Rb     |         C/   \E   |
       '---/\/\/\---*---------'     '---'
                                T2
 o--------------------------------------------o
GND                                          GND
All Transistors are NPN types

This circruit is not very complicated. Transistor T₁ gets its base current from R_b. If we assume, that the circruit is not active, because we have no overload condition, the voltage across Rs is below 0.7V. T₂ doesn´t conduct. If the current rises to about I=0.7V/R_s, the voltage across R_s rises and T₂ begins to conduct. T₂ draws the useless base current away from T₁ and T₁ gets a higher resistance. The higher it gets, the more T₁ blocks the current. Because of the transistor´s input characteristic this is not a real current source, but limits quite well. The disadvantages are, that there´s also a voltage drop across T₁ and R_s if the circruit does not limit and that the U_BE voltage of T₂, which we assumed with 0.7V depends on the transistor and drops about 2mV/K.

Electronic "fuse"


            Rs       T1                         I
  o---*---/\/\/\---.     .--------------------*->-o
      |            C\   /E                    |
      |             -----                     |
      |               |                       |
      |     Rb        |                       |
      *---/\/\/\------*                       |
      |               | T2                    |
Uin   |               |     -----*------------* Uout
      |               C\   /E    |            |
      |                -----   ----- C1       |
      |                  |     -----          |
      |        R1        |       |     R2     |
      '------/\/\/\------*-------*---/\/\/\---'

  o-----------------------------------------------o
 GND                                             GND
All Transistors are NPN types

This is another quite useful circruit. It behaves like a fuse. But unlike a fuse, you can reset it. by disabling and reenabling U_in after eleminating the error. The behaviour is a little bit tricky. As you can see, T₂ monitors the voltage, again. But the voltage is divided down by R₁ and R₂ and the monitored voltage is also the whole across R_s and T₁. This is the reason why it acts like a fuse. Think about any load connected to it and everything is normal. No overload. We have some voltage across R_s, lets say 0.3V and forward a voltage across T₁ of about 0.8V. These 1.1V are divided e.g. by two by R₁ and R₂. So we have 0.55V U_BE on T₂. T₂ doesn´t conduct. Now, if the current rises, The voltage drop arcoss R_s and T₁ gets bigger. Once it reaches 1.4V, U_BE of T₂ is 0.7V. T₂ begins to conduct somewhat. T₂ draws some of the base current of T₁ (which is supplied by R_b) away. T₁ begins to cut off a bit. Because of that, the voltage drop increases, U_BE of T₂ gets bigger, it conducts better and T₁ cuts off more and more. - The "fuse" is "blown". Note, that there´s still some current flowing through R₁, R₂ and R_b, which gets to the load, so it´s an aim to keep them high. The disadvantages are, that there´s also a voltage drop across T₁ and R_s if the circruit does not limit and that the U_BE voltage of T₂, which we assumed with 0.7V drops about 2mV/K. The trip current is determined by the U_CE of T₁ (and therefore a bit by R_b), the U_BE of T₂, by R_s and by R₁ and R₂. C₁ is for oscillation damping. If you make C₁ too high, it slows down the circruit, because C₁ has to be charged to get U_BE of T₂ to rise. On the other hand, you can use C₁ to slow down the raction and simulate a slower fuse.

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