Current regulating diode
A constant-current diode ( English current regulation diode , CRD ) or Strombegrenzerdiode ( English current limiting diode , CLD ) is an electronic component, which leads in one direction from a certain voltage, a constant current and stabilizes this value. It represents a design of constant current sources.
In terms of structure, current regulating diodes are not actually diodes , but an electronic circuit made up of a JFET (this is a special field effect transistor ) and a resistor for negative feedback . Since these circuits only have two connections to the outside and only limit the current in one direction, they are called "diodes".
Mode of action
The function is described here using the example of an n-channel JFET: An n-channel JFET introduced at small negative voltage U GS between source and gate, in this case at a low voltage across the resistor R S . If, on the other hand, one assumes that the current through the resistor would be too high, a more negative voltage U GS would result and therefore the JFET would reduce the conductivity between drain and source. If the current is too low, the opposite is true: the JFET would increase the conductivity. This results in a stable operating point , i.e. a stable current. Another perspective sees the JFET as a feedback source follower . The current is limited by the JFET channel; this can be seen in the output characteristic field (explanation see JFET ).
The temperature dependence of the current can be minimized: From the input characteristic curve (see figure) is evident that in U GS = 0, a current I DSS flows; however, this is strongly temperature-dependent. This also applies to other working points; at R S (-TK) shown as an example with “I D change”. The point of intersection of the characteristics is the operating point at which the temperature dependence is minimal. This operating point can be set by a suitable choice of R S (TK = 0). The resulting current I D is thus largely independent of the temperature, but is nonetheless dependent on the model, i.e. it fluctuates from component to component. Manufacturers of current regulating diodes therefore select the components during production in order to be able to offer customers the narrowest possible tolerances.
properties
Current regulating diodes have a "knee voltage" of around one to a few volts. Above this voltage is kept constant as the specified current (see FIG. Constant current source ), including this current is not reached. At low voltages, the component behaves similarly to an ohmic resistor . In the reverse direction (when the cathode is more positive than the anode), current control diodes conduct in the forward direction, similar to a normal semiconductor diode. In contrast to the Zener diode , the connection that is positive in normal operation is called the anode of the current control diode.
Current regulating diodes are not suitable for precision applications because the model variance (difference in current between different components of the same type) is relatively large (± 30%); the current also depends relatively strongly on the temperature (up to approx. 1% / K). Current control diodes are manufactured for currents of approx. 0.05 to 15 mA and maximum operating voltages of up to 100 V. Well-known types are 1N5283 to 1N5314, J503 or J511.
See also
- Thermistor
- Soft start (starting current limitation)
Web links
Individual evidence
- ↑ Low Power Discrete Data Book. Siliconix Inc., 1989