Summary :
Many electronic devices require a steady, controlled supply current. A practical solution is using Current Regulative Diodes (CRDs). A different name in the technical literature utilized: Current Limiting Diodes, which refers to the same element. CRD is a diode that supplies a constant current to an electrical circuit, whether connected to a power supply or the load’s impedance fluctuates. CRD is utilized for stabilizing current as well as limitation.
The CRD can regulate the current within the 1-100V input voltage range. As the construction regards, CRD is an n-channel JFET-transistor with the gate shorted to the source through a resistor. The J-FET functions like a two-terminal current limiter or current source in this operation mode, analogous to the voltage-limiting Zener diode. CRD allows a current through itself to rise to a particular value and then level off at a specific pinch-off (Up) voltage. Unlike Zener diodes, these diodes keep the current instead of the voltage constant. The constant current is kept unchanged when the voltage changes until the so-called breakdown voltage is reached. The contemporary regulating characteristics can be followed in Figure 2.
If you apply the power source to the circuit, a “drain” current is created that causes the voltage to drop across the channel. When the drain-source voltage has reached the minimum level known as the”pinch-off voltage” (Vp) will be gone, the J-FET will reach saturation, and the current will be constant. This configuration is known as a constant-current diode since it functions similarly to those that operate with a continuous voltage Zener diode that is used in essential voltage sources.
Due to the diverse saturation currents in J-FETs, the configuration typically contains a source resistor that restricts that current to the specified value, as seen in Figure 1. The drain connector assumes the function of a cathode (A) and acts like the cathode (C). In reverse, the CRD can behave as a standard low-signal diode. The unique capability to limit current and the typical dynamic impedance in the M range and the significant low-temperature drift give essential advantages compared to the existing regulation solutions based on transistors. Conventional circuits have at minimum five components that provide the current source, but CRD is just one item to accomplish the same function. (See Figure 3.).
Semitec-created CRDs that typically range between 0.1 and 15 mA regulated current to a maximum of 100V nominal voltage with 16 different models. The sensitivity to temperature fluctuations is low over the entire operating temperature range, between 30 and 150 deg C. An oblique corrector resistor can mitigate the increasing voltage and the reduction in current caused by self-heating. Utilizing more paralleled CRDs will enhance their capability to handle current as well, as it is possible to expand the voltage range using a Zener diode connected to the line, which provides continuous current across a more comprehensive voltage range. Semitec CRDs are available in Semitec CRDs come in Leaded (E) or SMD (S) varieties.
Applications
- The PLC (programmable logic controller)
- Digital and analog inputs’ current limiters
- Sensor applications
- LED control circuits
CRDs are used in LED applications.
The brightness of LED can change due to tiny fluctuations in voltage. One option to address the issue would be to provide a constant voltage source for LEDs. However, the different forward voltages of the various LEDs could also cause a variation in brightness. In most cases, LEDs require a steady current source.
CRD is a method to improve the performance of LED and regulates the current even if the circuit’s voltage fluctuates. On the I-V curves of the LED, it’s apparent that around the working point, a slight variation in the voltage applied can result in a dramatic variation in the forward current. This can lead to thermal runaway. In addition, the temperature at the junction of the LED can exceed the limit because of the self-heating and causing issues with its lifetime. The current will be controlled when using a resistor connected to the LED, but the I f will continue fluctuating due to the fluctuating voltage. The rise in forward current can be due to a change in its thermal coefficient that is reflected by an I-V curve that is different; the point at which the LED is working initial angle will shift into the turn of the newly created, which could result in more significant forward currents on the same voltage of f. CRD could be the solution for both scenarios. The f will be stable regardless of the voltage supplied by adjusting the voltage of the f.