Current Limiting

Auto poweroff for power supplies circuit

Probably the easiest way of doing automatic switch off is with relay logic. In the diagram, the box marked RL1 is the coil, the 2 in the coil box tells you there are two sets of contacts somewhere on the circuit, operated by this coil. In this simple circuit, it's easy to see them!Sw1 is a momentary push switch. When it's pushed (contacts closed), voltage from In is applied to the coil so the relay operates. RL1/1 (the first set of contacts on RL1) closes and applies power to Out. RL1/2 closes at the same time and shorts out Sw1, maintaining power to the relay coil. So Sw1 is a push to switch on and the circuit is a relay bistable - bistable as it has two stable states, on and off...

Current Booster

Volt regulators such as the LM708, and LM317 series (and others) sometimes need to provide a little bit more current then they actually can handle. If that is the case, this little circuit can help out. A power transistor such as the 2N3772 or similar can be used (See the project on the 3-part variable power supply). The power transistor is used to boost the extra needed current above the maximum allowable current provided via the regulator. Current up to 1500mA(1.5amp) will flow through the regulator, anything above that makes the regulator conduct and adding the extra needed current to the output load. ..

There are a number of ways to obtain the low voltages required to run small projects from the wall power outlet. The simplest way is to buy a factory-built molded supply which is designed to plug directly into the wall outlet. Some such supplies have an internal voltage regulator and need no additional parts, others provide an unregulated DC voltage and many are simply AC transformers in a box. The regulated types offer less power output for a given size with currents limited to a couple of hundred milliamps but the AC transformer types can provide several amps. ..

The following are current regulator circuits that can be used to drive light emitting diodes. As always, take time to do some testing before attempting to use these circuits in actual applications...

Current-limit switches are virtually ubiquitous in system controls. They provide a safe means for regulating the current delivered to a load circuit. The switches allow the load current to increase to a programmed limit but no higher. This application note illustrates the implementation of a digitally programmable current-limit switch using MAX890L a current-limited, high-side P-channel switch and MAX5160 digital potentiometer..

Figure 1 shows a 5V to 12V boost converter ideal for USB applications. The converter has a selectable 100mA/ 500mA input current limit, allowing the device to be easily switched between the USB low and high power modes. Efficiency, shown in Figure 2, exceeds 85%...

Current-limit switches are virtually ubiquitous in system controls. They provide a safe means for regulating the current delivered to a load circuit. The switches allow the load current to increase to a programmed limit but no higher. Typically, the current limit is a function of the voltage across an external resistor, produced by the current from a fixed source internal to the switch IC. This voltage serves as the reference for an internal current-limiting amplifier...

The comparator circuit in Figure 1 provides detection of a pending latch-up condition. The comparator senses whether the load voltage sinks below a given threshold. The comparator can then generate an interrupt to a supervising controller. You can use the circuit to effect board-level latch-up detection and power cycling...

The current-limit circuit in Figure 1 allows you to establish the setpoint by applying a voltage to one of the amplifier`s pins. With this design, it is possible to set the current-limit point with the output of a DAC, possibly under the control of an embedded µC. The OPA547 is a true op amp; thus, it does not need a connection to power ground...

The circuit in Figure 1 shows how a thermistor applies temperature compensation to the circuit`s output-current limit. The MAX1714`s linear current-limit (ILIM) input range at Pin 6 of IC1 spans 0.5 to 2V, which corresponds to current-limit thresholds of 50 to 200 mV, respectively...

The versatile 5 pin L200C regulator offers both voltage and current regulation in a single package. The IC also features thermal shutdown and input over voltage protection up to 60 Vdc. The package is also available as L200CV which has straight pins for mounting onto a PCB...

Although this short-circuit protection is often sufficient, some applications require a true current limit, such as the implementation in Figure 1`s circuit. A low-value resistor, R1, performs current sensing. The circuit applies the voltage across this resistor, through R2 and R3, to the inputs of the comparator IC2A. R4 and R8 create initial offset voltages to set the current-limit trip point...

The three major types of current-limit-protection mechanisms are constant, foldback, and hiccup. Hiccup current limit performs the best of the three types; however, the implementation is rather complex. In this scheme, upon detection of an overcurrent event, the whole power supply shuts down for an interval before it tries to power itself up again. The cycle repeats until the overcurrent fault disappears...

The circuit is based on IC1, an LTC1430 PWM controller. The current-limiting feature of the IC operates by sensing the voltage across the high-side MOSFET and compares it with a threshold voltage developed across R3...

You can add a simple, two-transistor circuit to a standard, step-down dc/dc converter to provide an adjustable limit for the output current (Figure 1). You can apply this method to most step-down converters that provide access to the feedback node...