In many sensitive electronic circuits or loads, limiting current beyond a set threshold could be one of the most crucial requirements for ensuring a fail proof safety for the various vulnerable devices and components. A simple method of implementing using current sensing resistors has been discussed in this article.
Integrating a Current Sensor Stage
The current from a power supply is mostly rated at much higher levels than the required safe value for a particular circuit under test or under operation.
Some high grade power supplies may include an adjustable feature for controlling current at their outputs, however normally we don't see this facility within ordinary or homemade units.
A simple current sensor configuration which could be built and used with sensitive circuits is shown below.
The resistor current sensor circuit utilizes just a single BJT and a few resistors.
Since most circuits would include an IC, or a configuration which would have a shutdown point somewhere within the circuit, this sensor module could be rigged with this "weak point" in order to shutdown or restrict the circuit from functioning in case the current intake rises above the danger mark.
How the Circuit Works
The functioning of the current sensing module can be understood as given under:
For circuits which may have a ground shutdown feature, the circuit using the NPN device could be employed as shown in figure on left.
Here Rx is selected such that a potential difference of about 0.6V is developed when the amp consumption at the output or by the load tends to go above the marked danger threshold.
The 0.6V is the optimal conduction voltage level for the shown BC547 or any low power general purpose BJT, thus as soon as this level is reached, the BJT conducts and grounds the available shutdown pin, switching of the supply to the load instantaneously, and the process keeps repeating at the threshold making sure the consumption is not allowed to exceed the set level.
Calculating the Sensing Resistor
Rx may be selected with the help of the following Ohms law, and as per the circuit requirement:
R = 0.6/I
The preset at the base may be used for fine-tuning the cut off region.
For Positive Shut-Down Feed
In cases where the current sensor circuit facilitates a positive shutdown option, the current sensor circuit at the right could be used, which is exactly identical to its NPN counterpart with its functions, except the polarities which are configured to produce a positive drive for the shutdown pin connected across the collector of the BJT.
Now let's learn a few of the example circuits and ICs which facilitate a shut down feature within its allotted pinouts.
Integrating with IC 555
For IC 555, the reset pin#4, or the control pin#5 can be used as the shut down inputs, the NPN sensor above may be wired up here for the intended results.
Integrating with IC LM317/LM338/LM396
For LM317, LM338, LM350, LM396, the ADJ pin functions as the shutdown pin, again the NPN module would work with these devices for the desired current restrictions.
Integrating with IC 4017/IC4060
ICs such as 4060, 4017 may be stopped from functioning by applying a positive reset voltage at their pin#12 and pin#13 respectively. Thus for these examples the current sensor with the PNP device will suit perfectly for the required amp control.
Integrating with IC SG3525/IRS2453
Other examples include IC SG3524/5 = pin#10, positive trigger shutdown.
IRS2453 full bridge driver = pin#5, latched shutdown, positive trigger (2V)
For opamp protection circuits, the input pin which may be responsible for inhibiting the power device at its output may be targeted as the shut down pin and appropriately wired up with either of the above current sensing modules.