Current sensing circuit

how does a current sensing circuit look and what are the design steps?

It usually involves using a small resistor (< 1 ohm) in series with the load. The resistor (0.1 to 0.5 ohms) is chosen such that it develops a small drop across it (100mV to 1V depending on the load voltage and sensitivity required). The drop across the resistor is directly proportional to the current through it (and therefore the current through the load), and can be used to determine how much current is flowing. For example, if the sensing resistor is 0.1 ohms, and develops a 100mV drop, it means that the load is drawing a current of 100mV/0.1 ohms = 1000mA or 1A.

You can connect a voltmeter (calibrated in mA or A depending on the expected max load current and sense resistor value) across the sense resistor to get a direct readout of the current. Or you could connect this to a protection circuit that trips the load when the current exceeds a certain level and so on.

Current-sensing circuits are essential for protection and control of the switching regulators.

This is a non-contact version of current monitor and can sense the flow of current in high-current appliances from a distance of up to 30 cm . It uses a standard step-down transformer (0-9V, 500mA) as the current sensor. Its secondary winding is left open, while the primary winding ends are used to detect the current. The primary ends of the transformer are connected to a full-wave bridge rectifier comprising diodes D1 through D4. The rectified output is connected to the non-inverting input of IC CA3140 (IC1).

Article compares high- and low-side amplifiers to measure battery charging currents, recommends selection criteria for current-sense resistors,

I’m recommending you to build a design strong but perhaps you could check out this design allegation which is guide for you.

Hello. What are you trying to do with the current sense circuit, is it AC or DC voltage and what current levels are involved? Is it just for testing in the lab or is it for a protection circuit in a product?

i am designing a voltage regulator for a school project and we have to incorporate a current sensing circuit in the design to protect it when a current of 1 amp or more is seen.......we are working with a 9 volt supply

Thanks, that helps. Conceptually, you would just need to monitor the voltage across the sense resistor sized as Rohit explained in his comment and when that voltage exceeds some threshold it would disable the regulator.

TI has a part that is very useful for this, the INA139. You would need to take it's out to a comparator to generate the enable/disable signal to your circuit. I would add a small RC circuit on the output of the INA139 to filter small current spikes or an in-rush surge at power up.

You could also do the same with op-amps and comparator(s) but there are several problems there. The main problem is if the circuit is on the input side you would have to power the op-amp with the 9 volt supply but then the op-amp's input would be too close to the supply voltage for the circuit to work without dividing down the inputs and inducing some amount of error.

Good luck with your project.

The first design step for a OVER CURRENT LIMIT circuit is to write a specification so you have something to design to. Questions need to be answered before starting a schematic.

1. Current limit?
2. Maximum voltage burden?
3. DC or AC current to be limited?
4. Speed of the current limit?
5. Required current limit accuracy?
6. Shut down until reset, foldback limit, constant current limit?
7. Power supply voltage(s) available to power it?
8. Power supply current available to power it?
10. Cost target for the circuit?