Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

What is Current Sensing?..What is it used for?

Status
Not open for further replies.

Haru

New Member
I reed a bit on current sensing techniques but I still don't get what is the point of it, and what is it used for?

Anybody knows?

I ask, coz I think it might be useful for my Led driver….humm! don’t know really, not sure.
 
it is used to detect current presence and/or current value.
it is often used in power supplies to detect short circuits
(too high currents) to prevent damage.
there is also voltage sense which is used to measure voltage
from the power supply at the remote end (where the load is).
this way power supply can compensate for voltage drop on wires
regardless of current drawn by the load.
 
What about the extremely low resistance (milliohm range) current-sensing resistors?

How do they work in a circuit?

I've seen them in a circuit where the main input voltage is split, with it going to do one thing, and going through a current-sensing resistor to another part of the circuit to do work there. But how does that allow the resistor to give away current draw? Something after the current-sensing resistor must work it out, comparing between the input voltage which didn't go through the CSR and the voltage which did, but what and how?
 
That sounds like a current-shunt to measure the current flowing.
 
It is sometimes useful to know how much current is flowing in a circuit. One way to do this is to insert a resistor in series, such that the voltage drop across it is either insignificant or can be compensated for (e.g., by feedback). The voltage drop across the sense resistor may need to be amplified and/or level shifted to make it useful. The resulting voltage can be used for various purposes, such as comparing it to a reference and using feedback to keep the current constant, to limit the current to a maximum (or minimum) value, to trip a circuit breaker, etc.
 
This diagram shows simplified parts of power supply circuit.

Looking at the left hand circuit first.
A1 is the voltage sensing amplifier which compares a fraction of the output voltage at the output terminals with an internal reference voltage. Any difference between them is used to control the output so that the difference (the error) is minimised. The output will therefore be constant in value.

A2 is the current sensing amplifier which looks at the voltage across the (small value) series R and compares this with another reference voltage.
The result of this comparison is used to limit the current which the supply will deliver.

n.b. As they say, "there are more ways to skin a cat". This is representational. There are many variants and this is to give some idea of what generally happens in a power supply.

Now, looking at the right hand circuit.
Some suppplies have the inputs to the voltage-sensing amplifier brought out to separate terminals on the panel. Links are provided so that they and the corresponding output terminals may be joined together so the supply is just like that at the left.
If the load is some distance from the supply there may be an unacceptable voltage drop in the leads which connect the supply to it.
By removing the links and running separate wires from the sense terminals to the load, the error amplifier sees the true voltage at the load and not the voltage at the output terminals. This compensates for the drop in the leads. Since the load current is in the leads from the output terminals, not in the sense wires, the latter can be of a very light gauge.
 

Attachments

  • sense.gif
    sense.gif
    4.9 KB · Views: 984
Fruit machines (slot machines) often use current sensing to detect when bulbs are blown, or a meter (for accountancy purposes) is disconnected.
 
Current sensing is also applicable in industrial control systems where it is used with sensors to detect whether it is a digital sensor (limit switch) that is giving zero signal or has the sensor transmission line broken up. This is done by measuring in live zero standard (4-20mA).
 
Hi,
In our applications (designs for automotive) we use small shunt resistors for measuring the current through solenoids/motors which are controlled by our control units.
We put a very low value (.1ohm) resistor in series at the return line from the solenoid and then use some signal conditionaing circuit and feed the voltage to the microcontroller and thus we know exactly how much current is flowing through the load.
All such solenoids/motors are normally current controlled loads so current sensing is important in our case.
 
need help

instruite said:
Hi,
In our applications (designs for automotive) we use small shunt resistors for measuring the current through solenoids/motors which are controlled by our control units.
We put a very low value (.1ohm) resistor in series at the return line from the solenoid and then use some signal conditionaing circuit and feed the voltage to the microcontroller and thus we know exactly how much current is flowing through the load.
All such solenoids/motors are normally current controlled loads so current sensing is important in our case.

Hello instruite.

Well i am kinda having a problem on how to sense the current from a load using a micro-controller. But i thought maybe you can help me out on this matter.
My questions are,
1)where should i put the shunt resistor if let say i am going to control a 12v lead acid battery charger,
2) what is the signal conditioning circuit,
3)do i have to make another circuit for the signal conditioning circuit or is it possible if i just connect it straight away to the controller.

thank you for your help, much appreciation.
 
If the load requires a common ground then put the current sense resistor in the positive side and use a differential amplifier across it to get the voltage drop and thus the current flowing through it.
 
Just trying to understand. Suppose my motor is connect using an H-bridge driver (L298N). This chip has current sensing pins, 1 and 15 for two channels. So, is it enough to connect a resistor - I'm using a .5 ohm , 12W resistor - to pin 15 and ground, and measure the voltage across this resistance?

Thanks
 
Just trying to understand. Suppose my motor is connect using an H-bridge driver (L298N). This chip has current sensing pins, 1 and 15 for two channels. So, is it enough to connect a resistor - I'm using a .5 ohm , 12W resistor - to pin 15 and ground, and measure the voltage across this resistance?

Thanks
You have it right. But you would probably connect pin 1 to another resistor if you're also using that channel.
 
Google "Current Sense Amplifier".

For your 12V charger example, I would use a high-side current sense amplifier with a very small current sense resistor in series with the load. MAX4080 is a good part that I've used before. It's app note will tell you how to use it. It's essentially a part made to do what Hero999 said.
 
Last edited:
Good Morning Mr. David
I have question about the connections of differntial amplifiers across small resistor to detect the current that flows through the wire. I really thank you what you do to help me aout.
 
Good Morning Mr. David
I have question about the connections of differntial amplifiers across small resistor to detect the current that flows through the wire. I really thank you what you do to help me aout.

This thread is over a year old. Things may work better if you start a new thread and ask your question in detail. Things like how small of a resistance and how much current? Is the current low side or high side of the load?

Ron
 
If anyone is still here and has some interest, download LTSPICE from Linear Technologies. It's a totally free unrestricted SPICE program. Pick an LT6105 and select the reference circuit for it. Now you can imagineer to your hearts content without spending a dime or soldering a wire.

Linear also has some very good application notes for current sensing. Analog Devices and Texas Instruments (TI) have good literature too.

We use current sensing to shut off jammed conveyors before they rip themselves out of the floor. We also use them on robots to detect collisions. The most novel use for them was at the water purification area of a steel mill. A hugh set of slowly rotating paddles constantly kept an equally hugh tank full of dirty water in suspension. A mechanism would slowly lower the paddles in the tank untill the current drawn by the paddle rotating motor exceeded a high limit. This would stop the lowering mechanism. The paddles would stir the water untill a minimum current was met which would restart the lowering mechanism untill the high current was met. etc,etc.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top