measuring load current (current sensor)

[crutschow]

I did search on https://www.ti.com/ but I couldn't find an input rail to rail opamp which is works at 24volts.
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You need to work on your search skills. ;-) Here's a listing.

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Current limiter circuit which I attached isn't work as I expect
Q2 gets HOT while limiting the current load.it is because of P=Vce*Ice=20volts*300mA=6Watt
What I'm suppose to do?How can I correct this circuit to limit the current(at 300mA)
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It is limiting the current and the heat is a normal result of that limiting. Three choices I can think of for the power dissipation:

• Add a heat sink.
• Go to a fold-back limiter.
• Go to a hiccup limiter.

 

[Tunelabguy]

Since your current limiter is in the ground leg of the load, you obviously have no problem separating the load return from ground. The difficulty of interrupting the low-side of the load is the chief reason why high-side current measurement is done. So since you can interrupt the ground leg, why not do your current sensing there too and save yourself all the trouble of doing high-side current sensing with different power supplies or rail-to-rail op amps and all that stuff?

 

[ftsolutions]

I think that your circuit above is actually better described as a constant current load (or source) - the transistors in the bottom are setup to try to draw approximately 260mA through Q2 regardless of what the top connected power supply voltage is, with the only other limiting factor besides the output level of the power rail being the resistance of the load. Do you need to have so much current draw? I actually use a circuit topology similar to this when I must implement a cheap floating power supply reference for powering ICs at high DC voltages - using a zener diode and fitlering caps/resistors and setting the constant current load to be some few milliamps above what the ICs require and to keep the zener in the proper part of its operating curve. I think I would implement a current limiter circuit using a power rated MOSFET device with low Rds and a small series sense resistor on the high side connection to the load instead of this - there are many examples of these circuits on the net for you to search.