Milliohm Resistors are used for current sense or to act like a fuse , why?

How do you use milliohm resistors to do current sense or act like a fuse?

I see them used on transistor emitter to ground and FET source to ground to do current sensing or to act like a fuse, but for what reason?

Where else would you use milliohm resistor?

In a class AB amplifier , where do they put the milliohm resistors mostly? they either use the milliohm resistor to do current sensing or to act like a fuse

Do you know any other circuits that uses milliohm resistors like a fuse or current sensing?

mΩ resistor is often used as current shunt - current sensor. Usage is to sense current flow at all or to measure exact current flow.

To use a milliohm resistor as a fuse is a bodge. Fuses are designed to "burn out" at a specified current. Resistors are designed to made to a tight resistive tolerance and no thought is given to their actual fusing current or time so these parameters are uncontrolled. Also it is useful for fuses to be pluggable, if they are soldered in it is likely that their performance will be degraded by the heating up during soldering. Likewise to have a pluggable milliohm resistor will lead to an uncontrolled contact resistance at its contacts in the holder.
Frank

Where would u put the Milliohm resistors at to do current sensing?

Why would you want to do current sensing? for transistors or FETS?

Why would a designer want to do current sensing? For what reason to do current sensing?

Current sensing is used for control purposes or maximum power point tracking of a solar panel.

Current sensing is used for control purposes

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What do u mean by control purposes? as in how so?

What are some examples of using Current Sensing?

This is fundamental thing in electronics.

I suggest reading some electronics tutorials about Voltage, Current, Power, Resistors, ... Ohm Law,...


Current sensing is used to measure current in circuit or in some part of circuit. There is various reasons why this is needed. Battery chargers, solar photovoltaic systems, inverter systems, cars, power supplys, ...

Read this article and most of your doubts will be cleared.
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Its good to know that we also have other ways and methods for current measuring, such as Hall sensors and current transformers. Hall sensors is good they dont make influence on current in circuit during measuring process (check Allegro A1302 sensor or similar), or can have minimal influence such as Allegro ACS serie of sensors. Unlike this shunt resistor have very small resistance in mΩ range (0,01mΩ-0,1mΩ) but still have enough resistance to make impact on current in circuit and to develop heat and losses, specially at higher current ratings.

ACS712 Hall sensor serie - Very small influence on circuit



A1302 Hall sensor - No influence on circuit
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Shunt Resistor - Pro maded expensive resistor, high impact on circuit, developing heat and loss, for higher currents


Shunt Resistor - Cheap wide usage power resistor, high impact on circuit, developing heat and loss, for lower currents

Why do battery chargers need current sensing? for what?
Why do inverter systems need current sensing?
Why do power supplies need current sensing?

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Why do battery chargers need current sensing? for what?
Why do inverter systems need current sensing?
Why do power supplies need current sensing?

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The Current Sensing I have seen or use are milliohm resistors Not using Op amps

The Milliohm resistor is the Sense Resistor or SHunt resistor which is for low side sensing or Direct, sense resistor is in series with the load

My question is how is the milliohm resistor doing current sensing when there is no op amp connected to it?

danny davis said:

Why do battery chargers need current sensing? for what?
Why do inverter systems need current sensing?
Why do power supplies need current sensing?

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You can see progress and at which current rate your battery is charged.
In inverter systems current sensing will show production or draining of current rate.
Power supply with current sensing and showing on LCD/7-seg will tell you how much your load/device use current.

danny davis said:

The Current Sensing I have seen or use are milliohm resistors Not using Op amps

The Milliohm resistor is the Sense Resistor or SHunt resistor which is for low side sensing or Direct, sense resistor is in series with the load

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Shunt resistor usually is in miliohm range and make smaller voltage drop. Opamp is used to make voltage in wider range such as 0-5V, 0-3V, to adjust better reading with uC ADC or some analog instrument.

Shunt resistors are also used on high side.

danny davis said:

My question is how is the milliohm resistor doing current sensing when there is no op amp connected to it?

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In that case you need better instrument for reading smaller values (smaller voltage range).

Shunt resistor only make voltage drop, and you read that voltage drop with some analog instrument, uC ADC, or on some other way.

You can see progress and at which current rate your battery is charged.

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This Shunt/Sense Resistor is in series with the battery?

Why you want to know the current rate of the batteries being charged?

In inverter systems current sensing will show production or draining of current rate.

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What u mean by show production?

Draining of current rate? what do you mean by this, how so?

What's the difference between High side current sensing and low side current sensing?

They said the low side current sensing will not protect the circuit if there is a load short , so the shunt/sense resistor will not open like a fuse

I do see Milliohm resistors on the emitter of transistors going to ground or a milliohm resistor on the Source going to ground

They seem to use this milliohm resistor like a fuse? but how does it work?

I suggest you to search EDABoard and Internet for answers on this elementar questions.

Danny Davis this questions are one example why must have practical practical learning and not only theoretical or virtual learning in some simulators such as Proteus,...

Schools which dont have practical teaching and lab experiments are good for poets and philosophers, and its not good for electronics learning.

Take 100-200g of electronic parts in hands and start experimenting. Take digital unimer instrument, can be even cheap Chinese, soldering iron, battery source or some power supply with different voltages (for start take 12V car battery or smaller 4Ah 7Ah such as for UPS devices, to feel currents), cheap power resistor with different values, try to blow fuse in some circuit scenario, ... Simple try to feel purpose of each part in circuit.

What you think why Tesla and other big world scientists had big labs where they had many experiments, before they make some conclusion ? Tesla has put all of his life and money in experiments, why you think that we are smarter with some simulator?


;-)

Don't know if I a correct about this but if the small resistor is being used at the emitter of a transistor in a Class AB amplifier it may be for thermal compensation. (don't know if it is called that). When a transistor heats up its Beta increases so more current passes through the transistor. When adding a resistor at the emitter a voltage is created across this resistor. The higher the current the larger the voltage. Now as the temperature of the transistor increases and more current passes the voltage will be bigger. Now when the voltage gets bigger acros the RE resistor the base emitter voltage of the transistor decrease reducing the current through the transistor. In short it is a type of feedback that stabilize the current through the transistor. For Class AB amplifier it is probably very small due to the transistor being a Darlington transistor. The same applies for Mosfets by decreasing the voltage between gate and source.

The resistor is not there for current sensing and neither as a fuse. No need for the confusion.

jmsattard said:

Don't know if I a correct about this but if the small resistor is being used at the emitter of a transistor in a Class AB amplifier it may be for thermal compensation. (don't know if it is called that).

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I'm not sure of the correct term either, but it is to prevent thermal runaway. Also, if more than one power transistor is used in parallel, then the emitter resistors help ensure equal current sharing. Common values for audio amplifiers are between 0.1Ω and 0.47Ω.