current measuring

[DarkZero]

Im trying to build ampere meter and i built one that works with a op-amp configured as a differential amplifier but as the voltage goes up on the sense resistor i found out that i would need to find op-amp with higher vcc rating.

now my question is how does multi meter does this or even better how is meter that works on 5v for example doing this i cant find anything on the internet on this or i just lack terminology

[joeqsmith]

very low shunt resistance.

[Kalvin]

The multimeter measures the voltage across the shunt resistor relative its own ground reference which is floating as the multimeter is connected into the circuit using only two measurement probes.

[coppice]

If you have problems with the voltage across the shunt being too big you are making some odd choices about your shunt value. You want to keep the shunt as small as possible, to avoid wasting power in it, to avoid changing its value by toasting it, and to minimise the effect you are having on the rest of the circuitry by introducing a voltage drop.

[DarkZero]

im using 0.1 Ohms shunt resistor to measure 0-20v 2A PSU the problem is i have to supply op-amp with a 22v in order for it to work so if i had 0-50v psu i would need 52v supply for the op-amp and such op-amp

so my question is how does those cheap amp meters do it of the 5v power supplies that they use and whats the practical circuit to do it 

[TheAmmoniacal]

It's very easy to calculate the best value shunt resistor to your needs, if you need 22 mOhm or 100 µOhm even..

V = I*R

[joeqsmith]

So 0.2V is is into the op-amp.  Something is wrong but I guess you knew that already.   Are you within the common mode range of the amplifier you are using?   

im using 0.1 Ohms shunt resistor to measure 0-20v 2A PSU the problem is i have to supply op-amp with a 22v in order for it to work so if i had 0-50v psu i would need 52v supply for the op-amp and such op-amp

so my question is how does those cheap amp meters do it of the 5v power supplies that they use and whats the practical circuit to do it 

[DarkZero]

i attached the pictures in effort to make my question more clear im trying build stand-alone meter for some monitoring

for example if i set PSU to 20V,   0.05 ohms  will develop some small voltage drop which will give some result on the output of the op-amp but op-amp needs to be supplied with more then 22v to be able to do that's and that where my problem is i would like my current meter to work for example on 5v rather then always higher then what  load is seeing.

[joeqsmith]

Those both don't look right.   Where is the center for the +/- supplies?  Tie your ref to that. 

[DarkZero]

Isn't the GND of the power supply my ref ?

[Jeroen3]

http://www.ti.com/lit/ml/slyb194a/slyb194a.pdf

You need to be aware of the common mode voltages in your circuit.
A multimeter does not have this problem because the multimeter does not share the same ground.

[Brutte]

You have applied 20V to a comparator that is Vcc = 5V powered.
What about "absolute maximum ratings" chapter?

[DarkZero]

You have applied 20V to a comparator that is Vcc = 5V powered.
What about "absolute maximum ratings" chapter?

op-amp i used is mc1458 i dont know how picture ended up with that part number

[Brutte]

You have applied 20V to a comparator that is Vcc = 5V powered.
What about "absolute maximum ratings" chapter?

op-amp i used is mc1458 i dont know how picture ended up with that part number

Ok, then you have applied 20V to an op-amp that is Vcc = 5V powered.

The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less.

[rstofer]

What you need is a high side current sense amplifier and a low resistance shunt:
https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4172.html

These devices are all over the place.  In addition, Maxim has an app note on how to use this device up in the 200V range.

[danadak]

http://www.analog.com/library/analogDialogue/archives/48-02/diff_amp.html

http://masteringelectronicsdesign.com/differential-amplifier-calculator-2/

These might help.


Regards, Dana.

[suicidaleggroll]

There's no reason your op-amp ground needs to be the same or 5v below your power supply ground.  All the op-amp cares about is if V+ minus V- is within the required supply voltage range and the voltage at its two input pins (both sides of the shunt resistor) are within the common mode voltage range, that's it.  Who cares where the ground reference for your power supply goes?  Also, why do you need to use a high side shunt resistor?

If you don't need a high side shunt, then put it on the low side and give your op-amp +/- 5V relative to the power supply ground.

If you do need a high side shunt, then consider using an isolated +/- 5V supply for the op-amp, and tie the common to either side of the shunt resistor to center it in the +/- 5V range.