If the current is going to ground you could use this simple op amp application. Since the - input is a virtual ground it will have no resistance, and have a voltage out for very low current applications. The better the op amp the better it works.

Could you help me out in this setup please? How should I connect it to determine actual current? I am using opamp OPA335

Thanks in advance!

 

We frown on hijacking someone else's thread. Now you have your own.

 

You will notice that that circuit is an inverting amplifier. Hence a positive voltage on the input will result in the output trying to go negative if the power supply will allow it to do so.

What you want is a difference amplifier that will amplify the DC voltage across a resistor.

 

Often it is done like this using a single supply.

 

Could you help me out in this setup please? How should I connect it to determine actual current? I am using opamp OPA335

Thanks in advance!

normally this circuit is used to measure low currents such as from a photo diode...
e.g.

 

normally this circuit is used to measure low currents such as from a photo diode...
e.g.

This is called an transconductance amplifier true?

 

This is called an transconductance amplifier true?

transconductance amplifiers are voltage controlled current sources e.g.

http://www.ti.com/product/opa861

 

transconductance amplifiers are voltage controlled current sources e.g.

http://www.ti.com/product/opa861

Transimpedance amplifier sorry.

 

Transimpedance amplifier sorry.

yep.

It is not a supply line current measurement device. Best to use a current shunt with a diff amp. e.g.
http://forum.allaboutcircuits.com/b...eed-an-amplifier-difference-or-isolation.693/

Peter

 

transconductance amplifiers are voltage controlled current sources e.g.

So it is like the mosfet Vs the BJT. The mosfet is an voltage controlled current source, whereas the BJT is an current controlled voltage source. I've never used an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) -- before. Memorized the anocrym for an exam years ago and have never forgotten it.

Seems a bit strange / expensive / specialized using the OPA335 OPAMP for such an fundamental building block. The more common TL072 JFET dual OPAMP is perhaps more appropriate?

 

So it is like the mosfet Vs the BJT. The mosfet is an voltage controlled current source, whereas the BJT is an current controlled voltage source. I've never used an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) -- before. Memorized the anocrym for an exam years ago and have never forgotten it.

Seems a bit strange / expensive / specialized using the OPA335 OPAMP for such an fundamental building block. The more common TL072 JFET dual OPAMP is perhaps more appropriate?

there are 4500 different types of opamp on the Farnell website. You could choose any one of them, as appropriate, for your application.

 

there are 4500 different types of opamp on the Farnell website. You could choose any one of them, as appropriate, for your application.

That's enough to send you mad. Farnell in Australia is called Element 14 now. Don't think business is so crash hot today. Old money, BIG profits from the 80's and the 90's maybe paying the bills.

 

We frown on hijacking someone else's thread. Now you have your own.

Sorry for that. Didn't know.

 

IMHO, the easiest way of measuring current in DC circuits is to use a dedicated IC like a ZXCT1009 and its cousins.

 

yep.

It is not a supply line current measurement device. Best to use a current shunt with a diff amp. e.g.
http://forum.allaboutcircuits.com/b...eed-an-amplifier-difference-or-isolation.693/

Peter

Hi Peter,

First of all thank you for yor interest to reply to my difficulty.

I have just realised what you have just said, that a transimpedance amplifier are not intended for supply line current measurement.

I intend to build a circuit that is able to capture the current drawn in the ranges on mA and uA. The main reason for this is to observe how power is consumed in battery powered device.

Would you be so kind to guide me please seeing that you are proficient in this area? I have modified my circuit to match the circuit which you provided earlier. [http://forum.allaboutcircuits.com/b...ed-an-amplifier-difference-or-isolation.693/]

I am using OPA335 opamp. R1 0.8ohm. R2,R3,R4 and R5 are all 10Kohm .1% resistors. R6 is a 220ohm resistor in series with a red LED (for testing purposes).

I am measuring the output voltage by hooking the probe to Vout and Gnd.

I appreaciate your help.

Thank you in advance!

Malcolm.

 

I have found this document. http://www.ti.com/lit/ml/slyb194a/slyb194a.pdf

Referring to Current Shunt Monitors Low-Side Measurements, what does a 50mV shunt mean in practice please?

Thanks,

Malcolm.

 

I have found this document. http://www.ti.com/lit/ml/slyb194a/slyb194a.pdf

Referring to Current Shunt Monitors Low-Side Measurements, what does a 50mV shunt mean in practice please?

Thanks,

Malcolm.

The resistance of the current shunt RShunt produces 50mV at full scale current. So:
RShunt = 50mV/FullScaleCurrent

for 100A, RShunt = 0.50mR
1A, RShunt = 50mR
10mA, RShunt = 5.0R

In each case, at full current, the shunt drops 50mV; you need to remember that this volts drop may affect your circuit. Take a look at the TI webpage you found and another blog I wrote http://forum.allaboutcircuits.com/blog/cursns2-current-sense-resistors.696/

 

........................whereas the BJT is an current controlled voltage source.
...................

No. For large signals (switching and DC bias calculations) a BJT can be considered a current-controlled current-source (the collector has a high impedance).
For small AC signal applications, the voltage-controlled current-source (transconductance) model is often used.

 

The resistance of the current shunt RShunt produces 50mV at full scale current. So:
RShunt = 50mV/FullScaleCurrent

for 100A, RShunt = 0.50mR
1A, RShunt = 50mR
10mA, RShunt = 5.0R

In each case, at full current, the shunt drops 50mV; you need to remember that this volts drop may affect your circuit. Take a look at the TI webpage you found and another blog I wrote http://forum.allaboutcircuits.com/blog/cursns2-current-sense-resistors.696/

I have a 5V 1amp power supply. So does that imply that I need to use a 0.05ohm resistor please?

 

I have a 5V 1amp power supply. So does that imply that I need to use a 0.05ohm resistor please?

for 1A yes. But how low a current do you want to measure, I think you mentioned microamps?