High side current sensing with variable gain

[phaedrus]

Hi all,

Is there any way to change the value of resistor R3 so that I can change the output voltage ? I could do it with a relay and change the value of R3 by switching in the values I need,but I would prefer a more elegant solution if one exists.

TIA.

 

[canadaelk]

This circuit is from Analog Devices app. note AN32. You propably will need an amp or comparator at the output. An alernative could be the Zetex ZXCT-1009 ic. E

 

[crutschow]

You could use a FET switch in series with a added resistor to switch it in parallel with R3. Just be aware that the FET will have a small "on" resistance that will add to the resistor value.

 

[phaedrus]

You could use a FET switch in series with a added resistor to switch it in parallel with R3. Just be aware that the FET will have a small "on" resistance that will add to the resistor value.

Thanks, I will use this idea.I think the on resistance value can probably be calibrated out.

 

[phaedrus]

This circuit is from Analog Devices app. note AN32. You propably will need an amp or comparator at the output. An alernative could be the Zetex ZXCT-1009 ic. E

Thanks Elk,
Actually I want to avoid using special parts,since I would have to import them.

 

[ericgibbs]

Thanks Elk,
Actually I want to avoid using special parts,since I would have to import them.

hi,
This is your circuit in LTspice sim, I have used a transistor to switch in a parallel resistor.

 

[phaedrus]

hi,
This is your circuit in LTspice sim, I have used a transistor to switch in a parallel resistor.

Thanks much Eric. I too modelled it in LTSPICE but using 2N3904 in place of your J1 and BS170 in place of your 2N3904.Any particular reason you chose a BJT instead of a MOSFET for the switching ?
In my app i need to use a 1E sensing resistor to meet the standard requirements. And the current would be from 0~200mA in one range and 0~20mA in the other one.
Thanks for all the friendly replies.

 

[ericgibbs]

Thanks much Eric. I too modelled it in LTSPICE but using 2N3904 in place of your J1 and BS170 in place of your 2N3904.Any particular reason you chose a BJT instead of a MOSFET for the switching ?
In my app i need to use a 1E sensing resistor to meet the standard requirements. And the current would be from 0~200mA in one range and 0~20mA in the other one.
Thanks for all the friendly replies.

hi,
It sims ok with two 2N3904's and a single supply.
No special reason for BJT versus MOSFET use.

 

[phaedrus]

One clarification

1)Suppose I connect the circuit as attached , why does the measured voltage R4 become much higher ? It works correctly up to about 60mA and then the values become very high.If I connect the way Eric has connected,then I get the proper values.


Thanks !!

 

[ericgibbs]

1)Suppose I connect the circuit as attached , why does the measured voltage R4 become much higher ? It works correctly up to about 60mA and then the values become very high.If I connect the way Eric has connected,then I get the proper values.
Thanks !!

hi,
Your image shows a LF411 opa, the common mode voltage is limited to approx +V /-V - [+/- 1.5V ] so its not suitable for the high side monitoring as shown in the circuit.

The LM301 accepts the full supply range common mode input voltage.

 

[phaedrus]

Hi Eric,
Thanks for the reply.
I made a mistake.
The circuit I had shown in my previous post works in the simulation as well as in the the wire wrapped board.
However the circuit I have attached now does not work.After about 60mA (2.98V) it goes to 7.67 volts for greater than 60mA.It happens in the simulation as well as practically.
The common mode voltage for LF411 is shown as +10V for 9V supply and -7.5 for -9V supply.Graph attached.Is that what you were mentioning ?

hi,
Your image shows a LF411 opa, the common mode voltage is limited to approx +V /-V - [+/- 1.5V ] so its not suitable for the high side monitoring as shown in the circuit.

The LM301 accepts the full supply range common mode input voltage.

 

[ericgibbs]

hi,
Which is the current sense resistor in your circuit.?? and the take off points for the LF411.?

 

[phaedrus]

R1 is the current sense resistor.
R2 is the resistor which simulated the connected load.
I did not understand the take off points part.Can you amplify that a bit.

hi,
Which is the current sense resistor in your circuit.?? and the take off points for the LF411.?

 

[ericgibbs]

R1 is the current sense resistor.
R2 is the resistor which simulated the connected load.
I did not understand the take off points part.Can you amplify that a bit.

Where is the top of R3 connected too.?? [take off point or }

 

[phaedrus]

R3 is connected to only 2 points,as show in the schematic. One is directly to +9V and the other to the N-INV input of the OpAMP.

Where is the top of R3 connected too.?? [take off point or }

 

[ericgibbs]

R3 is connected to only 2 points,as show in the schematic. One is directly to +9V and the other to the N-INV input of the OpAMP.

hi,
Sorry about the slow reply, had an overriding priority.

Are you trying to use the lower N MOSFET has a variable ''linear' resistor.?
Or as a switch to select the 20mA and 200mA ranges.?

 

[phaedrus]

Thanks for taking the time out to reply.
I am using it as a switch to select the 20mA and 200mA ranges.

hi,
Sorry about the slow reply, had an overriding priority.

Are you trying to use the lower N MOSFET has a variable ''linear' resistor.?
Or as a switch to select the 20mA and 200mA ranges.?

 

[ericgibbs]

Thanks for taking the time out to reply.
I am using it as a switch to select the 20mA and 200mA ranges.

hi,
This LTS.asc file shows a 200mA load, with the Vout switched between 5K/10K.
You can see the 2V and 4V change in Vout.
Ensure when choosing the Rsense resistor that you dont drive the LF411 into saturation.

 

[phaedrus]

Hi,
Is it ok then to use the LF411 for this application ? I can also get the LM301 of course.

hi,
This LTS.asc file shows a 200mA load, with the Vout switched between 5K/10K.
You can see the 2V and 4V change in Vout.
Ensure when choosing the Rsense resistor that you dont drive the LF411 into saturation.

 

[ericgibbs]

Hi,
Is it ok then to use the LF411 for this application ? I can also get the LM301 of course.

hi,
Rechecked the LF411 you were correct.
This LTS sim is close to what you are looking for.