hi,
Use this procedure,
Change R6 to 3k5 , this will increase the Gain.
1. Set R1 to 330R, increase the value of R3 to ~330.18R. [as you can see it is very small change in R3]
Vout should now read ~1V [ this is the base voltage which gives an Iout of 4mA]

2. Set R1 to 330.7R, Vout should read ~ 5V, so Iout should read =20mA

You may have to repeat from Step 1 around the loop until you get Vout =1V and 5V

Note: You could reduce R4 by a very small value instead of increasing R3 [ it may be easier, by adding a high value resistor]

E

okay eric,

i will work on it... i vary the R3 it will be easier one..
but is that any solution for my PCB board...

 

hi,
Use this procedure,
Change R6 to 3k5 , this will increase the Gain.
1. Set R1 to 330R, increase the value of R3 to ~330.18R. [as you can see it is very small change in R3]
Vout should now read ~1V [ this is the base voltage which gives an Iout of 4mA]

2. Set R1 to 330.7R, Vout should read ~ 5V, so Iout should read =20mA

You may have to repeat from Step 1 around the loop until you get Vout =1V and 5V

Note: You could reduce R4 by a very small value instead of increasing R3 [ it may be easier, by adding a high value resistor]

E

EDIT:
This LTS sim shows the Vout and Iout for R1= 330R [ 1v, 4mA] and 330.7R, [5v,20mA]
Note R3 value.

okay eric i do it on test board

 

hi,
What you are doing to zero and calibrate your bridge and amplifier is a normal procedure, especially when you have built your own bridge.
There is always some level of bridge/amplifier offset signal which has to be nulled/zeroed out and a gain/span control adjustment to get the correct output slope.

Concentrate on the bridge and amp, get that Vout right and the V2I circuit should be very close to 4-20mA.

E

 

hi,
What you are doing to zero and calibrate your bridge and amplifier is a normal procedure, especially when you have built your own bridge.
There is always some level of bridge/amplifier offset signal which has to be nulled/zeroed out and a gain/span control adjustment to get the correct output slope.

Concentrate on the bridge and amp, get that Vout right and the V2I circuit should be very close to 4-20mA.

E

okay eric i will do the procedure for balanced the bridge

 

hi r,
OK,
Another common method for balancing or adding an offset signal, is this image.
Use a 20 turn Trim pot.
The PDF is a bit of bed time reading for your reference data bank.

E

 

hi r,
OK,
Another common method for balancing or adding an offset signal, is this image.
Use a 20 turn Trim pot.
The PDF is a bit of bed time reading for your reference data bank.

E
View attachment 151420

okay eric..thanks for your all time support....... i do the circuit and will share my output values...

 

hi,
Use this procedure,
Change R6 to 3k5 , this will increase the Gain.
1. Set R1 to 330R, increase the value of R3 to ~330.18R. [as you can see it is very small change in R3]
Vout should now read ~1V [ this is the base voltage which gives an Iout of 4mA]

2. Set R1 to 330.7R, Vout should read ~ 5V, so Iout should read =20mA

You may have to repeat from Step 1 around the loop until you get Vout =1V and 5V

Note: You could reduce R4 by a very small value instead of increasing R3 [ it may be easier, by adding a high value resistor]

E

EDIT:
This LTS sim shows the Vout and Iout for R1= 330R [ 1v, 4mA] and 330.7R, [5v,20mA]
Note R3 value.

[/QUOTE]

dear eric,

As per your procedure i did the circuit here i share my output with circuit.
but in one place (R6) i used 2.2K.
kindly review it. and give me a comments

 

hi Rajit,
Those results look very good.
Do you need to trim out the 4.1mA to 4.0mA and span the 19.77mA to 20mA.???
E

 

hi Rajit,
Those results look very good.
Do you need to trim out the 4.1mA to 4.0mA and span the 19.77mA to 20mA.???
E

eric,
yes i need to trim it and also have to modified my circuit based on below condition
1. whether the load is 330.7 or greater than that i should have 20 mA at my output
2. at balanced condition or without input my circuit should generate the 4 mA output.

is it possible can i fix my output at 20mA when my input is >= to 330.7( => 5v at amplifier output)

 

hi,
To give 20mA at 330.7R you need to increase the Gain, by lowering R5 value.
You can make R5 a fixed resistor in series with a 500R Trim pot you can set the Gain precisely.
To Set the 4mA Zero point you need to add a high value trim pot across R3 of the bridge.
You may have to adjust the gain and zero trim pots a number of times to get a precise 4mA and 20mA
Do you follow that OK.
E

 

hi,
To give 20mA at 330.7R you need to increase the Gain, by lowering R5 value.
You can make R5 a fixed resistor in series with a 500R Trim pot you can set the Gain precisely.
To Set the 4mA Zero point you need to add a high value trim pot across R3 of the bridge.
You may have to adjust the gain and zero trim pots a number of times to get a precise 4mA and 20mA
Do you follow that OK.
E

yes eric. i am okay with that. i can get 500r trim pot
but can we get 20 mA output even though my load increase more than 330.7 ohm.
for example, my output would be,
zero point - 4mA
330.2 - 9mA ( approx.)
330.7 - 20mA
330.8- 20mA
330.9 - 20mA
output level should maintain at 20mA level from 330.7 load to more than that?

 

hi,
Are asking for the output current to be clamped at 20mA, so that it never exceeds 20mA, even if the 330.7R value is exceeded.?
E

 

hi,
Are asking for the output current to be clamped at 20mA, so that it never exceeds 20mA, even if the 330.7R value is exceeded.?
E

yes eric. thats my requirement.

 

hi r,
That could be a little difficult, I will run some simulations and post a solution.
E

 

hi r,
That could be a little difficult, I will run some simulations and post a solution.
E

okay eric.

is it possible if we add the zener diode at amplifier output side.. so saturate the output at 5V once the load exceed 330.7.
because the previous circuit provide 20ma at 5V...?

 

hi r,
Try a series 470R in the output of the bridge amplifier U3 , connect a 5.1V shunt diode to 0V from the end of the 470R. [U4 +Inp pin]
Due to the 'soft' knee voltage of a zener you will see a sharp clamping action at ~5v.
Post your test results.
E

 

hi rajit,
This is a LTS sim plot.
If it is clamping to early, try a 5,6V Zener.
E

 

hi r,
Try a series 470R in the output of the bridge amplifier U3 , connect a 5.1V shunt diode to 0V from the end of the 470R. [U4 +Inp pin]
Due to the 'soft' knee voltage of a zener you will see a sharp clamping action at ~5v.
Post your test results.
E

View attachment 153143

okay eric i can understand. what is the part number for shunt diode

 

hi rajit,
This is a LTS sim plot.
If it is clamping to early, try a 5,6V Zener.
E

eric,
i saw your simulation.. but this circuit did not satisfied the my another condition... for zero input output should be 4mA

 

hi,
eBay is one source of the 5.1Vz.

https://www.ebay.co.uk/sch/i.html?_...&poi=&adpos=1t4&device=c&crdt=0&ul_noapp=true