build own circuit with op amp or voltage current converter IC

Thread Starter

kevin0228ca

Joined Jun 5, 2015
30
I am trying to create a constant current output for my project between 0 - 10 mA.
My output signal is 5-30v square wave and I am trying to convert to 0-10 mA.
I read we can convert voltage to current by using op amp
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/voltage-to-current-signal-conversion/
but I also found IC http://www.ti.com/lit/ds/symlink/xtr110.pdf
so for voltage current conversion do I have to use a IC like xtr110 or should I use a op amp?
Assuming I am new to general electronics, and have budget.
Also, is there anything I should keep in mind when using IC

Also, I am using AD822 op amp, and I am seeing weird behaviour with a simple circuit.

for inverting I get correct Vout = (Rf/Rin) x Vin
but for


My Vin is 5v, Rg = 10K, Rf = 20K, should Vout should max at Rf/Rg x Vin + 1 = 11V
but my Vout is exceeding 11V as Vcc increase, and - pin is also increasing with Vcc but max at 5V.
I read that op amp +pin -pin should have same voltage.

Sorry for my newbie question.
Can anyone clarify?
Thank you.
 

Thread Starter

kevin0228ca

Joined Jun 5, 2015
30
oops, just verified and Vout max at 15v so should be correct.
also, I just read wiki
Equilibrium will be established when Vout is just sufficient to "reach around and pull" the inverting input to the same voltage as Vin.
So I guess is normal that - pin voltage is changing and max at 5V?
 

Thread Starter

kevin0228ca

Joined Jun 5, 2015
30
Thank you, I think my op amp is working.
However, when I build

I use 200 ohm instead of 250, and my Rload is 20K, Vin = 5V
But V at -pin becomes 0, and 0 mA instead of 5/200 = 25mA.
So my op amp cannot act as voltage current converter?
 

Bordodynov

Joined May 20, 2015
3,177
Use not 200 Ohm, use 20kOhm resistor.
If you used a perfect operational amplifier with supl. voltage 550V,
your electronic circuit would have worked.
In load would be allocated power
P = (25mA * 20kOhm) * 25mA = 500V * 25mA = 12500mW = 12.5W.
 

Thread Starter

kevin0228ca

Joined Jun 5, 2015
30
Thank you.
I just read that Vcc should be enough to give Rload 25 mA.
So suppose I want 25mA, and my Rload is 10K, My vcc should be 250.
Since my Vcc is 30V, my Rload max is 1200.
 
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