# build own circuit with op amp or voltage current converter IC

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 5, 2015.

1. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
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
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.

2. ### ericgibbs Senior Member

Jan 29, 2010
2,413
369
I would recheck your non inverting equation.
E
Vo = Vin(1+Rg/Ri)

3. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
oops, just verified and Vout max at 15v so should be correct.
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?

4. ### ericgibbs Senior Member

Jan 29, 2010
2,413
369
The AD822 is a R2R OPA, so with 5Vin and Vcc of 15V, the Vo max should be close to +15V, when Rf=20K and Rg=10K.

5. ### Bordodynov Active Member

May 20, 2015
599
174
Do Rg = Rf = 10kOhm.
Vin = 5V-> Vout = 10V.
Operational amplifiers often use to align the dynamic range.

6. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
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?

7. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0
oops 0 mA should be correct assuming all current flow through loop.
but I get 1 mA loop instead of 25mA.

8. ### Bordodynov Active Member

May 20, 2015
599
174
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.

Jun 5, 2015
30
0