Voltage to current converter circuit

[Bharanidharan]

Hi guys,

I didn't have access to internet for the last two days. So, could not reply to you. I apologize for that.

I experimented few cirucits, one of them included tuned circuits(Kris, I have no idea if the capacitor of 0.47uF is a common preferred value till now).The schematic diagram is attached to this post.

The idea was to make the circuit stable to any load. But I am not sure if i have done it right.

The result of the experiment was I was able to drive a current of 75mA(using DMM) to the inductor using the tuned circuit. It is nowhere near the result i want it to be. But if i measure the voltage across the series resistor of 4ohms, it is about 3V and by ohms law it should be I =V/R=> 3/4 = 0.75 = 750mA. but it is ten times lower than that value.

There are few things i analysed from the circuit, it produces a constant output voltage when input range is from 2.5V to 10V ac voltage. Stable in the frequency range of 15kHz -80KHz.

I am unable to understand what I am doing wrong here.


Bharani

 

[(*steve*)]

Why the connection from the output of the second op-amp to the output?

 

[Bharanidharan]

makes it stable... Tried removing that and there was no output signal for the circuit i posted.

Do you know any free electronic simulation software?

 

[(*steve*)]

Try LTSpice.

 

[Bharanidharan]

is this really impossible to achieve?

 

[Arouse1973]

I have re-drawn you circuit to make it a bit clearer, hope you don't mind.
Adam

 

[Bharanidharan]

Hi Adam,
Thanks for redrawing the circuit for me.


Bharani

 

[Arouse1973]

Ok I am still in the dark about what you actually want the circuit to do. Can you explain again so I get it. You know you can't have a selectable constant current that varies between certain values without changing at least one other thing. If you want to cancel out the reactance of the coil you will need to use a series tuned circuit. The parallel tuned circuit you have drawn normally has a very high impedance at resonance.
Adam

 

[Bharanidharan]

As you know the source is a AC 2.5 volt at 20KHz frequency. I want to convert this voltage to 200mA - 1A current and supply it to the Inductor and drive it. so, i tried a simple by changing the resistors, but the currents were really low.
Then, i moved to opamp and experimented (as you can see from the above circuit diagram) and that increased the current to 75mA as of now. But it still needs to be increased so that i can drive the inductors with high current.


Bharani

 

[Arouse1973]

Ok let me see if I can sort something out.
Adam

 

[Arouse1973]

Right, sorry for the delay. I have been busy the last few days. I have an Initial simple solution which I have attached, Input is 100 mV sine @ 5 KHz but should work at 20 KHz . R3 is your current adjust resistor. It has only been tried with a resistive load and works reasonably well. The main bug bare at the moment is having to switch in different value resistors or using a large expensive variable resistor. its funny it turned out very similar to your circuit. You may need a buffer on the output if you can't find an op-amp that can output high enough current. I'll work on the other design and try and work out a bidirectional electronically adjustable low value resistor . The simulation shows stepped resistor values of R1(Edit: R3) to show the increase of current as this changes.
Cheers
Adam

 

[Bharanidharan]

Hi Adam,

thanks for the Circuit.

i tried the connection and I am still getting 65mA nothing more that. R3 is 4ohm and R7 is 10 ohm, as i don't have 1 ohm.

opamp is AD817.

And how do i make a series tuned circuit. I have no clue in how to make it. Can you please tell what simulator you used?

i also looked at the opamps I found few

http://www.linear.com/parametric/High_Output_Current_Amplifiers

Not sure which one will be more efficient for driving the inductive load. Can you also help me with this?


Bharani

 

[Arouse1973]

I used Tina, I had to pay for it but they do Tina Ti which is free but very limited. Series tuned circuit is when the capacitor is in series with the inductor. The LT1210 might be ok, why don't you try it in LT Spice?
Adam

 

[Bharanidharan]

I used Tina, I had to pay for it but they do Tina Ti which is free but very limited. Series tuned circuit is when the capacitor is in series with the inductor. The LT1210 might be ok, why don't you try it in LT Spice?
Adam

Hi Adam,
Just downloaded Tina TI, and made a circuit. Did an AC analysis, I am getting 883mA current.. I need to know how to get a graph though... I couldn't find any option for making graphs..

I am attaching the circuit with this,, can you please help me how to make the AC signal graphs for this current.

Looks like i cannot send a schematic here. just attached it as image.

The values are
R1-4 ohm
R2-4ohm
C1-470nF
L1-135uH
R5-475ohm
Vs1/Vs3 -15V
VS2/VS4 = -15V
VG1-+5V 20KHz



Bharani

 

[Arouse1973]

For the graph you need to go to Analysis and then transient, choose you time base and ok. If you want to add further points to the graph you need to add a meter. Click the meter tab and choose the far left one called voltage pin. Add these to the schematic and then run analysis again.
Cheers
Adam

 

[Arouse1973]

Final circuit of mine showing graph of output load from 1R - 5R. P1 set to 70%. Vin 2.5 V Sine.
Thanks
Adam

 

[Bharanidharan]

Final circuit of mine showing graph of output load from 1R - 5R. P1 set to 70%. Vin 2.5 V Sine.
Thanks
Adam

View attachment 14550

View attachment 14549

Tried to simulate the circuit with opA552 and the load inductor and the capacitor, the max current I get from it is 100mA @2.5V and 201mA @5V..

Is there a way to make this circuit load independent meaning any load will work with same current flow...

 

[BobK]

Why are you not using an audio power amp?

The OPA552 is limited to 200mA.

Bob

 

[Arouse1973]

Tried to simulate the circuit with opA552 and the load inductor and the capacitor, the max current I get from it is 100mA @2.5V and 201mA @5V..

Is there a way to make this circuit load independent meaning any load will work with same current flow...

Hello
You don't seem to have looked at my circuit at all, this is what it does. It's a bi-directional current source. Perhaps you want to have a go at designing your own Op-amp. Here is just one I simulated, your going to need three so it might get a bit complex I understand you want to have a go yourself but I have done the hard work for you. And as Bob has mentioned already look at audio amps which can easily be made constant current
Adam.

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[Bharanidharan]

Hi Adam,

Thanks for the circuit.

I did have a look at your circuit, but the thing is I don't understand how it can produce so much current under loaded conditions.

let me explain myself what I understand from your circuit and what I do not:

My understanding is :

you have used an integrator amplifier circuit, differential amplifier circuit with a a voltage follower feedback. The input from the integrator is higher than the voltage follower making it inverted signal and there is a high current produced at the resistor end with an ideal opamp.

What i don't understand is :

How is this circuit will work under loaded conditions? I did make a simulation of the same circuit with different opamps and with my inductor load, but none of them produced values as close as to your graph.
I also tried using audio amplifiers, the best one i could find was LME4972 (high output current 350mA), and it produces 91.9mA @2.5Vand that is based on the circuit design I first posted.

I also tried the design you gave me. but the simulation results showed me that the output current is no more than 60mA for same LME4972.

Sorry, but I really am dumb.

I also tried op552,op551,THS3121/THS3122. Max current of 100mA was produced in op552...this is also based on the design i posted.

I also thought may be changing the simulation environment will make some difference but the LTspice is so hard... I need some components outside of it like the buffers and other opamps, but it is hard to import to them and also most of them do not support LTspice.

Final circuit of mine showing graph of output load from 1R - 5R. P1 set to 70%. Vin 2.5 V Sine.
Thanks
Adam

View attachment 14550

View attachment 14549