Voltage controlled current sources and resistors

[Skier4Life]

I have two circuits that I need help designing.

The first is a voltage controlled current source. The voltage source is an Analog Output board running in my computer that can output ±10V and can supply ±5mA. I need to be able to control current in the range of ±2A. I have a 30V 3A power supply available.

The second is similar, I need to be able to control resistance with the analog output board. The range is 25Ω to 300Ω, preferably the resistance would be logarithmic but if it is linear that will be fine, I can take care of the steps in the software.

I can answer any other questions you may have. Thank you in advance for your help.

 

[yjkwon57]

Hi~

I think you can solve the first problem simply. You will use an OP amp, a high-current NPN transistor, and a high-current PNP transistor, so as to make a current limit extended. The OP amp is used as a voltage buffer and to drive the two transistors, i.e., the output pin is connected to the bases of both transistors. The collectors of the NPN and PNP transistors are connected to +Vcc and GND, respectively. The emitters of the two transistors are connected with the negative input pin of the OP amp and used as the output. Of course, the voltage coming out of your computer is applied to the positive input pin of the OP amp. This technique is usually used in the computer-controlled measurement systems.

By the way, which one do you want in the second problem:
V = log I or I = log V?
I think a voltage-current converter will be required, i.e., the input voltage is sensed and and the corresponding amount of current is extracted from the input. If you want a logarithic scale conversion, you may use a diode with the threshold voltage elimated.

Bye~~

 

[Skier4Life]

Sorry it took me so long to reply. Just so you know, I am not an Electrical Engineer, I am a Computer Engineer. I have studied transistors and the like in the past but I wouldn't rate my skill very high.

I have wired up the current control circuit as you described and it works great for the 0 to 2 A range, but when I try to go from 0 to -2A it doesn't work. Am I missing something?

Also, I made a mistake when describing the resistance control circuit, it isn't logarithmic, it is linear. Just so you have a better idea of what I am trying to do, I am trying to control instrument panel gauges from the computer. The gauges are an Oil Pressure gauge and a temperature gauge, by varying resistance from the signal pin to the ground pin the needle in the gauge moves. The range of resistance needed is 40 Ohms to 250 Ohms for the oil gauge and 30 Ohms to 180 Ohms for the Temperature gauge. Having a slightly larger range then that would be preferred, but not required.

<Edit> I guess I should've made mention of this, the Voltage Current Converter you describe sounds like what I need. I don't think I have ever used this before and more details would be greatly appreciated. </Edit>

Thank you for your help so far.