Is it safe to use PC analog output card as current source?

Question

I have a very PCI analog output card which could output a voltage ranged from 0 to 5V. I need a circuit to output a current ranged from 10mA to 90mA. The load is about 45 Ohm.

So is it safe to connect directly the analog output to the load and change the voltage until we get the desire current? Will it burn the card if it is not correctly connected?

I am sorry that I don't have much experience in circuit building. But I read other article about using op-amp to control the output current by input voltage.

^{simulate this circuit – Schematic created using CircuitLab}

I wonder if the above schematic is a correct setup to apply a current to a load? What's the advantage to the op amp for this purpose?

Answer 1

Your circuit makes no sense - feedback resistor connected to some point half way along the op-amp is the problem but, even if it were connected to the output this would be a voltage buffer. If you want a ground referenced current source consider this: -

The voltage across R1 dictates the current thru the load (shown as a box with "current out" written in the middle. However, it is a little inconvenient to have current out controlled by the voltage across R1 and so people tend to make a positive rails referenced current source that feeds R1, then your input voltage (ground referenced) controls a ground referenced load with respect to current.

The circuit directly above controls a current from a ground referenced control voltage. Where it says Iload, this feds into R1 in the upper circuit and R3 is discarded.

There are plenty of different ideas for current sources as can be found here.

Answer 2

Your schematic shows a "buffer" circuit, the opamp just copies the input voltage at it's + input to the output, provided it can drive the output to this voltage and provided the output is loaded such that it can deliver the current.

If you would not use such a "buffer" circuit, all the current to the load would have to come from the analog PCI card in the PC. These are usually not designed to deliver much current, 10 mA will usually be problematic already. So you need something to fix that, this is called a buffer.

You do not need the 200 ohms R1 resistor, you can connect the - input directly to the output of the opamp.

You say you want to control the current but actually this circuit controls the voltage ! Now if the load is constant, for example 45 ohms then this makes no difference as I = V / R. If this is OK for you, meaning that the load will always be 45 ohms, then this circuit is fine.

BUT 90 mA will be too much for many opamps, however there are some power opamps that can handle this you will have to look at the opamp's datasheet. You can also let the opamp control a small power transistor so that the power transistor does all the hard work. That would look like:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Technically speaking your circuit is fine. You've build what is essentially a voltage follower, i.e. a unity gain amplifier. I don;t know why you have a 200E resistance in the feedback; using a wire would work the same. Your PCI card sees a very high impedance and does not source a lot of current. The op-amp will source the current to the load, and the voltage across the 45E Load will equal the PCI card output.

In reality, you need to take care of: 1. Isolating the power supply of the op-amp from the PC power supply, at a minimum. 2. Look at input impedance of the op-amp, output impedance of your PCI card and see how they match up.