Draw an op amp circuit with explination

[Myrddin]

In a pirani vacuum gauge the pressure is determined by the rate at which heat is conducted away from a resistance wire by gas moleculs. The heat is supplied using a constant current source and the rate of heat dissipation determined by the temperature induced change in the resistance of the wire.

Draw an op amp circuit that can be used to deliver a constant current to an external fixed load resistor. Explain how it works in detail.

Have very basic knowledge of op amp circuits and this question has thrown me, any help will be appreciated.

 

[vk6kro]

[PLAIN]http://dl.dropbox.com/u/4222062/current%20regulator.PNG

This is a current regulator.
The diode D1 is a 4.7 volt zener (fed by R2) and R1 is the load which we want 100 mA to flow in.

The opamp will produce base current into Q1 which is just enough to produce 4.7 volts across R1 as this will then produce equal voltages on its inputs.

It works well, but it has a disadvantage.
The actual current depends on R1 staying constant, so, if it varies, so will the current.

So, the current is regulated against variations in input voltage but not for variations in the load.

 

[skeptic2]

Before designing the circuit let's think about the requirements a little. The resistance of the wire changes with temperature and the stronger the vacuum the hotter the wire will get with the same current because less heat will be carried away by the air. If the measurement were always to be made at the same air temperature the circuit would be fairly easy. But it seems you will need compensate for the air temperature. To do that you will need two identical lengths of resistance wire with one inside the vacuum and one outside. Then you pass the same current through wires by connecting them in series and compare the voltages across them.

Think about how you might measure the voltage across a component that is not referenced to ground. Also about how two such voltages may be compared to each other and about how that difference might indicate the amount of vacuum.

You might want to search for some opamp circuits in a subtraction configuration.

 

[vk6kro]

Draw an op amp circuit that can be used to deliver a constant current to an external fixed load resistor. Explain how it works in detail.

"external fixed load resistor" means that the load does not change.

 

[skeptic2]

You are correct vk6kro, that's all that was asked for. I made the assumption, perhaps unwarranted, the the circuit should be accurate at more than one temperature and further that it might be manufacturable in that multiple units might be accurate with little or no calibration.

Using a zener with a negative temperature coefficient as a reference for an RTD with a positive temperature coefficient would not be accurate over a very wide temperature range. Furthermore there is enough variation between zeners to require that every unit be calibrated.

On the other hand if an identical unit is used as a reference, the only difference being that one is inside the vacuum chamber and the other is outside, the difference between the two will mostly be due to the difference in air pressure.

Note: there still will be some inaccuracies with the method I suggested. First the change in resistance with respect to temperature is proportional to the temperature in deg. Kelvin so a smaller difference will be seen at colder temperatures.

Second at harder and harder vacuums more of the heat loss will be from radiation instead of conduction or convection and black body radiation is non-linear with respect to temperature.

 

[vk6kro]

No worries.

It would be fun to explore this a little, but maybe not in the Homework section.

That circuit was submitted by Bob S some months ago.
I couldn't find his post about it, but I did have a LTSpice model I made of it. So, I dug that out and decided it would be educational at least to re-post it with a caution.
It actually models quite well.

At least it is easy to explain and it does answer the question, yet the application really needs something better, as you point out.

In my model, I substituted an LT1021-5 precision reference for the Zener, and that does give an exact 5 volts out. I don't like Zeners much, mostly because they generate a lot of noise.

 

[Myrddin]

Thanks for the circuit chaps, and a lot of the jargon you used allowed me to find more info online x