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The Thermistor Circuit and Zener Diodes.

The Thermistor Circuit.

The thermistor circuit is complete! There are a few resistor values I want to check before I show all of of the values, but essentially the circuit is complete.  This circuit will do everything we described in the last thermistor post, “One More Step Closer to Completing the Thermistor Circuit”.  There are 3 adjustments in this circuit.  R3 and R4 summed together will be adjustable from 9000 to 11000 ohms.  This will permit calibration to match the 25 deg C specification for the thermistor.

The adjustment will be performed with the thermistor at 25 deg C.   The voltage at the output of the first op-amp is measured and then the thermistor is disconnected.  The voltage ideally will be exactly twice the voltage with the thermistor connected.   If the voltage is less than twice then R4 is adjusted to increase the voltage.  This is repeated until the twice is obtained.

To complete the final adjustments an ice water bath (0 deg C) and a 70 deg bath are created.   The thermistor is immersed in a bath and after about 2 minutes the correct adjustment is made.  Although the two adjustments should have very little effect on each other, each will have some effect on the other and it will be necessary to repeat the measurements several times.  The settings are summarized below:

R6  – Gain Adjustment – Use to adjust the 0 C value to 3.94 V out.
R10 – Offset Adjustment – Adjust the 70 deg C value to 0.220 V out.

The reason for the 2 minute wait is the thermal time constant of the thermistor is 20 seconds – so 5 X 20 = 100 seconds or a little over 1 1/2 minute for the thermistor to stabilize.

Parallel Resistor set to exactly the same value as the Thermistor 25 deg C resistance.

In the last post I calibrated the circuit so 5 V was produced at -20 C and 0 V at 70 deg C.  This produced the output shown to the left.  This time around I did two things differently.  The A-D converter is expecting 0 V at 80 deg C and 5 V at -30 deg C.  This provides some “head room” if things are not right so the output does not go negative or above 5 V.  This discarded less than 0.5 V of the A-D range.

The error curve with 0 deg Calibration

The other difference from last time was the low calibration point is at 0 deg C instead of-20 deg C. -20 deg C could probably be obtained by using an anti-freeze solution and dry-ice, but that would require more time. 0 deg C is very easy to obtain and obtain for a long time by filling a container with ice and then adding cold water. As long as ice is still in the container the temperature will be exactly 0 deg C. The ice water should be mixed to make sure there are no temperature gradients in the container.

A diode and a zener diode in a simple regulator circuit

In the circuit I have added one new component, a zener diode, and we need to discuss it.  A standard diode basically operates like a check valve in electrical circuits.  Current can flow in one direction but will not flow in the opposite direction.   The standard diode in the picture to the right will pass current if the bottom lead is more positive than the top lead. This is called forward bias and the arrow of the diode shows the direction that conventional current (+ to – current) will flow through the diode.

However, everything has limits. If the diode is reversed biased, there is a voltage where the diode “breaks down” and starts conducting.  This is called Peak Inverse Voltage on standard diodes and is often 1000 V or higher.  Zener diodes are designed to operate in this breakdown mode.  Z1 in the diagram has the + voltage so that current would want to flow in the direction opposite the arrow.  The interesting thing about a zener diode is that once it reaches breakdown voltage it will maintain that voltage and draw whatever current is necessary to maintain that voltage.  The power consumed by the diode would quickly be enough to burn out the diode if the current was allowed to climb too high.   The purpose of R1 in that circuit is to limit the current.

The balancing act is making sure there is enough current so the diode operates well and not too much current so the diode will burn out.  The best way to describe all this is to use a real example.   The zener I chose has a zener voltage of 5 V and a maximum power dissipation rating of 400 mW  The zener was tested with 20 mA and recommendations are it should draw at least 5 mA.  R1 will have a voltage across it of 15 – 5 or 10 V.    If R1 was approximately 2K then 10 /2000 is  5 mA.    The problem now is if we draw any more current through our load, R2, R1 would attempt to drop more voltage across it and the current through the zener would drop below the minimum value.  Now lets try 500 Ohm for R1.   This would give 20 mA flow through R1 for the 10V drop and if R2 is not connected this would all flow through the zener..  Now let’s assume R2 is 2K, then 5V/2K = 2.5 mA  through  R2 and  the zener would have to have 17.5 mA to maintain the 20 ma through R1.  This would work!   R1 would dissipate 10 X 0.02 or 0.2 watts and the zener would dissipate 5 X 0.02 or 100 mw with no load connected.  Again this would work!

The description of the operation of a zener was a little simplified, but not a lot.  A minimum current is required through the zener because it is somewhat unstable at very low currents.  The zener does have some internal resistance so the voltage across it changes slightly with the current through it, but this is often not enough to affect the performance.   In our case we will be having a constant load because the load is the fixed input impedance of the op-amp circuit.

This was a long  and somewhat detailed post,  It may be necessary to review some of the previous posts.  The category “Electricty”  should bring up all the past posts and refresh you on the theory.   If you have any questions or concerns please contact me via e-mail listed on the “contact me”  menu item on the home page or use the @ icon also on the home page.

I will be ordering an Ardruino this week and making sure the circuit will work with it before I release the final diagram with all the component values listed.   At that time we will discuss options for assembling the circuit.  Meanwhile I will start the discussion of Capacitors and Inductors because we will be needing those in future circuits.

We have come a long way since we started talking about electricity.  We are very close to having a fairly complicated circuit operational.

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