Help me to design a comparator using LM339

Heyy.. can you give me some more info about the thermistor section? about setting the cutoff temperature and all?

Nikhil V said:

Heyy.. can you give me some more info about the thermistor section? about setting the cutoff temperature and all?

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Sure, take a look at Alec's drawing. The temperature section is comprised of U1D. Trim 2 (a 10K pot) determines the trip level by setting a reference voltage at the (-) Negative input of U1D. Alec used a 10K Thermistor as the temperature sensor. Thermistors have lookup tables where we can see the temperature curves. This way we know what the thermistor resistance will be for a given temperature. In the drawing the 10K thermistor is in series with R2 a fixed value 10K resistor so R2 and the thermistor form a voltage divider. The divider output labeled therm runs into the Positive (+) input of U1D. So when Vtherm exceeds the reference voltage set by Trim 2 the output transistor in U1D will be off and the output voltage of U1D will swing High as a result of R3 the pullup resistor.

So using a 10K thermistor you look at the temperature table for your thermistor. So for example if I use this thermistor from Spark Fun and look at the data sheet, which is this data sheet the thermistor resistance value at 25C or 77F will be 10,000 Ohms. So at 25C the divider output Vtherm will be 5 volts. At 100C the thermistor will have a resistance of 677.3 ohms. From here you can do the math but should have an idea of how the circuit works.

Alec designed this circuit so V+ is 12 Volts. Therefore if Rtherm is 677.3 Ohms we have a 10K fixed resistor in series with the thermistor 677.3 ohms so Rtotal is 10,677.3 ohms. The voltage drop across the thermistor is about 0.761 volt so if Trim 2 is adjusted for 0.761 volts at the Negative (-) input of U1D the system will trip at 100C or 212F.

Ron

Friends, i need some more help pls.. i found that in this circuit if both the three phases goes below the desired value the relay doesnt turn off, because as the all the three phases goes below a a certain value at the same time, the reference voltage for comparators will also be lowered. So i've decided to set reference voltage for the comparators from an external dc supply, like battery. Is that a feasible way? Suggestions please.. And also help me to give an LED indication for the faulty phase..

If you want a turn key solution a Google of "phase loss detection" will bring up a dozen hits of commercial products designed to do exactly what you want to do. Yes, LEDs could likely be connected to the comparator outputs but remember the comparators are open collector and require pullup. Yes, a battery scheme or power supply can be added to provide circuit DC power and provide the references.

Ron

I've heard horror stories where the phases were re-connected wrong and lots of stuff was affected. Solid state motor starters have a lot of the easy stuff built in.
How much time to wait before a trip and how much time to wait when reset may not be on a solid state starter. I was responsible in selecting protection for the expensive to repair equipment after we suffered damage.

KeepItSimpleStupid said:

I've heard horror stories where the phases were re-connected wrong and lots of stuff was affected. Solid state motor starters have a lot of the easy stuff built in.
How much time to wait before a trip and how much time to wait when reset may not be on a solid state starter. I was responsible in selecting protection for the expensive to repair equipment after we suffered damage.

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I've been to that Rodeo.
**broken link removed**. It works well and at about $200 USD is not difficult to replace. These are some of the pumps I worked with. They cost upwards of and exceeding $200,000 and they are difficult to replace. If the pump is connected with a phase reversal it will crash the impeller and a rebuild is about $100,000. Before I retired all of these pumps were placed on smart VFD drives which was a heck of a good move.

When I suggest things for anything industrial or commercial I gravitate to the higher end turn key systems. I guess it is in my nature.

Ron