Help me to design a comparator using LM339

[Nikhil V]

Hii, i am now working on my mini-project "Induction motor protection". For that i need four comparator circuit driving four 12V relays, in which 3 of them are for protection against single phasing and 1 for over temperature protection. i am using a 7805 regulator IC so that i can get constant 5V for driving comparator IC LM339. Can anybody help me to design the comparator section,please? Thanks in advance.

 

[audioguru]

A comparator is simply an on-off switch that compares the voltages on its two inputs.
The datasheet for the LM339 shows that it is low power so its minimum output current is only 6mA with a huge voltage loss of up to 1.5V so it might not be able to drive a relay.
Its datasheet shows that its inputs do not work if their voltage is within 1.5V from its positive supply voltage.

 

[alec_t]

What are the parameters of the four input signals to be monitored by the comparators?

 

[Nikhil V]

3 comparators should compare 12V output from three bridge rectifiers, and remaining comparator should monitor the voltage corresponding to temperature ( from thermistor). In first 3 comparators, the output should turn off when the corresponding rectified input voltage goes below a certain value, and the remaining comparator output should be turned off when temperature goes above a particular value.
I am also using a regulator IC, which is fed from any of the three rectifiers and gives a constant 5V output. The each comparator output should be also able to drive a 12v relay. So in my circuit there are four 12V relays.
Is there any means like using transistor for driving the relay? pls help me

 

[alec_t]

Do you really need four relays?
How about this as a slight variation on the theme.
The four comparators are connected such that if any one phase fails (volts drop below a percentage set by Trim1), or the temperature goes over limit (set by Trim2), the transistor Q1 turns on to activate the relay. Note there is no voltage regulator; the system is ratiometric, i.e. the reference voltages are fixed fractions of the supply voltage. Simulation shows that the temperature trip point varies by only ~0.2C with a supply voltage change from 9V to 12V.

 

[Nikhil V]

Yeah, i need four 12v relays and also a main 3 phase relay, which should interrupt the supply to the 3 phase motor whenever any one of the small relay gets turned off. Can you please help me on that design?

 

[alec_t]

What is the purpose of the four 12V relays? The single relay I showed could be used to control a 3-phase contactor to switch off the motor when any one of the four comparator outputs goes low.

 

[Nikhil V]

Ok.. I need some more clarification please. From where should we take the +V dc voltage for Vcc and biasing for LM339? Does it need a regulator for that? Does each comparator of LM339 need a pull up resistor to obtain the output? it is missing on this circuit i guess

 

[Nikhil V]

One more please, in the circuit shown by you the four comparators are connected such that if any one phase fails (volts drop below a percentage set by Trim1), or the temperature goes over limit, the transistor Q1 turns on to activate the relay. I want to get the relay activated all the time when circuit runs at normal condition and relay should turn off whenever any fault occurs, thereby isolating the 3 phase supply from motor. Is that possible to do so?

 

[alec_t]

From where should we take the +V dc voltage for Vcc and biasing for LM339?

It's shown as V+ in my schematic and is derived by using diodes D5/D10/D15 to "OR" the 12V outputs of the three rectifier bridges (shown in the three dotted-line rectangles), so that as long as one or more phases is active there will be ~12V across C1. I have assumed transformers are used to drive the bridges. You may have other ideas as to how to get ~12VDC voltages from the three phases.

Does it need a regulator for that?

I don't see the need for one. As I said, even a 9V-12V variation in V+ gives rise to little variation in the trip point.

Does each comparator of LM339 need a pull up resistor

No. They are wire-ORed and share a single pull-up R3.

I want to get the relay activated all the time when circuit runs at normal condition

That wastes power IMO, but is doable. I'll revise the schematic to show how.

Here you go:

 

[Nikhil V]

Thank you so much

 

[Nikhil V]

i have one more doubt pls.. will this circuit turns off the relay when any one of the 3 phase voltage goes below the preset value?

 

[KeepItSimpleStupid]

Here's a product matrix of a brand I used http://www.symcom.com/products.php?cat=63 Note the options,

I picked a model (it ended up slightly custom) that one could select the voltage and the re-start time was selectable. The re-start time varied the start time of some of the motors. The condition had to exist for a while, but I think that was fixed. I never replaced one. One got upgraded to to an electronic starter. Some options, but not all were available. The elctronic starter was more concerned about current.

 

[Reloadron]

i have one more doubt pls.. will this circuit turns off the relay when any one of the 3 phase voltage goes below the preset value?

**broken link removed** provides a very good write up on the LM339 comparator family. I suggest you read it and understand how the circuit Alec was kind enough to provide works, after all this is your mini project. Look at Alec's circuit closely and note the Vref (reference voltage) for U1 A, B, C and D is applied to the (-) Negative inputs. The open collector output transistors of the comparators will be Off when the (+) Positive input voltages exceed Vref. Resistor R3 serves as a pull up resistor for all 4 comparator outputs and will hold M1 turned on and the relay energized as you requested. If the (+) Positive input voltage to any of the 4 comparators drops below Vref the output of that comparator will go low as its open collector output transistor turns on. That low from any of the 4 comparators will turn off M1 and the relay. If any phase voltage drops below Vref or if the temperature exceeds Vref the normally energized relay will drop out. The circuit as drawn per Alec does require 3 transformers to reduce the phase voltage so when the phase voltage is correct the transformers output voltages will be about 9 volts. If this is a schoolwork assignment I would have to give Alec an A+. Keep in mind circuits like this will shut down a motor on a phase failure but will not indicate which phase failed.

For industrial applications, aside from mini learning projects there are simple solutions like the one KISS mentions and linked to. I use The Macromatic Line as an example. Typically around $80 USD.

Ron

 

[Reloadron]

Removed my double post. Sorry.

Ron

 

[alec_t]

will this circuit turns off the relay when any one of the 3 phase voltage goes below the preset value?

Yes. Ron has explained it nicely. If you wanted, the circuit clould be modified to give an LED indication of which phase is low/absent. If this is indeed a school/college project then I leave it to you to work out how that can be done. For safety's sake you will also need to consider how to prevent the motor suddenly starting up unexpectedly once the relay has dropped out and the weak/absent phase returns to normal.

 

[Nikhil V]

Thank you Ron and Alec.. thank you so much for the help.. Sorry for my limited knowledge

 

[Nikhil V]

Can you suggest me a free circuit simulation software to simulate this circuit?

 

[alec_t]

LTspice (a free download from Linear Technology) is what many in this forum use. It has a fairly steep learning curve, but worth the effort.
The Yahoo LTspice User Group has free downloads of libraries of lots of third-party component models which can be added to those that come in the standard LTS download.

 

[Reloadron]

Per Alec, LTspice IV Download Page. Free and there also plenty of free online tutorials. The Yahoo User Group is also very good and helpful.

Ron