Relay Problem

[Allycat]

Ive built this circuit to keep wine at a constant temperature while fermenting. The problem Ive got is the relay, which is switching 240v chatters when turning off. There`s a nice clean click when turning on. Can anyone suggest how to correct this?

 

[GonzoEngineer]

Your circuit is not complete. What are you doing with the + Input to the op amp?

Google "Comparator Circuits" and use a Comparator with hysteresis.

 

[Allycat]

Thanks, will look it up.

 

[Allycat]

Your circuit is not complete. What are you doing with the + Input to the op amp?

Google "Comparator Circuits" and use a Comparator with hysteresis.

Thanks, will look it up.

Looked it up, fried my brain. any chance you could alter my circuit and explain what all the resistors are doing, this would be a great help to me.

 

[duke37]

You will need two resistors in series from the 12V to ground. The junction should be connected to the + terminal. A couple of 10k resistors would probably do.

The junction would be at 6V. The voltage of the junction of the sensing circuit will be above or below 6V so the amplifier will switch to high or low.

In order to get a strong switch without dither, hysteresis can be added. This could consist of a 1M resistor from amp output to the + pin. This gives rapid switching and a dead zone.

A fried brain should not stop you posting the entire circuit.

 

[Allycat]

You will need two resistors in series from the 12V to ground. The junction should be connected to the + terminal. A couple of 10k resistors would probably do.

The junction would be at 6V. The voltage of the junction of the sensing circuit will be above or below 6V so the amplifier will switch to high or low.

In order to get a strong switch without dither, hysteresis can be added. This could consist of a 1M resistor from amp output to the + pin. This gives rapid switching and a dead zone.

A fried brain should not stop you posting the entire circuit.

I managed to get bo this stage before I read your postDoes exactly as i want in simulation, so looks like im on the right track. will add the 1M resistor and hopefully good to go. will let you know how I get on. Thanks for your help so far.

 

[duke37]

I do not see how this will work, You need to connect the thermistor and potentiometer as in your original circuit.

 

[Allycat]

Hi again Duke, Ive changed the circuit but with the 1M fitted
either R6 or and the transistor blow?????????????
BTW do you use circuit wizard/

 

[Arouse1973]

Remove the 100R and connect the collector to +V. You need two potential dividers connected from +V to 0V as Duke mentioned and one will contain your NTC connected to 0V. Centre tap them to the inputs of the Op-amp as in your first diagram. I don't know how well the hysteresis resistor will work with the resistor(NTC), The hysteresis resistor is supposed to modify the lower resistor by being place across this when the out put is low. As the NTC changes in value, the hysteresis value will also change. But it might be ok.

Adam

 

[Merlin3189]

May I just ask, what the relay is switching on?
As I see it, the thermistor has a negative temperature coefft. So when it gets warmer, the resistance gets lower and the Voltage at the - input goes lower making the output go high and switch on the relay. So a higher temperature switches on the relay.
So the relay does not power a heater. Maybe it powers a fan or refrigerator?

 

[duke37]

I have never met circuit wizard.

The relay driver circuit is not the usual one. Usually, the transistor emitter is connected to ground. A resistor is added to control the base current from the 741. The relay is connected between collector and positive supply. A diode must be placed across the relay to suppress high inductive voltages on switch off.

Tell us the specification of the relay (coil voltage and current) and of the transistor (maximum voltage, maximum current and gain).

If the operation is the reverse of what you want, then interchange the thermistor with the variable resistor.

 

[Allycat]

Thanks Adam, My previous circuit diagram shows this config i think.

 

[Allycat]

Hi Merlin. The relay is configured as normally open so switches off when temp rises.

 

[Allycat]

I have never met circuit wizard.

The relay driver circuit is not the usual one. Usually, the transistor emitter is connected to ground. A resistor is added to control the base current from the 741. The relay is connected between collector and positive supply. A diode must be placed across the relay to suppress high inductive voltages on switch off.

Tell us the specification of the relay (coil voltage and current) and of the transistor (maximum voltage, maximum current and gain).

If the operation is the reverse of what you want, then interchange the thermistor with the variable resistor.

Hi Duke. Heres the coil & transistor specs http://www.rapidonline.com/pdf/60-4680.pdf - http://www.alldatasheet.com/datasheet-pdf/pdf/22012/STMICROELECTRONICS/BFY51.html

 

[duke37]

You could look up the data to save us the trouble.
The relay data you refer to has a host of different coils so it is not clear what the requirements are. Assuming the coil is for 12V and run on 12V, the current will be 37.5mA. With a bit of voltage drop across the transistor it would be OK to use 35mA for calculation.

My computer threw a wobbly when trying to get the BFY51 data so I looked it up in my data book.
30V 1A beta 40
so, to get 35mA collector current, you will need 35/40mA base current. double this to ensure that the transistor is fully on so the base current will be 35/20ma = 1.75mA

A resistor from the 741 will need to supply this current. There will be about 10V available so the resistor will need to be 10/1.75m = 5.7k. Use 5.6k as a preferred value.

The BFY51 emitter should be connected to ground and the relay coil should be connected between collector and 12V.
Remember that you must have a diode across the relay coil or you will damage the transistor.

 

[Allycat]

Simplified version, what do you think?

 

[duke37]

That is rubbish.

 

[Allycat]

That is rubbish.

What would be your choice of doing this Duke?

 

[(*steve*)]

I'd go for the one in this post, except I would take the advice from this post, and this post, and this post, and of course this post.

If there is anything you don't understand about the advice, ask.

 

[duke37]

OK, this is what I suggest. Please tell us how you think the various parts of the circuit works. There is little point in copying without understanding.
Start with the bridge, what does the 741 do, why put a base resistance in, why connect the transistor this way. How does the feedback work.

Even I am not perfect! Will need to charge my brain cell.

The feedback resistor should be connected to the + input of the 741
The transistor should be BFY51