Control Relay - using a Variable Resistor

[pradoartz]

I have created a relay based circuit.

My circuit consumes 12v DC 30amps load.

Please find the image attached below in that there is a "circuit i need" box. That is the exact circuit i need. Help me to build a professional Schematic for that circuit i need.
I already got a variable resistance(1.3k to 0.1k), the relay and the buzzer.




The point 86,87 in the relay needs 12v positive and negative supply to energize it, which in turn turns the connectivity ON to the point 30,85 and the buzzer beeps.

Exactly the variable resistor i used here is a "coolant temperature sensor".



It senses the heat and reflects it in resistance. When the temperature sensor senses the heat is too high then it actually supplies 0.3k ohms and more heat and more heat the resistance becomes lesser than 0.3k ohms.

Thanks

 

[d123]

Hi,

Might help to know the 85, 86 voltage and current ratings, and a part name or datasheet for the relay. 85 + 86 are the relay coil? 87 and 30 are COM and either NC or NO connection?

First thing that comes to mind is a comparator, or op amp configured as a comparator, to detect the 0.3K point (as I imagine that is a specific, repeatable voltage), followed by a BJT or MOSFET from the comparator output to the relay.

 

[pradoartz]

Hi,

Might help to know the 85, 86 voltage and current ratings, and a part name or datasheet for the relay. 85 + 86 are the relay coil? 87 and 30 are COM and either NC or NO connection?

First thing that comes to mind is a comparator, or op amp configured as a comparator, to detect the 0.3K point (as I imagine that is a specific, repeatable voltage), followed by a BJT or MOSFET from the comparator output to the relay.

Relay : Single pole, single throw, automotive type. Coil, 12V, 80 Ohms. Contacts, 30A minimum

 

[pradoartz]

The buzzer I mentioned is just an example. Actually it's a radiator fan. If I mention it as the radiator fan, and it will automatically gets switched on using ECU and blah blah, I am not able to get what I planned, instead of that I am getting purely brainwashed to go with some other circuit. So to make the plan simple I made a logarithm what I want.

 

[d123]

Hi, thanks, so the relay coil is 12V/80R = 150mA?

I guess the power supply for the variable resistor and "circuit I need" is 12V/?A, and is not going to be drawn from the same battery in the picture (a car battery?)?

Pure interest: how come the buzzer needs 30A?

- - - Updated - - -

Hi, this is a basic version of the general idea. There may be mistakes in the calculations or concept related to your circuit, so get a second opinion.

If you use a comparator like the LM339 (instead of an op amp as a comparator), it has an open collector output, so needs a pull-up resistor, and the circuit would need reconfiguring a bit, perhaps just by changing the NPN to a PNP.

Probably want to limit the op amp output swing so its output doesn't hit the supply rails.

Might find that a +12V supply is not quite enough for the relay, and it will chatter, and +13V or so may be necessary.

 

[FvM]

A comparator circuit like suggested in post #4 is basically the way to go.

Some necessary modifications:
- need hysteresis for the resistance threshold
- drive the transistor with sufficient base current to saturate the switch, e.g. Ic/Ib = 10..50
- should clarify how the resistance sensor has to be connected to achieve switch-off with rising resisatnce

A relay with 12 V rated coil voltage has usually a drop-in voltage of e.g. 7 - 10V, should be no problem to power it by a 12 V battery. Need to review the specification.

 

[pradoartz]

Already a friend from Somerset UK, had done a schematic for this project. Since that schematic components is too tough to find in my area, i am searching for circuit with basic components.

here is the link below.
https://www.electro-tech-online.com...fan-control-switch.149254/page-2#post-1276513

 

[d123]

Hi,

That's a nice circuit in the link you posted, certain to function correctly.

You could use an online component retailer to locate the parts and have them sent to you, I think that's a good way of shopping as you get a wide range and can compare suppliers and delivery costs, or try searching online for "equivalent part for ("xxxxxxx component", e.g. LM393N).

Limited experience tells me that - unfortunately - quick-fix circuits with less parts or bargain bucket-style component selection procedures are usually not as accurate/reliable as circuits that seem to have a lot of "unnecessary" components. If you can, I'd look at mouser, digikey, farnell, rs-online for the parts and have them sent.