Help with LM358 thermistor circuit

Thread Starter

bigjoncoop

Joined Feb 1, 2019
125
I came across this circuit which is almost exactly what I was looking for. It's using 100k thermistor and a lm358 to control a relay.

heat-sensor-switch-using-lm358-ic.gif my question is how would I have to change this in order to control a mosfet instead of a relay and I need the circuit to turn off the mosfet after a certain temperature. I understand how to change the desire temperature but in this schematic it is turning on the relay.
It also says the circuit is good for up to 12 volts or as low as 5v. I would have to use this circuit for lower than 5v. Anywhere between 3v and 42v... would I be better off powering this circuit separately from my main project it would not be difficult to use this with any of those voltages?

Thx
 

Audioguru again

Joined Oct 21, 2019
1,785
1) You are using an opamp as a comparator. Most people would use a comparator.
2) The base-emitter of the transistor is shorting the output of the opamp since it is missing a series current-limiting resistor. Then the opamp and/or transistor could be damaged.
3) The circuit has a voltage gain of at least 15,000 times and could be 300,000 times which would cause the output to quickly switch on and off and on and off when the input voltage becomes close to the switching voltage. Then it needs a little amount of hysteresis (positive feedback) so that the on and off switching voltages are a little different.
4) The minimum supply for an LM358 dual opamp is 3V and for an LM393 dual comparator is 2V. But the output high voltage of an LM358 with a 3V supply is only +1.8V and most Mosfets need an input of 10V but some need only 4.5V.
5) It is easy to swap the turn on temperature for a turn off temperature by swapping the positions of the thermistor with the 100k resistor in series with it.
6) The important spec's for a Mosfet switch are the voltage and current it is switching. What are they?
 

Dodgydave

Joined Jun 22, 2012
9,322
I came across this circuit which is almost exactly what I was looking for. It's using 100k thermistor and a lm358 to control a relay.

View attachment 197816 my question is how would I have to change this in order to control a mosfet instead of a relay and I need the circuit to turn off the mosfet after a certain temperature. I understand how to change the desire temperature but in this schematic it is turning on the relay.
It also says the circuit is good for up to 12 volts or as low as 5v. I would have to use this circuit for lower than 5v. Anywhere between 3v and 42v... would I be better off powering this circuit separately from my main project it would not be difficult to use this with any of those voltages?

Thx
You're better using a comparator op amp like LM339 series, as you need Hysteresis to stop relay chatter,, here is one example of mosfet driving..a0yBQ.jpg
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
125
hey guys thanks for the quick reply.

@Audioguru again so it seems like you're saying that this is circuit that I found at circuit Digest is completely invalid and will not work at all... Correct?

And that circuit at circuit Digest whoever posted it explained that it would work from 5 volts to 12 volts.
http://www.circuitdiagram.org/heat-sensor-using-lm358-ic.html

I'm trying to use an LM358 since I have a ton of them. And I have a bunch of 100K NTC thermistors. Since I'm always buying so many different things for different projects I would like to use what I already have. When it comes to IC's like the LM358 I don't really have many other IC's besides voltage regulators but I do think I have a couple types of comparators or something of the sort...

So if it circuit will not work for what I'm trying to do could you guys suggest a fairly simple circuit to turn off a mosfet or cut the power going to the battery while charging...

again I'm trying to use what I already have. I don't know why the font is now different I didn't change it LOL


I'd rather not use an IC if I can help it. Since normally only pushing no more than a few amps of current if that at the most I was hoping I could do something simple like have the thermistor cut power to the gate of a mosfet if the temperature rises above a certain level. Is there a very simple way of doing this?

I've been charging lithium ion batteries without temperature sensing for a long time but it would be a good idea to add it but I use 10 different charging stations at any given moment and don't want to spend $10 for temperature control relay boards for each one if I already have a ton of spare components
 

Thread Starter

bigjoncoop

Joined Feb 1, 2019
125
Or can I still use the circuit in question with some minor tweaks? Also I don't mind providing the circuit a separate power source at a lower voltage then my main project
 

Audioguru again

Joined Oct 21, 2019
1,785
@Audioguru again so it seems like you're saying that this is circuit that I found at circuit Digest is completely invalid and will not work at all... Correct?
Correct. The circuit has the faults that I mentioned. It might work for a short time before its overloaded parts fail.
The website has other bad circuits (I saw a class-A audio amplifier with a high DC current in the speaker), maybe because it is a new website (it began in 2015) and maybe posts bad projects from students in its country.
 

Audioguru again

Joined Oct 21, 2019
1,785
I could not find the project with two transistors and a thermistor.
An NTC thermistor reduces its resistance as it gets warmer which will cause the transistors and the relay to turn on. Simply swap the positions of the thermistor and the trimpot P1 to turn off with heat.
Transistor T2 should have a resistor from base to emitter (use 22k) to turn it off when the first transistor turns off.
 

ci139

Joined Jul 11, 2016
1,684
transistors biased with low resistance value resistors are supply dependent in many ways especially the BE junction is ""constant"" 0.65V
so if the power floats your transistor thing gonna trigger differently

the op amp is a bit better because it would react to the relative (supply independent) difference of the 2 voltage dividers

to gain the result with transistors goes complex (the R2 is varying as a thermistor)
NTC - Draft - 1.png
 
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Thread Starter

bigjoncoop

Joined Feb 1, 2019
125
I could not find the project with two transistors and a thermistor.
An NTC thermistor reduces its resistance as it gets warmer which will cause the transistors and the relay to turn on. Simply swap the positions of the thermistor and the trimpot P1 to turn off with heat.
Transistor T2 should have a resistor from base to emitter (use 22k) to turn it off when the first transistor turns off.
Here's the link. Didn't realize it was a link the picture. This is the correct one...

https://www.electroschematics.com/temperature-relay-circuit/

So you're saying that schematic will work for what I'm trying to do? I just need to flip flop those two components and add that resistor? If that's the case what is the maximum and minimum voltage I could use for the circuit? I know it shows 10v. And I'm assuming that I should power this is circuit independently of my charging circuit for whatever battery I'm charging since it will be CC / CV. And at that link to that schematic and the info it does say exactly what you'd mentioned about switching those two components in order to have the reverse effect. Okay so I'm going to try that circuit when I get home in a little while from work. I will let you know how it goes thank you

And

@ci139

So you're saying I would be better off using lm358? And that schematic does look pretty involved. Not too bad but... and R2 would be the thermistor correct?
 

Alec_t

Joined Sep 17, 2013
11,549
I would have to use this circuit for lower than 5v. Anywhere between 3v and 42v...
That could be a problem if you want to power the circuit from the same supply that you are monitoring. It would be better to power the circuit with an auxiliary fixed supply voltage of, say, 12V so that you have a good choice of op-amps/comparators/mosfets/relays.
 
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