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build a thermostat

AMIRAAM

New Member
Hello
I don't have an electrical or electronics engineering but i know some basics about electronic

I'm trying to build a thermostat please help
i want to add capacitors on this diagram, can you tell me where and why please
 

Attachments

gophert

Well-Known Member
Most Helpful Member
Hello
I don't have an electrical or electronics engineering but i know some basics about electronic

I'm trying to build a thermostat please help
i want to add capacitors on this diagram, can you tell me where and why please
hi, could you tell me why you want to add additional capacitors?

Without more information, I can only suggest that you print out the schematic tape some colorful capacitors to each corner.
 

dr pepper

Well-Known Member
Most Helpful Member
The parts in the circle are resistors, are these what you mean?
 

DrG

Active Member
i want to add capacitors on this diagram, can you tell me where and why please
That sounds a little difficult to respond to because you are literally saying that you want to add something and you are asking how and why.

So, I am looking at the circuit and it looks simple enough for me to understand. You have a comparator with two voltage dividers as inputs. One input is dependent on a thermistor's resistence (i.e., the temperature) and the other is dependent on the setting of a pot. When triggered, the comparator operates a relay that turns on the heating element (the resisters inside the circle. There are also a couple of LEDs

You have three capacitors on the power line.

Others here will know more, but there are only two places that I see: One is at the comparator Vcc to ground (again, you already have three smoothing the power supply. The other is across the relay coil. You have a diode there, which is appropriate but I have seen RC snubbers used as well (search terms - RC Snubber relay noise suppression).

Maybe I am missing something, but that is what I see.

You will get a better response if you provide more information.....where did that schematic come from? Why do you think you need to add capacitors? Homework assignment or what?
 

AMIRAAM

New Member
hellow gophert, dr pepper , DrG

thank you for help


hi, could you tell me why you want to add additional capacitors?
Without more information, I can only suggest that you print out the schematic tape some colorful capacitors to each corner.
i should say : I want to add or remove capacitors on this diagram, can you tell me where and why please !?

like dr pepper said :
The thermostat is for the egg Incubator; a comparator lm339 with two voltage dividers as inputs. One input is dependent on a thermostat’s sensor (thermistance NTC for 37 °C) and the other is dependent on the setting of a pot (reference for 37 °C). When triggered, the comparator operates a relay that turns on the heater resistor


he parts in the circle are resistors, are these what you mean?
yes it is heater resistor

You will get a better response if you provide more information.....where did that schematic come from? Why do you think you need to add capacitors? Homework assignment or what?

I don't have an electrical or electronics engineering but i know some basics about electronic, while i am searhing on google i found a lot of schemas , they add two type of capacitors : the polarized and non-polarized capacitor
can i
replace a polarized capacitor with non-polarized capacitor !?

please tell me why you add each capacitor and why and also their values are they correct or no !?
 

Attachments

dr pepper

Well-Known Member
Most Helpful Member
The capacitors on the drawing are all 100uF, and they all connect to the same point, this might seem confusing.
It is however where they are positioned that is important, each capacitor decouples a section of the circuit, and should be placed close to that section of the circuit.
This is because of the pcb tracks & wiring resistance affecting the circuit.

You can replace a electro with a non electro so long as the voltage ratings etc are the same, although for 100uF a non electro would be very large.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The capacitors on the drawing are all 100uF, and they all connect to the same point, this might seem confusing.
It is however where they are positioned that is important, each capacitor decouples a section of the circuit, and should be placed close to that section of the circuit.
This is because of the pcb tracks & wiring resistance affecting the circuit.

You can replace a electro with a non electro so long as the voltage ratings etc are the same, although for 100uF a non electro would be very large.
And non-existent :D

If we're really been picky on the circuit, it certainly doesn't need four 100uF on the supply - I would suggest a 100uF on the incoming power, and a 1uF across the comparator, and leave the others out. The circuit presumably wasn't designed by anyone who knew what they were doing, and used pointless overkill on the decoupling.
 

AMIRAAM

New Member
hellow
thank you for help

i modfied the diagram ,please see it

another thing :
The led2 is on when the heating system is on, I want to add a led 1 to be on when the heating system and the led1 are off. Please how can I do that?



THERMOSTA05.png
 
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alec_t

Well-Known Member
Most Helpful Member
Add the LED1, with a 1k series resistor, between the relay's NC pin and GND.
 

alec_t

Well-Known Member
Most Helpful Member
You could modify the right-hand side of the circuit like this:
TwoLEDs.png
Note: I haven't shown the capacitors. The LED colours are arbitrary.
 

AMIRAAM

New Member
THERMOSTA05.pngthank you very much , I'm so happy , I had taken a long time searching for this sollution
so i modified the diagram as you suggest :THERMOSTA06.png
i want to give me your opinion for another thing please :
in my old diagram there was a voltage divider like is shown (red cercle) , i want to keep it if this is possible :

THERMOSTA07.png
 
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alec_t

Well-Known Member
Most Helpful Member
That should work.
Why the voltage divider?
 

AMIRAAM

New Member
Hello lec_t

thank you very mutch

Why the voltage divider?
i want to replace the PNP Tr by NPN Tr (red cercle)
i don't know , i think that it would be better with the NPN , also by the voltage divider ,i can have good control for the NPN Tr ! ?

THERMOSTA08.png

but please i want from you to give me another way to do this :
on my old diagram the hysteresis was done by big resistor (red ligne)
please are there any other method for this (for example a voltage divider ) because i don't like to use a this big resistor

THERMOSTA09.png
 

gophert

Well-Known Member
Most Helpful Member
Hello lec_t

thank you very mutch



i want to replace the PNP Tr by NPN Tr (red cercle)
i don't know , i think that it would be better with the NPN , also by the voltage divider ,i can have good control for the NPN Tr ! ?

View attachment 122463

but please i want from you to give me another way to do this :
on my old diagram the hysteresis was done by big resistor (red ligne)
please are there any other method for this (for example a voltage divider ) because i don't like to use a this big resistor

View attachment 122462
What do you mean "big"? High value resistors are generally very small in physical size (or the same physical size as any other resistor) because very few amps can get through them so very little wattage. I've never heard of anyone being concerned with using high-value resistors when a high value resistor is perfectly suited (needed) for the circuit.
 

alec_t

Well-Known Member
Most Helpful Member
For the above circuit with the NPN transistor:
1) The R8/R6 potential divider will prevent the NPN transistor from switching off. You can replace R6 by a short-circuit.
2) Depending on how much current your relay coil draws, R8=10k is probably too high a value. I'd suggest no more than 2.7k.
3) Without hysteresis the relay will probably 'chatter'.
4) Going from a PNP to an NPN means the switching logic is reversed. To compensate, you can connect the heating element to the relay's NC contact instead of the NO contact.

i don't like to use a this big resistor
The 'big' resistor is the same physical size as the other resistors. What is your objection to its use? This is the usual and simplest way to provide hysteresis.
 

AMIRAAM

New Member
hello
thank you again
i will keep the diagram as you suggest with the PNP :


TwoLEDs.png


but please another small question :
why the voltage divider is not good way : ( i will keep the PNP)


THERMOSTA11.png




THERMOSTA10.png
 

alec_t

Well-Known Member
Most Helpful Member
You seem to be obsessed with voltage-dividers :). Yes, your latest PNP versions will work with a voltage-divider, but what advantage do you think it provides? I see it as an unecessary complication rather than an advantage.
 

gophert

Well-Known Member
Most Helpful Member
You seem to be obsessed with voltage-dividers :). Yes, your latest PNP versions will work with a voltage-divider, but what advantage do you think it provides? I see it as an unecessary complication rather than an advantage.
The professor said his design must have at least 10 components? (Just guessing)
 

AMIRAAM

New Member
hello
thank you again

I know two or three things about electronic, the voltage divider is one of these things :cool:

A natural science teacher gives his students two paragraphs, one about the elephant and the other about Earthworm, the next day the teacher asks one of his student, what is an elephant, the student said: the elephant is a big animal, it has a nose like the Earthworm and the Earthworm is small animal, it has ......... and he completing by the paragraph about the Earthworm because he read only the paragraph about the Earthworm

That’s why I seem to be obsessed with voltage-dividers ;)
thank you very much
 

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