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# DESPARATE help, but an easy task !!

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#### gurzyb

##### New Member
Hi

I'm making a really simple school-project, a circuit that makes hot temperatures turn a fan on and off.

I'm using a resistor, a thermistor, a relay and a transistor.

I managed to get it to work ages ago, but now it doesnt. I can only get a voltage when I connect it across BOTH resistors, not just one (Vout = Vin x R1/R1+R2)

any clues? I've only got a week!!!!!!!!!

You should get voltage across the lower resistor of potential divider. The problem may be temperature which is not enough to develop a potential that is enough to bias the transistor or the other reason may be that the thermistor is open/short or the other resistor is open. Check their continuiy.

continuity?

in the end i'm gonna use a transistor

at the moment, i'm trying just two FIXED resistors and a single battery cell.

i can just about power an LED from the HIGHER resistor of the two, but not a relay/buzzer or anything like that (even though they operate at 3v, and the DMM reports there being like 5volts across the resistor).

there's no use in using the transistor till i can get a voltage across the resistor see.........

For biasing a transistor you just need 0.7V between base and emitter.
I don't know how your circuit is. If possible please post it here so that it will be easy to debug. Also let us know what is the type of your thermistor. NTC or PTC?

That's the basic circuit diagram - really simple!

The problem I'm having at the moment is that the resistors don't provide the right output (ignore the transistor/relay bit whilst considering this). The DMM says there is like 4v/2v across each of the resistors, which is right, but they only seem to actually provide 1volt or less.

I think I'm using the wrong type of resistor, something to do with metal film/power

Also would you know the model number of the transistor I should be using? I have a the transistor doesnt work too well either...[/img]

yeah by the way, I'm using an NTC thermistor

at the moment i'm testing just a potential divider circuiit with two fixed resistors, and that's where the voltage output problem bit comes into play

any help would be appreciated !!!!!! also regarding the transistor model number

He he: There is no image on that URL. May be you have deleted it. Just load it and check my above post. It will display the image.

Ok check this circuit: You need to play with the resistor and pot values to get the desired trip point.

That's great, thanks

Just a few things: why is there an arrow between the thermistor and R2?

Also, wouldn't you just need ONE resistor and one thermistor? On your circuit diagram there are 2 resistors......

BTW thanks, I hope that's the right model number of transistor, because I'm gonna be buying it tomorrow

I have found two transistors, BC547C and BC546; not quite right

When I try my ciruit (similar to yours), the relay STAYS on until i disconnect the power supply. When I tried a buzzer, it turned off when the thermistor cooled though........weird

also the buzzer came on SLIGHTLY even when at room temperature. When i heated it, it just got louder

The resistor with an arrow is called "POTENTIOMETER" ("POT" in shortform). It is a variable resistor whose resistance can be changed by rotating the shaft. It is the same thing that controls the Volume on your cassette player. It has 3 terminals out of which two terminals have fixed resistance between them and the third (middle terminal) is the vriable one). So to get a variable resistance you short one fixed and the centre terminal so it becomes a 2-terminal variable resistor.
Here POT is used to adjust the sensitivity of the circuit so that the relay is turned on only when the desired temperature is reached.!

could you just tell me, what is that trianglular diode thing across the relay?

why is needed, what happens if it's not there?

That diode is called a "FREE WHEELING DIODE". It protects the transistor and other components being damaged due to very high voltage spike generated across the relay coil when its magnetic field drops suddenly (normally during switching). This spike has a voltage of the order of several kilo-volts which is enough to send the transistor to heaven . The diode equalises this high potential across the relay coil terminals.
You can use any general purpose medium current silicon diode like 1N4007.

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