Bit of help needed with some electronics

[Neeyik]

Electronics isn't my greatest talent and it doesn't help that it's such a minor part of UK secondary level physics these days but anyway, the problem is this: one of my students is using a thermistor-based potential divider circuit to switch a transistor that, in turn, activates a fan circuit at a predefined temperature. No problems there - it works exactly as intended - however, he wants a second fan to switch on at a higher temperature (doesn't matter whether the first fan stays on or not) and he can't get it to work for love nor money.

Can anybody offer some circuit suggestions for him to try? I thought that I had a solution but in practice, it just doesn't work well enough.

 

[AlNom]

Q: To what kind of transistors does he have access?

hm... so with the thermistor voltage divider... higher temp = higher resistance = higher fraction of input voltage. Was your initial solution to just hook up a second transistor with a higher Vthres in parallel with the first transistor?

edit: Q2: BJT?

 

[Neeyik]

Q: To what kind of transistors does he have access?

Standard range of low power npn ones, in TO39 cases.

hm... so with the thermistor voltage divider... higher temp = higher resistance = higher fraction of input voltage. Was your initial solution to just hook up a second transistor with a higher Vthres in parallel with the first transistor?

Actually higher temp = lower resistance for the thermistor, but the voltage is tapped off the resistor in series with it, so the voltage still rises with temperature. And yes, the 2nd transistor in parallel was my initial solution.

edit: Q2: BJT?

Que?

 

[Bludd]

He is asking if the transistor is a BJT (bipolar junction transistor).

 

[Neeyik]

Ah, gotcha - in that case, yes I am.

 

[AlNom]

Standard range of low power npn ones, in TO39 cases.
Actually higher temp = lower resistance for the thermistor, but the voltage is tapped off the resistor in series with it, so the voltage still rises with temperature. And yes, the 2nd transistor in parallel was my initial solution.

Had a slightly different image in my head, but I guess it works out...

With the second transistor in parallel, does the first fan still work? If so, maybe it just needs more voltage to activate? How about boosting the voltage to the second transistor: add a second resistor in series and hook up the second transistor to both while keeping the first transistor connected to a single resistor. Resistor values and input voltage will need to be adjusted.


edit:
The problem might be that the emitter sides are connected to one another while the fans are too and the current directions are getting mucked up. hm.....Sorry, just thinkin "aloud".

oh right, there are two ways of setting up the two resistor thing I mentioned. One is where the first transistor is hooked up to the "top/first" resistor, and the second transistor is hooked up to both. So here, the Bs are still connected to the same point but the E1 connected to end of first resistor, E2 connected to end of second resistor. The other way is to have both E connected to one another at the end of the second resistor while each B is connected to the respective resistor.. Or I might have completely lost touch with reality...

Que?

already answered

 

[_xxx_]

I'd use a PT100 or such temp sensor and use some microcontroller for measuring and switching. All the info you need can be found on the net, here's an example:

http://www.analog.com/UploadedFiles/Application_Notes/2001119207465975025AN709_0.pdf
http://www.vishay.com/resistors-discrete/ultra-high-precision/res-tol-less-pt005/

 

[Neeyik]

With the second transistor in parallel, does the first fan still work? If so, maybe it just needs more voltage to activate? How about boosting the voltage to the second transistor: add a second resistor in series and hook up the second transistor to both while keeping the first transistor connected to a single resistor. Resistor values and input voltage will need to be adjusted.

With the second transistor in parallel, neither of the fans work - combinations of increased voltages and base resistors didn't work either.

The problem might be that the emitter sides are connected to one another while the fans are too and the current directions are getting mucked up. hm.....Sorry, just thinkin "aloud".

That's a distinct possibility - I'll have a look at his circuit again.

oh right, there are two ways of setting up the two resistor thing I mentioned. One is where the first transistor is hooked up to the "top/first" resistor, and the second transistor is hooked up to both. So here, the Bs are still connected to the same point but the E1 connected to end of first resistor, E2 connected to end of second resistor. The other way is to have both E connected to one another at the end of the second resistor while each B is connected to the respective resistor.. Or I might have completely lost touch with reality...

If you hook the second transistor across both of the resistors in the potential divider circuit, then the voltage across it will be constant no matter what happens to the thermistor.

I'd use a PT100 or such temp sensor and use some microcontroller for measuring and switching.

This isn't an option unfortunately - he's limited to using a simple potential divider circuit with a basic transistor arrangement.

 

[_xxx_]

In that case, the simplest option would be to use two PTC's/NTC's/whatever, and have two potential dividers next to each other, each one triggering one fan. Kinda just doubling the whole circuit. Or put additional voltage dividers (different values) in front of the transistors, so they get triggered at a different measured voltage level.

 

[_xxx_]

If you hook the second transistor across both of the resistors in the potential divider circuit, then the voltage across it will be constant no matter what happens to the thermistor.

Add one more resistor (or even better a thermistor to achieve more variance) to that, hook one transistor on the first resistor, the other on the second and the third one goes to the supply. Thus both transistors will get voltage changes.

 

[Neeyik]

Thanks _xxx_ I though that would be the case but as I mentioned earlier, electronics isn't my strength at all, so I tend to doubt my own ideas quite frequently!

 

[Rainbow Man]

he's limited to using a simple potential divider circuit with a basic transistor arrangement.

Curious.. Why is he limited? Is it a school project or somesuch with set rules of what stuff to use or is he stuck on a deserted island somwehere and pigeon airfreight can't deliver him a microcontroller?

If first why isn't he doing the work himself and if the second what use are temp switching fans on a deserted island?

Peace.

 

[Neeyik]

It's an experiment for A-level physics coursework and he is doing the work himself, but as I pointed out in the first post, my intial ideas weren't working and I had doubts over my others.