school project help

[teimes]

Hi everyone, Me and a friend are doing a school project about subzero refrigeration and are building everything from scratch and we wondered if anyone here could help us with a circuit that we need?
we have 0-12V DC signal in, and a load that takes 0-12V DC 8A, and a PSU that provides either 12v or 5v DC 100W 

[lapm]

And your problem you needed help was?

[CrashO]

And your problem you needed help was?

I think he wants us to do his homework and design everything needed to go from a 0-12V signal to refrigeration 

[dan3460]

I think you need to give us more information. Is the signal a square wave from 0v to 12V? how is this signal been controlled? I'm guessing the load is the refrigeration unit.

[teimes]

Funy rightCrashO?
But we said sadly nothing about homework just a school project for inpres our teacher, All we wondered was how it works and which components we should choose

[teimes]

ohh sorry dan3460, it varies between 1v and 12v equally on both in and out putt's

[Simon]

Funy rightCrashO?
But we said sadly nothing about homework just a school project for inpres our teacher, All we wondered was how it works and which components we should choose

the mention of school usually implies a task set by a teacher. If you give a bit more background it helps.

Hint: Don't include any form of number display that will be shown without warning, and no loose wires hanging around especially red ones both can make people nervous......

[teimes]

it is not a task we have been given by our teacher, we are two who like electronics and building and challenge ourselves.

we have no circuit yet, because we have not learned anything about this type of circuit yet and therefore we are here for geting help

there is no display on it and the only cables coming out of it is that seen on the "drawing", we have a signal that varies between 1V and 12V as its input, and the output is a load on 8A and have the samma output voltage ass the input signal, so we just wonder how we increase the current in the signal coming in so it be able to drive the load. 

[Simon]

What do these "signals" represent. You need to explain what parts you have so far and how they work so we can understand what you are doing.

[teimes]

I have a signal that varies between 1v to 12v, ie 1v, 2v, 3v, 4v, 5v, 6v, 7v, 8v, 9v, 10v, 11V, 12V, but the amp is so small that it can not drive the load in the other end so I have to increase the amp, so when the signal in is at 3 Voltage so is the utput on 3 Voltage 8A for example

[Simon]

Where does the signal come from, what does it represent ? You could use a buffer amplifier like a power opamp with a gain of 1 but 8A is a lot and this sounds like the wrong way to do it, you can also use a power supply of some sort and vary it's output in rsponse to an input.

[teimes]

"you can also use a power supply of some sort and vary it's output in rsponse to an input." how? 

[Zero999]

If all you need to do is vary the speed of the fan, depending on the input voltage, then you could use PWM (Pulse Width Modulation). There are plenty of ways to implement this.

http://electronicdesign.com/power/pulse-width-modulator-features-versatile-operating-parameters
http://www.almico.com/sfarticle.php?id=1

[teimes]

the fan is controlled by the temperature gauge, and we thought it could also be Controlled load that needs 8 amp's but the fan controler has a max utput of 2 amp's

[IanB]

I think from your comments so far you are trying to take on something beyond your current level of skills and knowledge.

You have a concept, which is good, but you don't yet have the skills to make it real.

You should consider taking on something less ambitious that you can build with knowledge you already possess.

[Simon]

To control "refrigeration power" using a peltier cell (I assume you are using a peltier) you can PWM the peltier itself, or you can use a thermostat circuit. Look up "comparator" and "Thermistor"

[teimes]

I think from your comments so far you are trying to take on something beyond your current level of skills and knowledge.

You have a concept, which is good, but you don't yet have the skills to make it real.

You should consider taking on something less ambitious that you can build with knowledge you already possess.

that's why wi want to learn it

[teimes]

To control "refrigeration power" using a peltier cell (I assume you are using a peltier) you can PWM the peltier itself, or you can use a thermostat circuit. Look up "comparator" and "Thermistor"

yes we are, we have a thermostat that is connected to the fan but it gives only 2amp
We thought that we should let the thermostat Control the temperature in it, so it does not risk geting to warm but also that it cools down the space quickly

[IanB]

that's why wi want to learn it

I understand. But there is a difference between trying to learn "everything" all at once, and learning particular details as you go.

This is why you cannot go to college and learn how to be an engineer in just one year. There is just too much to learn.

In this thread people have been giving you good answers, like "use a power supply with a control input", but you do not understand how to apply that answer. This puts you in the position of someone with a plate of nutritious food, but you cannot digest the food.

So, I do not want to discourage you, but I do think you are being too ambitious for your level of knowledge, and you need to begin with something simpler.

[tooki]

The problem is that we have no context. To provide any kind of useful answers here, we need to know a ton more about the concept, how it needs to work, etc.

[bitslice]

The power levels are the sticking point here, if you wanted to vary a 12v@1a load, then that is easy.
To vary a 100W load, not so easy.

And that fan is going to stop working at <6v

[Ian.M]

To control "refrigeration power" using a peltier cell (I assume you are using a peltier) you can PWM the peltier itself, or you can use a thermostat circuit. Look up "comparator" and "Thermistor"

Pelter heatpumps don't do well with direct PWM control because the unwanted Joule heating is equal to (I^2)*R and the cooling is proportional to I.  That means, assuming you want to run the module at half its heat transfer capability, if you run it at full voltage and  PWM it at 50% duty cycle, you get twice the self-heating that you would if you reduced the voltage to reduce the current to 50%.  Its therefore essential to filter the PWM to get a reasonably smooth current through the module.

Also, they dont do well with 'bang-bang' control  (simple on/off control with a thermostat) because the repeated thermal cycling causes differential radial expansion and contraction of the hot and cold faces, flexing the individual thermoelectric elements leading to fatigue and cracking resulting in partial or total module failure.

You therefore need to drive them with a variable voltage controlled power supply, and use a PID control algorithm tuned to be somewhat overdamped to avoid rapid cycling.

[teimes]

Thank you everyone, we have read a bit on what you have proposed have come up with a circuit that works in proteus, so thank you everyone 

[teimes]

that's why wi want to learn it

I understand. But there is a difference between trying to learn "everything" all at once, and learning particular details as you go.

This is why you cannot go to college and learn how to be an engineer in just one year. There is just too much to learn.

In this thread people have been giving you good answers, like "use a power supply with a control input", but you do not understand how to apply that answer. This puts you in the position of someone with a plate of nutritious food, but you cannot digest the food.

So, I do not want to discourage you, but I do think you are being too ambitious for your level of knowledge, and you need to begin with something simpler.

we do not learn as long as we do not try 

[Simon]

that's why wi want to learn it

I understand. But there is a difference between trying to learn "everything" all at once, and learning particular details as you go.

This is why you cannot go to college and learn how to be an engineer in just one year. There is just too much to learn.

In this thread people have been giving you good answers, like "use a power supply with a control input", but you do not understand how to apply that answer. This puts you in the position of someone with a plate of nutritious food, but you cannot digest the food.

So, I do not want to discourage you, but I do think you are being too ambitious for your level of knowledge, and you need to begin with something simpler.

we do not learn as long as we do not try 

Yes but there is aiming for something you can achieve and aiming for what is impossible for you now. You were unable to even explai what you had to work with. This project is not an absolute begier one.

If i was doing it I would read a temperature and then generate a PWM and low pass filter is with a SMPS output circuit to get a smoothish voltage to run the pltier on. There would ned to be a micro controller i there probably to adjust the PWM to correct the temperature. There may be a more analogue way of doing it.