Major Help Needed Please: Peltier and 2 channel fan controller needed :S

[MR Spade]

I have ordered a peltier and I have two fans that I want to be able to control. It all needs to be powered by a 4 pin molex connector.

I want to have 3 potentialometers so that I can control them all. I want to keep the voltage at 12v and be able to change the amount of amps going into the peltier.

So bascally:
12v x 4 amps
12v x 5 amps
12v x 6 amps
12v x 7 amps

Meaning I could have upto 84 watts. As far as the fans go it doesnt really matter but I just want to be able to change them from off up to 12v. Could anyone help me. (Ideally I would also like a tempurature sensor, display and auto and manual mode but I think that would be asking to much from you guys).

I will post some details about the project later.

Thanks alot, MR Spade.

 

[Nigel Goodwin]

You appear to have very little knowledge of electrics (never mind electronics), most devices take what current they need - and while you can limit the current to them the voltage will fall at the same time. As it's a heating/cooling device, probably the easiest method is 'burst fire' - turn it ON for 30 sends, then OFF for 30 seconds, to give half power. Adjust timing and mark/space to do what you want.

You might also try looking for other threads on peltiers?, generally they are far less useful than they initially look!.

 

[MR Spade]

I wasnt too worried on how much the peltier I have "took" its just I was worrying about the ability for my PSU to supply it. I didnt want to overstrain it and also I just want to be able to controll it never mind the voltage being at 12v.

I want to learn electronics but I havent really found a good guide nor class .

Is there any circuits that will allow me to vary the current going into the pelt.

 

[dknguyen]

You are still implying that you want to control the amps going into the Peliter device when you are appling a fixed 12V, which means you still don't understand that it only takes the current it needs for a given voltage. The only way to control the current is to reduce the voltage you are appling to the Peltier- you can't control both voltage and current at the same time. YOu have to sacrifice control of one for control of the other.

To control voltage at the currents you are asking for, you need a separate very big variable power supply which costs a lot of money. Or, if you thought you could build one, it is certainly beyond your ability pending a large chunk of time into gaining more knowledge and experience. In short if you want to work with your current computer supply right now, you cannot have variable current/voltage control, and your power supply probably cannot supply all that power along with the power for everything else. You just have to hook it up and hope it works- forget about variability. Like I have a 12V, 20A power supply sitting on my desk and it measures 25x10x 23cm (10x9x4 inches)...and it's fixed, not variable.

 

[MR Spade]

I do realise that I am asking what seem like silly questions to you.

This may also be silly. If i understand you correctly then the fact that I am using a PSU that is providing 12v (which I am happy with). In what way will it be possible to controll the current going in. Is it possible to have something simple like a variable resistor (I dont mind if this 12v is changed I just want to have some control of the current going in).

Also are there any really good online guide to start learning.

 

[dknguyen]

You cannot get variable resistors large enough for what you want. The only way is to make a discrete load resistor box that switches more resistors in parallel so the overall resistance decreases.

But there are 3 big problems with that:
1. The resistor load box will be BIG. I'm guessing at least half the size of your tower.
2. It's pointless because the resistor will be dissipating a lot of energy and heating up a lot (around the heat of a 40W light bulb).
3. It is a misguided way to try and conserve energy since you are just dissipating the "saved" energy as heat.

 

[MR Spade]

Well looks like I will just have to hope it works.

Would buying a different peltier be a better Idea something lower rated such as 80 watts.

Also to finish I really want to learn about electronics especially in more depth to do with computer interfacing (i know a long long way away). Could you point me in the right direction and what would be your advice.

 

[dknguyen]

WHy did you want to control the current going to your peltiers anyways?Peltiers are really inefficienct. An giant 80W fan would or heat pipe would be more efficienct and effective. Do you really need that much cooling? Or are you one of those people that just liks adding more and more cooling to their computer? In which case I would say water cooling since it's flashier, more effective, and more efficient.

If you still want to go with the Peltier, just don't bother with a variable resistor box. Just used a fixed value since it's worth more effort than the benefits it will bring. The more current going to the Peltier, the higher the temperature differential will be (and the more energy is consumed of course). So there's little point in limiting the current except to conserve energy (which it doesn't sound like you care about) or having it work within the limits of your power supply- in which case you should just find a fixed resistor value to get the whole thing to work or get a much lower rated device (going from 84W to 80W will do nothing).

 

[MR Spade]

I wanted to control the current because depending on the room tempurature depends on the ability for my heatsink to cool it down. I was making a unit that cools the hot side and vents the heat (I have a use for this later as a heater as well) then there is a heatsink on the coolside with a fan which pumps cool air into a case.


Question still remains. : Also to finish I really want to learn about electronics especially in more depth to do with computer interfacing (i know a long long way away). Could you point me in the right direction and what would be your advice.?

 

[dknguyen]

was editing my post when you responded. Please read it again.

So why would you ever want to reduce the current as long as your PSU was able to supply it? Even if you want a heater, using the Peltier is still misguided since fans would more effective and use less energy for cooling, and you could always just blow the hot air to where you needed it. Just because Peltiers cost more and are flashy doesn't mean that they work better than simple fans.

It would be far easier (amd more compact) to just control the speed of the fans to limit the amount of heat dissipating away from the Peltier if the CPU ever gets too cool (then that heats just ends up going back to the CPU), and using a single fixed resistor to keep the Peltier current draw within your PSU limits. But do you even need this cooling?

You're going to have to tell me what you mean by computer interfacing- you mean through it's serial ports? If so, I am not the person to ask about that.

 

[MR Spade]

Well I need to learn the basics first anyway.

Right the peltier unit that I have at the moment is rated at 110watts. I would be fine with being able just to have a single resistor (or maybe this would be equally as good be able to have a switch to put the resistor in. So i could either have it off , with resistor or just directly connected)

would it be possible to do something like that or even be able to do something like off 2 resistors, 1 resistor and then high. I am not to worried about being able to finely tune it. I just wanted a bit of control.

 

[dknguyen]

Forget about the resistor load box. It just occured to me that PWM might work, although I still don't understand why you want to use a Peltier though. It seems to me you just want to use it for the sake of using one (at least at first, although the reason now might be that you already spent money on it so you might as well use it). Make sure to overclock to make the whole thing worth it (or just run the Peltier below rated).

PWM on it's own does not let you actually "vary" the voltage/current in the analog sense. What it does is let you control the amount of on-time and off-time between 100% and 0%, which lets you control the overall power that the load sees (which in many cases, translates into control of the overall voltage/current being delivered to the load). For something liike electronic circuits that actually need a steady current/voltage, you need some extra components to actually average out this momnentary current that jumps between 0% and 100% as the voltage jumps from 0% to 100% (the amount of current being deteremined by the 100% voltage level and the load's resistance/impedance). But for things like motors or resistive loads (like your Peltier) where input power is more important than steadiness of voltage these extra components are not required. You would need a 555 timer where the timing resistors are variable (so you can control the PWM duty cycle). You could make the timing capacitor variable instead of the resistor, but variable capacitors tend to be less accurate, have less range, less stable, not as good, more expensive, and rarer than variable resistors. The 555 timer drives a power MOSFET to switch on and off to send current to the Peltier (since the 555 timer cannot actually deliver this current on it's own).

To add automatic temperature control you might and use a temperature dependent resistor for the timing resistor instead of a variable resistor. A thermistor is probably most practical and best, but the response would be non-linear but that's not too critical. Just make sure the resistor varies enough over your interested temperature range and the the slope of the temperature vs resistance graph is in the proper direction baesd on whether it is being used for discharging or charging.

Things like platinum RTD are very linear but their base resistance (and the temperature dependent variations) are so low that it's impractical for this case, not to mention they are hard to find and expensive.

PS. Whenever I mean charging/discharging, I am referring to how the 555 timer times itself by measuring the time it takes for a capacitor to charge up through a resistor- change either the resistor or capacitor and you change the time.

 

[MR Spade]

Okay I have used 555 timers before (a long time ago lol). I read something like this with a MOSFET and I didnt understand it. If you could give me like the basics of what I would need and possibly a circuit diagram I have a few friends that are alot more able than me that could assist me.

Rougly what parts would i need and what cost.

 

[dknguyen]

MOSFET THEORY:
So lets go with the N-channel MOSFET (NMOS) since they are cheaper, more efficient, switch faster, and easier to find than the PMOS.

Your voltage 12V, so you need one with a breakdown source-drain voltage of >12V (try not to cut it too close). Your peltier is rated for 110W, so at 12V, so the max current expected is 110W/12V = 9.2A. So it needs to be able to handle at least that much current.

Since the 555 timer can takea maximum of 15-18V, you should be able to run it directly off your 12V PSU. THis also means that the 555 timer output will be 12V which means you can use MOSFETs that do not have a logic-level gate threshold switching voltage (which are 2.5-3.3-5V or so, vs the typical 10V). It should be pretty easy and cheap to find such a MOSFET.

In an NMOS, the gate pin goes to the output on the 555 timer (duh). The NMOS can only block conventional current trying to flow from drain to source. This means that the source must be more negative than the drain (current flowing the other way will see a diode and will flow right past).

Since the triggering voltage on the MOSFET gate is referenced to the source and the 555 timer output voltage is refenced to ground, this means you shoudl connect the source to ground so that the NMOS can easily surpass the gate threshold voltage (so the driving signal has the same reference as the threshold voltage). ALl this means is that it's easiest to switch an NMOS on and off it is on the low-side than if it is on the high-side (you know what those terms means right?)

ie. if the source voltage is 7V higher than the ground because you connected the transistor closer to +12V than GND (high-side), then the 555 timer will need to output an extra 7V to turn the transistor on which it cannot do).

So connect the source-drain of the NMOS on low-side (closer to ground) with the Souce-drain polarity in the same way. The peltier goes on the high-side (closer to +12V).

Then you just set up the 555 timer to output PWM pulses using the astable circuit with variable resistor (manual or temperature) to control the PWM duty cycle which will switch the MOSFET on and off int he same way.

BTW, in case you haven't already figure it out, a MOSFET blocks current (going from drain-to-source only, the other directions just looks like a diode) when the voltage from gate-to-source is less than Vthreshold, and turns allows current to pass when the voltage is larger so it acts like a one-way switch.

OR BLINDLY DO THIS:
The connection between 555 timer and MOSFET looks like this:
**broken link removed**
like this except that your load is not a motor, but a Peltier device. The circuit I had in mind used output to drive the MOSFET. In this one they use the discharge pin and a pull-up resistor. There are a few ways to do it. Shouldn't be a problem for you to figure out the best one since you have used 555 timers before.

EDIT: The one I had in mind, I was not sure if it kept a constant pulsewidth and pulse frequency as the duty cycle changes (I'm pretty sure it didnt.) The provided circuit does. Keep the frequency in the range of the 555 timer and MOSFET. Maybe keep it low since it's just a peltier and doesn't really matter. I'd choose 1kHz.

IMPROVEMENTS:
Also, remember how I said thermistors are non-linear? I never thought about using them as a voltage divider for both charging and discharging like the circuit provided (which also keeps the pulse-width constant with changing duty cycles). Just a quick thought which is unsubstantiated but you *might* be able to make it linear if you use two thermistors as a voltage divider (or something similar) so that their non-linearities cancel each other out. you'd have to look into it more.

You might also want to put a fixed resistor somewhere (one at the end of the potentiometer will produce a minimum or maximum achievable PWM duty cycle depending on which side it is on. One at the center-tap of the variable resistor will produce equal max/minimum limits on the duty cycle. The upper limit is because if you realize that your PSU cannot handle the full Peltier current, then you need to physically limit it somehow so you (or the thermistors) don't acidentally turn it up too high. There might also be a minimum duty cycle required which would cause your CPU to overheat if you went below it.

For the record, I think you should a fan rather than the Peltier and if you really needed below ambient cooling, then I think you should use water cooling or a vapour-phase system. I just like the 555-timer PWM circuit.

 

[MR Spade]

So (I read the link half understood it) would you be able to change the pulse timmings? I am not woried about thermistors. How would I change the pulse timmings, im guessing (from the diagram in the link) that the pot is what does it.

Okay I hear you water cooling or a vapour-phase system would be so much better. None the less I want to continue with this,.

Thanks for taking the time for the long response. I am going to try and look through that site and see if there are any other tuts on it.

//EDIT: Looked through the site and there isnt anything more on there that isnt too advanced for me. Have you got a link to a site with basic tuts on it right through the very basics.

 

[dknguyen]

By pulse timings you mean duty cycle? The pot does that. It forms a resistive divider. THe capacitor charges through one half, and discharges through the other half controlling the % that each pulse is on and off.

The frequency is fixed in the circuit by the full R & C values. What changes is the proportion of the R that is used for charging and discharging which allows you to change the duty cycle.

PWM Example:
At 1000Hz, that means that one second is divided up into 1000 divisions. For each division (which turns out being 1ms long), if we have a 30% duty cycle, then for the first 30% of that 1ms the pulse is high, and for the remaining 70% of that 1ms the pulse is low. Once the 1ms interval is over, a new one starts and it goes high again for 30% of another 1ms and then low again. It keeps repeating.



You now what a comparator does right? There are two terminals + and -. Whenever the votlage at - is less than + then it outputs high, otherwise it outputs a low. You need to know that to figure out the tutorial which I find is a bit wordy, but it's thorough. Just read it carefully and slowly and a few times. I find the weirdest part is the description of all the pins which is really REALLY through since it refers to the schematic. Why don't you give it a try and then ask some specific questions about the pins. It's very easily described if you can give me somewhere to work from (otherwise it just becomes wordy).

I'll just try and give a brief explanation of the 555 pins ahead of time:

The 555 timer charges a capacitor through a resistor. The time it takes to charge is predictable. It then discharges the capacitor through another resistor. During charging the output is HI and the time it takes to charge reprsents how long the 555 timer output is high for. Discharging is the same way except the output is low.

Control voltage- ignore this and bypass it to ground with a cap. THe voltage here represents the voltage that the capacitor must reach before the 555 timer will try and discharge it by connecting the discharge pin to ground. It is internally set to 2V/3 to make the math easier. Just ignore this pin and connect it to ground through a capacitor. It gives you flexibility but jsut complicates most things.

Output- THis goes high while the capacitor is charging up and usually represents the "on-time" of whatever the 555 is outputting. While the capacitor discharges this pin goes low. Some circuits use the high signal from this output to charge the capacitor, while others just connect the capacitor straight to the power supply through the charging resistor.

Discharge- it doesnt connect to anything while the capacitor is charging up. When the capacitor voltage hits the control voltage (default 2V/3) then it connects to ground. It is meant to connect to the capacitor through a resistor to discharge the capacitor. While this is happening output goes low. This respresents the "off-time" of the 555 timer output.resistor (which sets the discharge time). Some circuits don't actually use this pin to discharge and use other methods.

Threshold- this is the pin that connects to the + side of the capacitor. It measures the voltage the capacitor is currently at. And controls everything else pretty much. Whenever this voltage reaches the voltage at the Control Voltage pin (Default 2V/3) it tries to discharge the capacitor through the discharge pin.

Trigger- when this pin voltage gets low enough, it makes the output high and disconnects the discharge pin. This will try and charge up the capacitor again. This pin will trigger when the voltage gets BELOW V/3. This makes the math easier in the same way the control voltage pin is 2V/3 default. You can think of this pin as the opposite of threshold. The capacitor voltage will bounce back and forth between the voltage thresholds for trigger and threshold (set by control voltage pin). Whenever it gets too high, threshold pin triggers and starts the discharge. WHen it discharges enough and the voltage gets too low, trigger pin will trigger and start the charge.

You can use these pins in many different ways (like you don't actually have to discharge through the discharge pin, you don't have to charge the capacitor directly through Vcc- some circuits have the output HI signal charge the capacitor). You can manipulate the functions of these pins to do a bunch of timing things (including constantly oscillating or just making a single pulse once, and stopping until the next time you apply a low voltage to the trigger pin through a button or something).

 

[MR Spade]

Hi, I am looking to order the parts that I do not have at the moment,
can you just confirm that the following circuit would be suitible for my project. Also that the potentiometers are correctly suitible.

Thanks alot, I knwo that this thread it quite old but I am just
looking at getting this done now. Again thanks for all your help.

**broken link removed**

This was the circuit that I originally intended to use. However would this circuit be better. If in these circuit diagrams any of the components need to be different can you tell me.

**broken link removed**