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# The enigma of Dt....

It is obvious that few here understand Dt.
It took a long time for it to sink in to me even after exchanging many PM's with an American engineer who used to frequent XS and used TECs in the line of his work....he knew how they worked....
It was him that explained to me negative Dt and once I understood that it all fell into place.

Before I start to explain there are a number of things you need to understand, these are all indisputable facts.

1.) A TEC moves heat.... that is all it does it moves heat from A to B or more importantly from one side of the TEC to the other.

2.) You watercooling boys need to understand TECs are different. Your Dt is merely the difference in temp between your coolants etc....Dt in TEC is far more explicit.... it is the actual temp of the plates that form the outside of the TEC not the coolant/air faffing about around it...THE ACTUAL TEMP OF THE PLATES. It is near impossible to measure the temp of the coldside because more often than not you have it smacked up against a heat source to create a load.

3.) All properties of a TEC are liquid, they change any time . You can only calculate using a chart any property at the point of turning on the power to the TEC...once you have turned on the power to the TEC any of the properties can and will change. Dt, Qc, current draw, everything will change the only thing thats static is the voltage you chosen to apply. I used to say the only that was static was the TECs physical size but no...the voltage is static at the point you have chosen.

4.) When you apply heat to the coldside you increase the load....when you increase the load to a TEC the Dt becomes lower and when you lower the load the Dt increases. This is an indisputable fact for anyone that has run a TEC.

5.) On a Qc/Amp chart the Qc shown at the operating point is NOT the amount of heat moved. It is the Qc which is the max capacity at that operating point (voltage set.) . The amount of heat moved is entirely dependent on the load applied to the coldside and the hotside cooling which can be any amount UPTO THE Qc at that operating point. It is only up to the Qcmax if you are applying the max input.

6.) The movement of heat is relative to the electron flow through the TEC. The electron flow is from +ve to ground. Any movement of heat follows this path. That's why if you reverse the electrical connections the hot and cold side exchange with one another.

The next bit is where everyone goes wrong. Yes...Dt is the difference in temp between the hot and cold plates of the TEC...thats how you qualify it....BUT what is Dt. It has a purpose....it is stated in the main specs for every TEC..why ? Dt is actually a measure of the TECs ability to move heat. The higher the Dt the easier/quicker that heat can be moved. Thats why a TEC with a Dtmax of 68 is not as good as a TEC with a Dt max of 71.... it has less ability to move heat.

So that brings us to the status quo......if Dt is the ability to move heat when Dt=0 the TEC has no ability to move heat....therefore heat is not moved. Since the purpose of a TEC is to move heat at dt=0 the TEC is not doing what it should....it is not moving heat...any heat...there is no heat exchange from the coldside to the hotside.

When running a TEC as the load on the coldside increases the Dt falls - the TECs ability to move heat heat falls, and the Qc the max capacity of heat movement rises. As long as the Dt continues to fall the Qc increases until such point as the Qc is at max and Dt=0. recognize that ? Qcmax = Dt 0Âº.
At that precise point heat movement stops because Dt=0Âº - there is zero ability to move heat.

Whilst the hot and coldsides are exactly the same temperature there is no heat movement. For heat to move there has to be a difference in the temp of the two sides.

Need more help to understand this ?

Here is a small anology...not a good one because it ignores negative Dt but I will come to that later....

Think of having a little cart and your pulling the little cart across your yard - the length of the string is your Dt, Your sibling is constant chucking dirt in the cart - the dirt is Qc. While the cart is light - low Qc you pull it fast, as it fills up you get slower and slower until it reaches your feet and there is no string left - 0Âº dt. The cart stops and you slump on the ground your job done - You do nothing you do move any more dirt - same as your TEC it stops moving heat.

I am going to go off a bit now and explain charts.

If you wanted a TEC to move 200w you look up the vertical axis for 200w then read across till you reach a Dt line obviously the first one will be Dt=0. This is no good to us because as I stated Dt=0 means no heat movement. carry on reading right to the next Dt line it will be Dt=10Âº where the line for 300w bisects the Dtline read straight down that is the current that the TEC will draw at that point then if you go to the Volt/amp chart find the same value of current along the bottom read up to stop at the same Dtline the 10Âº one then look left and that will tell you the voltage you need to apply to have an initial Dt of 10Âº and a Qc of 200w. (Now you see why a variable power supply is best if you are serious about using TEC.). The voltage you set is your operating point and the only parameter that doesnt move unless you move it !
The problem is the Qc you chose is the max wattage that can be moved at that operating point. It is highly unlikely that your heat source will be 200w (Not all heat sources will be static. CPU's certainly are not......) if the heat source you apply to the coldside is less than 200w the Dt will rise. If the heat source is higher than 200w the Dt will fall. The thing to remember is that when the load is higher than the Qc the Dt will fall and consequently the current draw will rise to accomodate the extra load - i.e the working parameters will completely change. Of course it is completely the same if the load is significantly less then the Qc the Dt will rise and the current draw will fall.
In the ideal world you need to pick an operating point with a Qc sufficient to give you a bit of headroom on top of your maximum expected heatload. Then as your heatload is lower than the initial Qc, Dt will rise which should, as the TEC runs, allow you to maintain a reasonable Dt. When your choosing a TEC in the ideal world you need one with a Qcmax roughly twice your maximum heat load to be able to maintain reasonable dt with sufficient Qc.

Now coming back to our problem.....
If there is no heat moving when Dt=0 what happens...there must be a heat build up...yes there is...the heat build up is at the point you are applying heat to the TEC which is at the coldside so the coldside becomes hotter than the hotside. If there is a build up of heat at the coldside i.e you are applying more load the Dt falls...but it is already at 0 ? The hot and cold side where exactly the same temp but now with no heat movement there is the extra load on the coldside
Yes it goes to negative Dt. Negative Dt is the opposite to positive Dt so the heat is moved slowly at low Dt and as the Dt becomes more negative it moves heat more easily/speeds up.
As with positive Dt , negative Dt is influenced by the load on the coldside together with the cooling on the hotside.
Dont go thinking that as negative Dt has occured the hot and coldsides change place they dont as cuckoo as it sounds your coldside is hotter than the hotside but heat still flows from the coldside to the hotside. The reason why it doesn't change is because the heat movement is determined by the current flow - which is still the same way round !
So you see Dt=0 generally only occurs for a small period of time because there should never be an occasion where there is neither a load to the coldside or cooling to the hotside.

So to recap what happens...

There is a build up of heat at the coldside because there is no flow through the TEC, the increased load causes causes the Dt to go negative and heat starts to move slowly again so the load at the coldside starts to decrease and the Dt rises as long as the load continues to decrease the Dt will continue to rise and go through dt=0 again and continue to positive Dt.

That is Dt in a very large nutshell......
When using TECs you need to aim for as high a Dt as possible whilst still maintaining sufficient Qc to allow you to cover your heat load. Getting a decent Dt whilst maintaining your Qc will be a major step to determining whether you have a good setup or a bad one.
Edited by zipdogso - 4/21/11 at 3:55am
+REP

What an excellent post on DT, and I have definitely learned more.

I vote sticky.
Quote:
 Here is a small anology...not a good one because it ignores negative Dt but I will come to that later.... Think of having a little cart and your pulling the little cart across your yard - the length of the string is your Dt, Your sibling is constant chucking dirt in the cart - the dirt is Qc. While the cart is light - low Qc you pull it fast, as it fills up you get slower and slower until it reaches your feet and there is no string left - 0Âº dt. The cart stops and you slump on the ground your job done - You do nothing you do move any more dirt - same as your TEC it stops moving heat.
That analogy doesn't really seem to fit.. try this analogy

A pumps pressure vs flow ( ie dt vs qc in watts)

when a pump is at max pressure, flow is 0 (ie at dtmax, qc=0)
when a pump is at max flow, pressure = 0 ( ie at dt=0, qc=qcmax)

Regards
Lichking
Edited by lichking - 4/20/11 at 12:05pm
 LKTEC (13 items)
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 LKTEC (13 items)
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What I've gotten from reading this is that when Dt goes negative, it raises the coldside temp slowly at first and then will increasingly get hotter faster?

If that's true, the only fault with a Dam representation of a TEC would be that when Qc = Dt, the flow through a flood gate would get slower and stop (negating negative Dt). I guess a Dam isn't a pump, but still, the focus if the analogy is to show backing up on the coldside.

For me to change my thinking, I would have to see proof that when the hot and coldside are the same temp, no heat is being moved. (I can only see this happening if the TEC is off). Also wouldn't resistance of the TEC become infinite or something to stop all current flow? (This would probably be the proof that would define it or just a test showing that the coldside gets hotter slowly then skyrockets)

Either way, it's going to stay in the back of my head whenever I experiment so I'll see it eventually.
Edited by Krow - 4/20/11 at 1:49pm
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