Power Amplifier for thermoelectic cooler with Op-Amp and Power MOSFET PROBLEM

[noLimits]

Hello there!

This is my first post, so please be kind if I do something wrong here
I have a regulated circuit for a thermoelectric cooler (112.7 W, Impedance: 4.7 Ohms, max. Current 6 A, V max 29.8 V, max Temp change 74 °C). I know that this worked but it is somehow broken. So i need to build it anew but first i really want to understand how it works.
I have a controller that measures the temperature via a PT100 (the controller is connected to the "-" of the power supply unit and to the Op-Amp at "Stetigausgang") and a power supply unit with 30 V and 3 A max output current. The Op-Amp is powered by this supply. I want the thermoelectric cooler to be at 18 °C but that didn't work anymore. I got the circuit diagram by measuring shorts and resistors with a multimeter so i can't guarantee if this is correct, therefore I need to know if this works in principle and thus repairing it by just building it anew according to the diagram in the attachment.

This is how i understand it:
The controller puts a voltage on the connector "Stetigausgang" (sorry for the german...) if the temperature is too low. Then there's a difference in potential at the Op-Amp which amplifies this difference so that the gate-source voltage goes above the threshold voltage and thus letting the current (but how big is it and does that even matter if it is regulated?) flow from +30 V through two resistors and the thermoelectric cooler which then cools a bit down. The two resistors down below the inverting input produce a voltage from the current and the difference at the inputs of the op-amp becomes zero so it stops the current through the MOSFET. Then everything begins from the start according to the measurement of the controller and its output.
I assume the controller is doing its job properly so i don't wanna go into detail to this thing.

My questions are finally:
-How do i get these values for the resistors?
-Why are their values as they are?
-How important are the value of these resistors?
-Why do i even need this amplifying circuitry?
-Can i use this MOSFET instead: https://people.ece.cornell.edu/land...s/s2012/tcj26_ecs227/tcj26_ecs227/F12N10L.pdf
or this one: http://www.irf.com/product-info/datasheets/data/irli530n.pdf ??


And before you ask: yes the MOSFET is on a gigantic passive cooling element


Please clear things up for me and I hope I answered possible upcoming questions in advance. And thank you so much for your help!

Greetings!

 

[duke37]

Firstly, the power supply is rated for 3A and the cooler requires 6A. Something will or has gone pop.

The control signal is attenuated by the two resistors so 10V input will give 0.483V at the op amp. The opamp will turn on the MOSFET until the other input also gets 0.483V. This achieved when the current through the 0.33Ω resistors is 2.93A. so the power supply may be adequate if the input control voltage does not exceed 10V.

Some practical application of Ohms law will explain the values of the resistors.

I do not know why the MOSFET is not switched off or hard on (switch mode), this would reduce dissipation in the MOSFET. The addition of one resistor would do this with hysteresis. Perhaps thermal delay does this job at least partially.

 

[noLimits]

Hello duke37,

thank you for your quick reply!

So if I got that right the maximum current is 6 A for this thermoelectric cooler. That should mean I can also run it with 3 A but it doesn't cool as much -> this should not be an issue?

I could reproduce your calculations fairly simple but I dont understand why to use such high-ohm resistors when just the ratio is important for the voltage at the opamp? And secondly why use two 0.33 Ω resistors when i could just use one 0.165 Ω resistor?

Man I feel like these are so basic things that I should know...

Am I right when I say that you could see this as a voltage regulated current source?
But why all this stuff in there when you could just regulate the voltage over the thermoelectric cooler or the current through it respectively and thus "switching" it on and off?

Thank you again!

 

[Alec_t]

why to use such high-ohm resistors

Perhaps the controller providing the 0-10V input can only source a low current, so requires a high impedance load.

And secondly why use two 0.33 Ω resistors when i could just use one 0.165 Ω resistor?

Probably easier to source a standard E12 series value.

 

[noLimits]

Perhaps the controller providing the 0-10V input can only source a low current, so requires a high impedance load.

It says in the data sheet of the controller that it provides 0-20 mA when the load is smaller than 500 Ω and if its greater than that, it switches to 0-10 V. So isn't it completely irrelevant for the opamp what the current in that circuit is for theres (almost) no current going in the opamp and its amplification is solely dependant on the voltage potential on that point?

Probably easier to source a standard E12 series value.

Makes sense!

It is still unclear to me why i even need this circuit, it doesn't even amplify anything? It just switches the current through the cooler on an off, doesn't it?

Also it would be great to know if I could use the above mentioned MOSFETS instead of the used one?

Thank you guys!!!

 

[duke37]

Probably easier to source a standard E12 series value.

The 0.33Ω resistors are even in the E6 series.
Using two resistors splits the power so smaller resistors can be used.
Have you tried to get a 0.165Ω resistor?

The three MOSFETs are very similar and any one could be used.
The IRL1530N has a slightly lower on resistance.
The RFP12N10L has a slightly more sensitive gate - not necessary here.
Chose the cheapest.

edit: It does amplify, from less than 1mA to 3A.
And switches a much larger voltage.

 

[Alec_t]

It just switches the current through the cooler on an off, doesn't it?

No. If it did you would have 0A or 6A (too much for the power supply). Instead it provides proportional/linear control of the current.

 

[noLimits]

No. If it did you would have 0A or 6A (too much for the power supply). Instead it provides proportional/linear control of the current.

Wait I thought it would be 2.93 A through the MOSFET?
And these could be provided by the power supply?

edit: It does amplify, from less than 1mA to 3A.
And switches a much larger voltage.

why less than 1 mA?
I hope I'm not annoying you too much with all these questions, I just want to understand this thing and not just knowing what it does before touching it.

 

[duke37]

10V / 205k = .0.05mA input.

 

[noLimits]

10V / 205k = .0.05mA input.

Ok i should go to sleep now I guess this thing confuses me more than it should.
What I don't get though is, it doesn't amplify this said current, this is rather just the current that controls the 3 A provided by the power supply or am I wrong again?

 

[duke37]

That is how all amplifiers work. The only component that can increase or decrease a voltage or current is a transformer which can trade voltage for current with some loss.
The power has to come from somewhere.