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Arduino and temperature PID control with Peltier chip: basic question about wiring 12V supply

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#1 Ivor Redtorch

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Posted 04 December 2020 - 01:09 PM

Hi All

 

This is a very basic question, but tutorials tend to leave out the obvious, which is a problem for those who are trying everything for the first time.

 

The following image is from a tutorial  by Andrei of electronoobs and was taken from https://electronoobs...ino_tut24_2.php

and it seems to me to be a really useful tutorial

 

schematic 2

 

It is for controlling a peltier chip using an Arduino, with MOSFET and Rotary encoder.

 

My question is about the 12V supply for the peltier itself. The Arduino has its 5V USB (or even 12V, which it will convert), but it can't deliver 12V to the peltier, so I assume that the right hand side of that circuit needs its own power supply.

 

The schematic has two red 12V DC arrows, both point the same direction (out of the circuit, up), and this is what is confusing me. It would make sense to me if the 12V was coming in at the resistor and going to the MOSFET first, and then to the peltier (or 12V heater in his schematic). In that case I would expect the arrows to be pointing down (towards the resistor) into the MOSFET, and the red arrow from the heater to point upwards (as in the schema), i.e. returning back to the power source (in practice the negative, or black wire).

 

Perhaps the arrows pointing in the same direction is some electrician's convention that I am unaware of, or perhaps Andrei assumed that it was obvious and wasn't too concerned about how he drew his arrows.

 

I know this is going to sound really silly to anyone who has wired a circuit before (I have wired circuits but not micro, or as PC builds where I only need to plug stuff into a mobo), but I need to be sure before I risk frying something.

 

We use to talk of "standard idiot proofing", well I am the standard idiot here!

 

Thank you for your tolerance

 

Ivor

PS I didn't email Andrei to ask him because his Q&A forum is full of spam and I am not sure what's up there, plus he has a monthly rate for answering questions, and I currently only have one. Also, I am planning to use this for an astro application (controlling bolt-on cooling for a ZWO camera - well I know it probably won't work so well, but really I just want to learn about what an Arduino can be used for) so if I ask astronomy people I might get more specific tips.


Edited by Ivor Redtorch, 04 December 2020 - 01:10 PM.


#2 Exciton

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Posted 04 December 2020 - 01:31 PM

Ivor,

In this case, the arrows do not have any special significance other than directing you to connect those two lines to the +12v supply. 

 

You are correct, the Arduino cannot supply 12v, so the two transistors on the right side of the schematic serve as your interface from a lower voltage (+5v or +3.3v) control to 12v control.  Wire the resistor and Peltier to 12v as shown and you should be good to go.

 

Exciton



#3 Exciton

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Posted 04 December 2020 - 01:41 PM

Also,  if you look around, you can find "teardown" pictures of the ZWO cameras.  In the case of my camera, the actual camera IC is nowhere near the back of the camera case.  This means you will be cooling the case without very much impact on the actual camera chip.  You can try it, but dont be too dissappointed if there is not as much change in temperature as you had hoped.    You really need to base of the camera chip up against the heating/cooling plate to get as much temp change as advertised in the cooled cameras.

 

Also, I am not that sure the uncooled cameras have a sealed cavity to prevent moisture condensation.  Mine is sealed by a simple piece of glass over a rubber o-ring.  I am not sure that is guaranteed to be airtight over the full temp range.  If you cool the camera chip far below ambient, an unsealed cavity might cause the camera to fog up. 

 

Only by experimenting do we learn....Have fun....

 

Exciton



#4 Ivor Redtorch

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Posted 04 December 2020 - 04:16 PM

Thank you Exciton

 

Condensation is indeed a problem, I have found a few threads on CN where reference has been made to that. One problem might be condensation on couplings close to the casing, so there could be an electrical problem even though the sensor itself is unfogged.

 

I recall that one ingenious member found a way to squeeze a thermal probe through the innards of the camera and closer to the sensor, to at least get better temperature readings, especially when coupled with a copper grub screw, as a cold-finger, in the 1/4" UNC 20 mount point in the center of the camera back.

 

Sounds like a good never ending project!

 

Ivor 




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