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Peltier project review

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arronar

New Member
Hi out there.

I'm coming from web dev background and I'm kinda newbie on electronics world. Here I will try to explain my first "big" project and I would like to read your suggestions/ideas/corrections.

So I want to build something that might sound simple and maybe it is. I want a controller to control a Peltier element, a fan and a temperature sensor. Since I have a background on web dev, I thought to choose as my controller a raspberryPi-zero. Install linux and build a web interface that will cooperate with some python code to control the previously mentioned elements. The user will have access to the web app through a WiFi AP( access point) that RP zero will serve.

Through this web app, a user should be able to set some kind of configuration on what temperatures and for how long the Peltier should develop. For example one could configure the following:

1 - Ramp up to 65 C and stay @ 65 for 20 min
2 - Ramp up to 95 C stay @ 95 C for 10 min
3 - Ramp down to 20 C and stop

What Peltier is going to warm and cold is a small volume (around 100 to 500 microliters) of liquids.

So far through my search, I ended up to the following construction (see picture below) and the corresponding element list. As I told you I'm kinda a newbie and the following might be completely wrong. But please be patient with me :)

Here is how I imagine the whole schematics


and here is the list of the elements

#1 16V power supply
https://www.ebay.com/itm/16V-4-5A-A...03:m:myFY9yzLow0ou5zt0uEJ1Vg&var=521727474621
#2 5V 1A regulator
https://www.dimensionengineering.com/products/de-sw050
#3 Raspberry Pi / Zero W
#4 Raspberry Pi relay
https://www.adafruit.com/product/3352
#5 H-bridge
https://www.ebay.com/itm/USA-L298-L...071129?hash=item4400ac5919:g:ROUAAOSwBOtZAOAG
#6 Peltier
https://gr.rsdelivers.com/product/a...eltier-module-162w-28a-157v-30-x-30mm/4901430
#7 Temp Sensor
https://www.ebay.com/itm/5PCS-DALLA...187157?hash=item20fbe73555:g:niUAAOSwNSxVUwed
#8 Fan
https://www.ebay.com/itm/40mm-DC-5V...341761&hash=item2a711938b7:g:R3cAAOSwOyJX3zzx

As you can see I tried to use only one power supply for the whole project as also I added and an H-bridge to reverse the polarity of the Peltier.

Any idea and suggestion on element selection or schematic are welcome.
Thanks.

P.S I would like to test the whole project without any PID controller at first.
 

Diver300

Well-Known Member
Most Helpful Member
That looks OK.

I can't see why you need the relay board. You could wire the H-bridge directly to the power supply, as the H-bridge will have an "off" state.

You need to know how much heat will be needed to overcome the losses, and how much power the Peltier cell can produce. Also the Peltier cell isn't quoted beyond +75 °C on the hot side, so I don't know how well it will work. I will basically become an electric heater at 95 °C on the hot side so a resistor would work almost as well for heating, and you wouldn't need the fan. However, you will need the Peltier cell to get the whole lot back to 20 °C unless you are always using it in colder temperatures than that, so you might as well use the Peltier for heating as well as cooling.
 

alec_t

Well-Known Member
Most Helpful Member

schmitt trigger

Well-Known Member
Surely you don't want to power the peltier cell and fan from the 5 volt they consume significant amounts of current.
Drive them from the 16 volt instead.
 

arronar

New Member
Surely you don't want to power the peltier cell and fan from the 5 volt they consume significant amounts of current.
Drive them from the 16 volt instead.
From 16V I drive them in the schematic.
Also the Peltier cell isn't quoted beyond +75 °C on the hot side, so I don't know how well it will work
In the datasheet, I'm seeing that it can reach 87 C maximum deltaT. So if the room temperature is around 25C I don't think that I will have a problem.
I will basically become an electric heater at 95 °C on the hot side so a resistor would work almost as well for heating, and you wouldn't need the fan.
I cannot understand what are you saying here.
 

Diver300

Well-Known Member
Most Helpful Member
Also the Peltier cell isn't quoted beyond +75 °C on the hot side, so I don't know how well it will work.
In the datasheet, I'm seeing that it can reach 87 C maximum deltaT. So if the room temperature is around 25C I don't think that I will have a problem.
The graphs in the data sheet have been produced for three hot side temperatures, 27°C, 50°C and 75°C. There is no clear indication that it is OK to run the hot side beyond that.

It will basically become an electric heater at 95 °C on the hot side so a resistor would work almost as well for heating, and you wouldn't need the fan.
I cannot understand what are you saying here.
In the graphs on the data sheet, the COP (Coefficient of performance) drops to very small values, around 0 - 0.2 when there is a larger temperature difference. If the coefficient of performance is 0.2, that means that for every Watt of electrical power put in, o.2 W of heat is removed from the cold side. In that case the amount of heat delivered to the hot side will be the 1 W of power input on top of the 0.2 W of heat that has come from the cold side, so for 1 W input, you get 1.2 W of heat output. A resistor will always give out 1 W of heat for 1 W of power, so the Peltier cell is more efficient, but only 20% more efficient, and a resistor is a lot cheaper and simpler.
 

arronar

New Member
A resistor will always give out 1 W of heat for 1 W of power, so the Peltier cell is more efficient, but only 20% more efficient, and a resistor is a lot cheaper and simpler.
I see. Can suggest such a resistor to see what do you mean?
 

Diver300

Well-Known Member
Most Helpful Member
I see. Can suggest such a resistor to see what do you mean?
https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/7890724/

I don't know what your heat losses will be but the Peltier element or the resistor will use around 3 A at 16 V, so generating 48 W of heat. If you are warming 0.5 cc of water from room temperature to 95 °C, that will only take about 150 J. If there were no losses heating the resistor or the Peltier element, that would take 3 seconds. There will be losses, and it will take longer, but I think that you will need less power than that.
 
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