I'm designing a TEC circuit with a Peltier element, LM35 as the sensor and Arduino uno as the controller. I simulated it with Proteus and to my knowledge it was working, I suppose. But when I tried to replicate the circuit on a bread board with the components, I had problems with PWM. Can you guys please have a look at my circuit and advise for any changes/improvements needed. Thanks.
Looks potentially OK. Transistor drive may be too low.
Place a reverse polarity diode across fan so it can freewheel during PWM off.
Fan needs to be DC brush motor and not brushless (BLDCM) for PWM control. .
You should say what "problems with PWM" mean.
What is desired Peltier current.
See other answers here re driving Peltiers. Olin has things to say about it :-).
Better diagram copy is needed. I assume diagram is available at better resolution and has been munted by downsizing. Imgur allows files of any sensibly desired size.
Assume 5A Peltier current.
Assume 5V Arduino.
TIP122 datasheet here
Beta (current gain) is 1000 min at 3A.
Use 1000 for now.
Vbemax (from datasheet) = 2.5V (!) At say 5A Peltier current Ib = 5A/1000 = 5mA.
Max Ib is about (Vcc - Vbe_darl)/Ib = (5-2.5)/1k = 2.5 mA = too low.
That's wity worst case Beta (at 3A) and worst case Vbe and you can HOPE real device may be better BUT you must always design to worst case.
Making Rb say 220 Ohms or 33 Ohms may help.
Please advise correct figures for assumed value above.
Another thing to note in your design: You also do not want to directly expose the TEC to your PWM due to it's thermal cycling limitations. When you PWM, you provide an "effective" rms voltage at the TEC, however, the TEC is actually still seeing voltages that go from 0 to VCC at various duty cycles, thus exposing the TEC to the max and min currents every period. Basically, this is bad and will reduce the life of your TEC. You need a passive second order low pass filter that has proper transient and frequency response based on your design requirements in order to make that voltage ripple go to somewhere less than 10%.