# Peltier circuit design for TEC

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.

• Could you please either: Upload a higher resolution image, or re-draw the schematic in the forum's schematic editor? I'm having a very squinty hard time reading values and names. That said, I think it's very likely to be your transistor choice. Jun 20, 2015 at 15:21
• @Asmyldof thank you for your comment. I'm on the road and I'll try to get you a better picture asap. Jun 20, 2015 at 17:22
• what if the motor is brushless. Then what to do? (posting on behalf of Ram)
– user79283
Jun 21, 2015 at 15:10

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.