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RC Filter for Web Based PWM Controller (NodeMCU + ULN2803)

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CrossX

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Hello friends,

i'm newbie in microelectronics, but it's very interesting for me :grin:

Best practice to learn something - work with real projects.

I want build Web Based PWM Controller (NodeMCU + ULN2803) for my home server room.

Requirements:
- web based PWM control
- temperature monitoring
- "silent mode" for night time
- adaptive fans work based on current temperature

Components:
- 4 x 12V 0.21A PC FAN
- 3 x DS1820 temperature sensor
- ESP8266 ESP-12E NodeMCU
- ULN2803APG 8ch Darlington Sink Driver


Now i have working prototype on breadboard, but i'm not sure in RC Filter. It's really needed? And same question about overvoltage protection.

In some resources people say that "Motor inductance is enough to smooth your PWM." And on this forum LINK offer clean overvoltage protection with 100nF ceramic capacitor, 10uF electrolytic capacitor and 20V zener.

Thanks!
 

Now i have working prototype on breadboard, but i'm not sure in RC Filter. It's really needed?
With regard to RC filtering, it would actually have some usefulness to reduce the generation of harmonics due to PWM switching on a typical reactive load like a Fan, but that would make sense only if the control were by analog variation of the current, which is supposed to be what you have in mind, in which case this filter would have to be located eg at the base of darlington ULN2803. However, as in this case the control is being made non-switched, the transitor would overheat, therefore you should have to consider not using the transitor array, but discrete power rated devices, instead.
 
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    CrossX

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As i understand, using ULN2803 for this case it's a bad idea? And you suggest using discrete MOSFETs, right? Now I tried to measure the temperature of the transistor array and it differs from the room temperature by only +2 degrees, with a load of two fans.
 

Although at first sight the motor 210mA lies within the specification of the ULN2803 datasheet - which is 500mA for each transistor - you will notice that there is a graph for the duty cycle which somehow delimits a global dissipation threshold. As it is specified for the 8 transistors in the array to be operating simultaneously, I admit that I cannot estimate how much it would decay for only 4 of them. Anyway, if your experiment has already proven that it doesn't heat up, I have no more remarks to give on this regard.

[EDIT]

Here, in another datasheet it is clearly shown the current limit for each amount of transistors, and the maximum allowed for 100% duty cycle conduction of 4 transistors working together is somewhat less than 300mA, which would suggest that this array would be comfortably suited for your application:

DutyCycle4Transistors.png

However I would concern on the fact that it is rated for the average, not the peak, and once you are switching on PWM it could easily reach this value, not mentioning that during rotation start, the peak current is much higher than at the steady speed.
 
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