What are the advantages and practical differences between these two speed controllers of DC motor?

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Yes, it's linear - so will get VERY hot, and will most likely require heat sinking on the FET.

The first circuit is 'digital', so switches the transistor ON and OFF very fast, producing little heat in the transistor.

However, both circuits are from one of the really poor sites on the Internet, and are very badly 'designed' by someone with little knowledge of electronics.
 
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Thank you very much for your answer. So in both cases the problem is heat dissipation, but particularly in the second one. I guess the proper way should be to have an actual board(speed controller) to control the speed of the motor, right? Which would be the best way to control the speed of a DC motor and why respect to these methods?
 
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The best way is PWM, like in the first example.
 
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The second one will likely have way less power to the motor because it is using an N-Channel Mosfet as a high-side switch (or variable resistor). The IRF540 in that design will deliver a bit less voltage to a small (< 1 amp) and a lot less voltage to a larger motor where you want the Vgs to be 10v as a standard design.
 
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Yes, it makes sense, the higher the load, the higher the power loss and so the so lower output voltage to the motor. In any case, in respect to what you have said you guys, I think in industrial application or heavy power application they wouldn't use a transistor to power a DC motor because even with PMW, the transistor can still have problems of heat dissipation/power loss. In any case Thank you!
 
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DaveElectronics6XT said:
I think in industrial application or heavy power application they wouldn't use a transistor to power a DC motor because even with PMW, the transistor can still have problems of heat dissipation/power loss.
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It's down to the transistors being used in the appropriate configurations. The circuit you show is just a poor design.


Bipolar transistors were used for many years in high power industrial DC drives, until MOSFETs / IGBTs and "brushless DC" types gradually became the preferred types.

The earlier brushless DC systems still used bipolar transistors, sometimes the early ones even had the same power boards as in the normal DC drives, but three half-bridge cards rather than two.

This is an example of a board from a Siemens DC servo - there are two of these plus a control board to make one complete drive; this is a "half bridge", the output can be switched to either positive power or 0V.

Each group of 16 power transistors plus other smaller drivers elsewhere on the board form one massive "transistor switch".

This normally has heatsinks fitted, I took the photo while repairing it after a fault in the machine wiring.

The drive was rated 75A at 200V, if I remember right.

 
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Thank you very informative! Really appreciated!
 
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