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Comments about this simple circuit that is teach in a university......
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In Vacuo Veritas:

--- Quote from: wilmer on June 12, 2018, 10:14:36 am ---
The I/O port don't have a resistance in series for the port. That MCU is a hero to support that, a PIC 16XX will blow immediately.

--- End quote ---

No one said it does. Internal resistance is just the model of the finite resistance of the parts, unless they are superconductors.

The internal resistance of a battery isn't actually a discrete resistor inside the battery, for example.

C is correct, it could just be a port configured with a weak pull up, a "resistor" to VCC  which is actually a mosfet strapped as a weak current source, equivalent to some high value resistor.

In which case, why add the extra diode? Who knows. Universities are the last place I look for reliable information these days.
wilmer:

--- Quote from: rstofer on June 12, 2018, 03:12:52 pm ---I don't like the circuit and I would never use it but...

Assume a motor current of 1A and an hFE of 100.  Base current is 10 mA and the input to the diode will be around 1.4V (2 diode drops).  So, for a 3V Vdd on a 16F877A, we need a Voh of 1.4V @ 10 mA.

This is pretty typical according to Figure 18-17 (document page 205, PDF page 207).  It might overdrive the base by a 5 mA but, again, when we do switching, we often overdrive the base to guarantee saturation.  The Absolute Maximum rating for sourcing through a pin is 25 mA (document page 173, PDF page 175).  With Vdd = 3V, the pin can't deliver anywhere near that much current.

http://ww1.microchip.com/downloads/en/DeviceDoc/39582b.pdf

Not my favorite circuit but there are many uCs where this can work without having the magic smoke escape.

I think it is a mistake to assume the circuit won't work without searching for conditions under which it will work.



--- End quote ---

You are right, In the real world, the student circuit work for a couple of hours after that: 1) the transistor blows(2n3904), 2)the MCU get very hot and 3) there is not complete saturation, the motor didn't work properly.
rstofer:
Likely the transistor went first and took out the uC.  If the transistor can't get into saturation, it will heat up badly.  Nothing in the schematic gave part numbers.  With a small enough motor and enough hFE to get into saturation, the thing might actually work.  An hFE of 100 seems marginal.  We really need that device in hard saturation, Vce of 0.2V.  Then we could calculate the heat dissipation from Vce and Ic - maybe a heatsink helps, maybe a more robust transistor.

As a practical matter, if the transistor doesn't short out, the uc should survive nicely and for a long time.  The circuit, as I calculated it, isn't overloading the pin.  But it could easily be overloading the transistor.

Even if I convince myself it should work, I'm not going to use that circuit.  There are motor driver chips made specifically for this purpose.
wilmer:
 :-/O

--- Quote from: rstofer on June 12, 2018, 10:45:48 pm ---Likely the transistor went first and took out the uC.  If the transistor can't get into saturation, it will heat up badly.  Nothing in the schematic gave part numbers.  With a small enough motor and enough hFE to get into saturation, the thing might actually work.  An hFE of 100 seems marginal.  We really need that device in hard saturation, Vce of 0.2V.  Then we could calculate the heat dissipation from Vce and Ic - maybe a heatsink helps, maybe a more robust transistor.

As a practical matter, if the transistor doesn't short out, the uc should survive nicely and for a long time.  The circuit, as I calculated it, isn't overloading the pin.  But it could easily be overloading the transistor.

Even if I convince myself it should work, I'm not going to use that circuit.  There are motor driver chips made specifically for this purpose.


--- End quote ---

That is exactly what happened to them. the transistors were 2n2222,2n3904 and of course, they blow. Some motors were asking 300mA but some other motors were asking 10A , I believe some motor have internal damage because students connect them directly to the battery bank. I assemble some circuits for them with TIP31 and TIP 32 and that circuit work perfectly.  :-/O
Belrmar:
for this kind of circuit i tend to prefer yo use a MOSFet, as it is less likely to overload the pin when correctly used, and i would add a resistor to the capacitor to prevent LC oscilation
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