There are so many mistakes, omissions and non-standard symbols used on that page, that one would be better off to NOT look at it.
There are no formulas anywhere for the reader to determine the correct resistors to be used. Simply using 1k Ohms for everything is not acceptable; as the transistors would either not be in saturation, or excessive base current would be flowing.
Let's start with the 1st schematic:
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The one on the left is fine. But could the one on the right be drawn any more cryptically? What was that person thinking when they drew that? Obviously, they were not!
Here is an improved schematic, which is much easier to understand; very important for a novice. I've also added a far better circuit which overcomes the shortcomings of the previous two circuits; it is at once simple, elegant and there are no combinations of the two switch settings that could cause a short:
Why didn't the author of that page include a circuit like that, instead of those with shortcomings?
Here's another one:
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I have never seen a relay (the lower drawing) drawn so cryptically. A novice would have a difficult time understanding it. When introducing people to new ideas, they could have at least used standard symbols.
Yet another:
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Non-standard relay symbols, AND the author omitted a couple of critical items - flyback diodes for the relay coils. Without them, the transistors would very quickly fail.
They should have drawn it more like this:
The upper schematic introduces them to the basic parts required, while the lower schematic adds a diode bridge across the motor to help prolong the life of the relays. As an alternative, a pair of Zener diodes back-to-back or a TVS diode across the motor would also serve.
Now this schematic:
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has NO base return resistors on the upper PNP transistors! As a result of this oversight, if the transistor that is ON gets close to falling out of saturation, the opposite transistor will turn on, which will create a short across the supply!
This one:
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compounds the error, because now there are FOUR transistors without base return resistors! Also, the author fails to mention that the logic level must match the bridge supply voltage, or the B input will not turn off!
Design 3:
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isn't so bad, as it was Steve Bolts' design. Except Steve Bolts used 74HC14's, which this person should have used, as it will get the transistors out of that transitional shoot-through more quickly.
Design 5:
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They swapped the PNP and NPN Darlingtons. Why use Darlingtons as voltage followers if you don't have to? That simply exacerbates the Vce, as neither transistor can saturate.
Also, the TIP12x series has a minimum hFE of 1,000. This is one instance where they could have used a much larger resistor, but they threw that opportunity out the window.
Design 6:
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10k for a P-ch gate pull-up resistor? Don't try PWM with those P-channel MOSFETs, unless you have some marshmallows handy.
And I can't help but wonder just HOW they calculated 22k for those base resistors? Qg is 39nC... 22k seems a bit high for anything that could be controlled by something faster than a hand on a switch.
And the hits just keep on coming:
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Here they used a couple of 555 timers which has a Darlington follower for the high side output, to drive yet ANOTHER pair of high-side Darlington followers - on both the high AND low side! Even in a no-load condition, they will lose 1.3v + 1.3v off the top, and ~0.3v + 1.3v off the bottom - I get 12v - 4.2v = 7.8v left - but wait, there's more! Add some load to that circuit, and your Vce
really goes in the toilet!
This is one of the few pages I've seen where I'd recommend an L298 over their suggestions - and the L298 is pretty awful.
Now, here's a shocker:
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Look at that up top! They apparently HAVE heard about base return resistors! They have
no clue on how to size them properly, but at least they made a feeble effort. That brings them up to.... let's see ... I can't decide between a D- or F+.
Rube Goldberg comes to mind here:
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Just one problem - there is no OFF!
Oh, I take it back - there actually IS some mention as far as base to collector current ratios - at almost the bottom of the page! Why wasn't this mentioned near the top?