Re: H-Bridge for Small Motors...

"nobody" <nobody@xxxxxxxxxxx> wrote in message
news:dtd6lo$uiu$1@xxxxxxxxxxxxxxxxxx
So 3 cells are required giving me 4.5-2.4=2.1V for the motors...

Is this correct? Or is there more? I've just increased the heat
generated, haven't I... I see heatsinks in my future, yes? B-)

Well, not really, 0.66A * 2.4V = 1.6W /2 = 0.8W per transistor, easily
handled by a non-heatsinked transistor, at least as long as the chassis
isn't too stuffy. (TIP31 is rated for 2W, at 25°C ambient, without
heatsink.)

It also assumes something else: note the parameters for Vce(sat), 3A Ic. A
graph shows typical Vce(sat) between 0.1 and 0.2V at 0.6A Ic, more typical
for your application.

Note also the condition that Ic = 10*Ib, so you need 60mA Ib as well.

If you use complementary transistors for your H-bridge, remember that PNP
transistors are generally worse. Interestingly, at 0.6A, the TIP32 actually
fares better, according to the graph, placing just under 0.1V saturation.
However, the curve is steeper, so it will vary more.

In both cases, Vbe is around 0.8-0.9V, which means, for an (ideally) 1.7V
supply, you have only 0.8V available to operate the transistors.
Considering 60mA can easily be sourced/sinked at that voltage by general
purpose types such as 2N4401/03, this shouldn't be a problem.

Nonetheless, TIP31/32 is 5x overkill for this project. Some smaller
transistors, for example the Zetex line I mentioned, easily place under
0.2Vce(sat) for similar conditions, coming in a smaller package (though the
price is comparable to a TIP31!). Zetex also picks up the ball with
MOSFETs, though a lot of them are surface mount packages. Other
manufacturers also have lines of MOSFETs for this sort of application.
Remember MOSFETs need some amount of turn-on voltage.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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