Re: H-Bridge for Small Motors...

Tim Williams wrote:
"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.

But Ic is 660 mA, not 60 mA. So the 2N4401 is too small.


Nonetheless, TIP31/32 is 5x overkill for this project.

He mentioned "Overkill" (which it is, as you point out) and
that word makes it sound like a bad thing, which it isn't.
The TIP31/32 can easily handle the maximum current, and won't
overheat without heatsinks - that makes it *good*, not bad.
"Overkill" by itself is not sufficient reason to change the
devices. Are there better devices? Certainly - as you point
out below, there are other devices that could be used.


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


Some excellent points, above. I am not familiar with the
Tamiya kit, so I am guessing he has the room/flexibility
for the following: Use his first approach with the TIPs,
and measure Vcesat with whatever the bridge driving current
is. Then build a motor supply that delivers Vout-(VcesatTip31+VcesatTip32) = 1.5 (or whatever he wants
from 1.5 to 3.0). A 317, resistor& trippot plus a couple of
caps...

Hopefully he could use that approach with whatever devices
he chooses - it would seem he has to develop a motor supply
in any event. There is a temptation to use 5 volts to supply
everything, and count on Vcesat to lower the voltage to the
motor by 2.4 (maybe add some diodes for an additional ~.6
drop) - but we don't know Vcesat at 660 mA, and it still
leaves motor voltage a bit higher than recommended.

Ed
.

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