Alt-BEAM Archive
Message #01433
To: beam@corp.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Wed, 10 Mar 1999 21:46:48 +0100 (CET)
Subject: [alt-beam] Re: Surface mount H-bridge chip
On Wed, 10 Mar 1999, Terry Newton wrote:
> >Um, what does `powered up without a driver' mean? I don't see a
> >`poof' state.
>
> No driver = nothing driving the transistors, bases floating with
> their resistors tied together. Connected like this, inputs (to the
> transistor pairs) must be (all the way) high or low, not floating.
> In other words, the driving chip must be connected to the transistors
> at all times.
It is generally possible to create unsafe situations by powering up
partially demolished hardware. This particular error would result
in about 200mA @ 3V, 300mA @ 4V (per driver pair). Certainly
undesirable, but not quite `poof' :)
> > The trick is to find plain transistors with a low VCEsat at a
> > low base current. Given reasonable motors, that isn't too
> > difficult.
>
> The Zetex parts have Hfe > 150 at full current, better than anything
> else I've tried. Most transistors lose gain over 100ma or so but not
> these, 3ma drive gives 500ma or more available to the motors.
The BC337-25 and BC328-25 have an hFE between 160 and 400 @ 100mA IC.
The VCEsat of these transistors is usually around 0.16V @ 100mA IC,
with a base current of 1mA; at 2mA IB you can saturate them a
little deeper, to about 0.12V VCEsat. At around 250mA IC you pass
0.3V VCE @ 3mA base current. They cost $0.25. And there
are -40 versions now, which do even better (250 < hFE < 600).
But their size/performance makes the Zetex bridges *very* sexy!
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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