Alt-BEAM Archive
Message #01432
To: beam@corp.sgi.com
From: Terry Newton wtnewton@nc5.infi.net
Date: Wed, 10 Mar 1999 11:41:54
Subject: [alt-beam] Re: Surface mount H-bridge chip
At 02:32 PM 3/10/99 +0100, Steven Bolt wrote:
>On Wed, 10 Mar 1999, Terry Newton wrote:
....
>> > http://www.xs4all.nl/~sbolt/Other/Hsmt-bridge.gif
(7961 bytes)
....
>> and if powered up without a driver then poof...
>
>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.
>> >But a darlington comes with a much higher VCEsat, and large voltage
>> >drops in series with the motor are not a good idea at our usually
>> >low supply voltages.
>>
>> Ouch! .2 volts loss to almost 2 volts... starting at 3 volts
>> there wouldn't be much left... oh well, better stick with what
>> I've got, a few hundred ma for a couple ma drive isn't bad.
>
>Darlington VCEsat in this type of application should be around 0.8V
>- far more than the 0.1V to 0.2V of a BC337/327.
The .2 volts I refered to was the .1 volts per transistor loss I get
with regular transistors, zetex or otherwise. The darlington smt were
listed as .95 volts Vce sat, or almost 2 volts for both Q's, WAY too
much for good use. Like you said...
> 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.
Terry Newton
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