At 04:56 PM 12/3/96 +0100, Martin Nilsson wrote:
>Ben L Wirz wrote:
>
>>         You have used the L293D or the L293 to 1 Amp?  The L293 is rated
>> at .6 A and the L293D is rated at 1 Amp, or they can be bridged for
>> double capicity.  Can I also ask where you found the L293's if thats what
>> you used, I haven't been able to find them in anything less then 200 min
>> order.
>
>I'm afraid you should not try to increase the current capacity by
>paralleling outputs of bipolar devices, such as the 293. If you do,
>the maximum allowable current will be not be the sum, but the minimum
>of the individual maxima.
>
>The reason is as follows:
>
>Bipolar transistors have negative temperature coefficient,
>i.e. resistance goes down as they heat up. For two paralleled outputs,
>the one with lower on-resistance will take more current, and will heat
>up more, resistance becomes even lower, etc. The positive feedback
>quickly causes current imbalance.
>
>MOSFETs are different, and have positive temperature coefficient, so
>from this point of view, their outputs can be paralleled. However,
>there can be other problems with paralleling MOSFETs.
>
>I have heard quite a number of reports of people blowing up their 293s
>by paralleling outputs, many of them from builders of the 6.270 board,
>where the assembly manual at one point suggested piggy-backing of two
>293s.
>
>Martin Nilsson                           http://www.sics.se/~mn/
>Swedish Institute of Computer Science    E-mail: mn@sics.se
>Box 1263, S-164 28 Kista                 Fax: +46-8-751-7230
>Sweden                                   Tel: +46-8-752-1574
>
>

You can increase the current capability this way but you need to put a small
resistor in series with each bipolar device to level the current sharing.
You are correct that simple parallel connections will cause the current to
not share equally and devices will blow up. Also the total capacity will be
less than the sum of the capacity. There is also a possible problem due to
switching times if one device turns on before the others but the technique
will work to a point.

Larry G. Nelson Sr.
L.Nelson@ieee.org
http://www.ultranet.com/~nr