>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 19 August 2005 17:04
>To: Microcontroller discussion list - Public.
>Subject: Re: re [EE] Design Challenge - A better H Bridge MK2
>
>
>Russell McMahon wrote:
>
>> Do you like the brake circuit ? :-)
>> 
>>     http://russell.servepics.com/temp/r100b.gif
>> 
>> Too many parts, but it works well.
>
>I'm sure it does -- if you say so :)
>
>I can't figure out why it should, though. How is the basis of 
>Q44 ever 700 mV more negative than its emitter?

When Q43 is conducting, C42 is charged via D43.  When Q43 is switched off, R41 pulls the negative side of C42 up to the positive supply rail, so the positive side of C42 (and also the emitter of Q44) is at a potential of around double the supply voltage.

It's a fairly classic voltage multiplier circuit, though used in a "one shot" mode here.

Regards

Mike

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