X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f
X-Recipient: geda-user AT delorie DOT com
X-Authority-Analysis: v=2.0 cv=SqgSGYy0 c=1 sm=0 a=6jktZp3dcHAl1vye2O6wCg==:17 a=O7eMSDLu23oA:10 a=_zQ4Cw4pZVQA:10 a=6WB07kdHjWAA:10 a=8nJEP1OIZ-IA:10 a=CkRcVXnbn8YahHUfFlgA:9 a=wPNLvfGTeEIA:10 a=6jktZp3dcHAl1vye2O6wCg==:117
X-Cloudmark-Score: 0
X-Originating-IP: 70.113.67.117
Message-ID: <4EF23C5C.70108@ecosensory.com>
Date: Wed, 21 Dec 2011 14:06:52 -0600
From: John Griessen <john AT ecosensory DOT com>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.24) Gecko/20111114 Icedove/3.1.16
MIME-Version: 1.0
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Design suggestions...
References: <CALSZ9godJsJDhBd_PaCiWPx9M8GyKz8PW1wVUs6NxVgtrOS-Fw AT mail DOT gmail DOT com> <201112211816 DOT pBLIGsaf016475 AT envy DOT delorie DOT com> <4EF229B4 DOT 8050008 AT ecosensory DOT com> <201112211909 DOT pBLJ9GKo018193 AT envy DOT delorie DOT com> <4EF2343D DOT 10601 AT ecosensory DOT com> <8D154A5F-9D7A-446E-AA98-8D0582C8B40D AT noqsi DOT com>
In-Reply-To: <8D154A5F-9D7A-446E-AA98-8D0582C8B40D@noqsi.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Reply-To: geda-user AT delorie DOT com

On 12/21/2011 01:55 PM, John Doty wrote:
> But if you turn the FET fully on, it will charge the cap up to the supply voltage, defeating the PWM.

Yes.  I was thinking of an R-C at the motor, but had Rob's meaning confused.

>
> Usually with PWM you use inductance (possibly intrinsic to the thing you're driving)

to smooth the current, and let the voltage swing rail to rail, with larger excursions prevented by diodes.

Yes, the R in series with the motor was just a goofy idea.  And the body diodes will have to take any
voltage from the motor inductance.  So there's no short cut around calculating the motor
inductance kick, or experimenting to see how hot your drivers get to be sure they are not burning up.