Re: Hey, Wingerr , got PWM ?? - Saab 9000 Bulletin Board - Saabnet.com
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Re: Hey, Wingerr , got PWM ??
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Posted by Wingerr (more from Wingerr) on Mon, 10 Sep 2007 07:20:47 Share Post by Email
In Reply to: Re: Hey, Wingerr , got PWM ??, udderslate, Mon, 10 Sep 2007 06:32:26
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My 93 has the continuously variable ACC control, instead of the dropping resistors, so I didn't measure discrete voltages at different operating speeds.
Are the blower motor dropping resistors wired on the high side or the low side of the blower motor? For the circuit to work it has to be in series on the low side; I'm not sure about the specifics of the non ACC Saab setup, but I would have figured it was connected on the high side. Just a thought, since I don't have any details on the older setup.
My current draw measurements on the blower motor at full speed on mine was about 18A, so you probably want to fuse it with at least a 20A rating. I replaced the stock 30A fuse with a 20A to better protect my controller, since I'd rather have advance notice of the motor draw going excessively high with a blown fuse rather than a blown controller.
All the heat would be generated at the transition time the transistor switches, so the drive should be a sharp as possible to turn it full on or full off quickly. If you do find the transistor heating up too much, you might try using a comparator like an LM339 instead of the LM324 op amp used in the circuit; switching time would then be a fraction of a uS intead of about 15uS. You can add external suppression diodes to better protect the transistor if that proves to be a problem too.

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