AeroElectric-List Digest Archive

Thu 07/18/13


Total Messages Posted: 1



Today's Message Index:
----------------------
 
     1. 08:54 PM - Battery capacity tester- Why isn't there a flyback diode? (Mark Todd)
 
 
 


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    Time: 08:54:16 PM PST US
    From: Mark Todd <motodd@frontier.com>
    Subject: Battery capacity tester- Why isn't there a flyback
    diode? I finally got around to building Bob's battery capacity tester and, as a long-time lurker on the Aeroelectric list, I'm primed to slap a 'flyback' diode on every switched inductive load I find. But, the schematic for the battery capacity tester ( http://www.aeroelectric.com/articles/battest.pdf ) doesn't appear to have one. My understanding of semiconductors is pretty weak but the datasheet for the NPN transistor I happen to be using (2N5961) says the max collector-base voltage allowed is 60V. Although the inductive load in this case doesn't appear to be huge (my relay coil draws about 36mA at 12.4 V), it seems to me like I might be able to do some damage to my transistor. Is there something about the behavior of the Zener or the transistor that moderates the flyback voltage effect? I don't see any harm in adding a diode, but I'm just trying to learn. Do I need to a diode across my coil before I actually use this thing? Mark




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