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motodd(at)frontier.com
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 Posted: Thu Jul 18, 2013 7:54 pm Post subject: Battery capacity tester- Why isn't there a flyback diode? |
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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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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Fri Jul 19, 2013 5:17 am Post subject: Re: Battery capacity tester- Why isn't there a flyback diode |
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There probably should be a "flyback" diode there. Actually I've been searching the antique shops for an AC plug-in clock and haven't been able to find one.
This issue of EDN has a plethora of battery-related designs, among which is a similar battery-tester. See attached.
http://www.edn.com/design/analog/4368112/Circuit-measures-battery-capacity?elq=e82db0a50e5244e5961468cc2f470404&elqCampaignId=258
ps: The 787 has yet ANOTHER battery problem. Yikes!
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Battery Tester.pdf |
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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