Re: Blown fuse puts circuit on ground?
- From: Timberwoof <timberwoof.spam@xxxxxxxxxxxxxxx>
- Date: Mon, 23 Jun 2008 20:54:56 -0700
In article <IqKdnSzBeeqgh_3VnZ2dnUVZ_vqdnZ2d@xxxxxxxxxxx>,
timeOday <timeOday-UNSPAM@xxxxxxxxxxxx> wrote:
Rob Kleinschmidt wrote:
On Jun 23, 11:29 am, timeOday <timeOday-UNS...@xxxxxxxxxxxx> wrote:
I installed a 12V plug last night and it took a lot longer than it
should have. I blew a fuse near the start of the job (by leaving my
voltmeter set to resistance when I wanted to measure voltage) but didn't
notice, and once it occurred to me, thought what I was seeing ruled that
out.
With fuse in replaced and ignition on, it is now 0V from positive
battery terminal to the lead, and 12V between the lead and ground. With
ignition off, 0V between lead and ground. All just what you would expect.
But with the blown fuse, it was 12V between the positive battery
terminal and the lead, and 0V between the lead and ground, with or
without the ignition on. This was with the other end of the lead
disconnected from everything (heated grips in this case). I kept
looking for a short but never found it. Finding and replacing the blown
fuse appears to have fixed it.
What gives? It's as if the blown fuse somehow connected the lead to
ground.
Sounds like it's just telling you it's not seeing voltage, same
as it would read if it were not connected to anything at all.
No, that's just it. With the fuse blown, I would have expected to see
0V between the positive battery terminal and the lead, as if it were
connected to nothing. Instead I saw 12V, as if the lead were connected
to ground.
It was: through the heated hand grips.
Check for continuity or ohms and I'd bet you see infinity.
Are there any special considerations checking continuity between
different voltages? If I measured resistance on a pair of battery
terminals, would that a) measure internal resistance of the battery;
b) give a nonsense result, since the multimeter applies a small voltage to
measure resistance but the battery already has a much larger voltage; or
c) effectively short-circuit the battery destroying both it and my
multimeter :)
Probably B and C. (But the battery had better survive having an Oh!
meter connected to it. It will just cause the magic smoke to escape the
Oh! meter.)
To measure the internal resistance of your battery, you have to get
comfortable with Ohm's Law. Then it gets complicated, for you have to
measure the battery voltage at two current levels. You can measure the
current your headlights (or just running lights) draw and the voltage
across the battery terminals in that state. Then measure the battery
voltage as you have the starter crank the motor. The battery voltage
will drop considerably (to ~9V), and somewhere I have notes on how to do
the math that translates that to batter internal resistance. But that's
just academic. If the battery isn't holding a charge, replace it: you
don't need to know it has a high internal resistance.
You don't want to check *continuity* between different voltage levels!
Turn the bike off before you do that. And use the Oh! meter's lowest
resistance setting; that will let you notice things like the resistance
of a headlight (~1 Ohm).
--
Timberwoof <me at timberwoof dot com>
faq: http://www.timberwoof.com/motorcycle/faq.shtml
Ten Steps to Fascism: http://www.guardian.co.uk/usa/story/0,,2064157,00.html
.
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- From: timeOday
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- From: Rob Kleinschmidt
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