Hi all,
All the recent talk about circuit protection made me remember one of my
high-school electronics lessons, and that led me to do some experiments
on my ammeter, which is one of the 50A range ones that are sold by Rick
and others.
I took my Flamethrower II 0.6 ohm coil and wired it in series with the
ammeter, then connected that combination straight across a 12V battery.
The voltage developed across the ammeter was 0.308 volts, average, and
the battery voltage was 12.5 volts before and about 12.4 volts after the
test. The needle on the ammeter did register about 20A draw, by the way
- all things more or less add up.
So what's interesting about that? Assuming that the resistance of the
meter movement is linear between the 20 and 50 amps, we can work out
that at 50 amps, we'll be losing about 0.75 volts across the meter
movement (and incidentally, generating about 37 watts of heat - we're
all agreed that with ammeters, you definitely don't get something for
nothing). But 50A charging current is not a normal occurrence, and
usually this is an indication of something bad happening elsewhere, so
this extreme condition normally wouldn't happen for long, and if we can
limit the ammeter current to below that range, then we can prevent both
damage to the ammeter, and the potential for having some very hot places
under the dash. As it happens, diodes start to conduct (depending on
their exact construction) in the range from 0.3 to 0.7 volts, and we can
use this feature to limit the ammeter current to something that's
non-destructive: we just place an appropriate diode across the ammeter
terminals. If the ammeter current gets too high, then the voltage across
the ammeter reaches the point at which the diode starts to conduct, and
the diode will then pass all the excess current, rather than letting the
ammeter take it. The ammeter will simply indicate a current that is
something less than full scale. To protect the ammeter in both
directions, we put two diodes in - one facing in each direction, so we
end up with the circuit depicted in
<http://members.shaw.ca/tsmit/alternator/ammeter.gif>.
I can order in diodes (I'm looking at the 70HF series from International
Rectifier, for those who want to look it up) in bulk, and I will build
modules that you can screw onto the back of the ammeter for $40 US,
provided there is any interest.
By the way: Some people were apparently confused by my comments about
fusible links. They are not a replacement for anything you're currently
using in your wiring harness - everything used to manage current
distribution, from fuses to relays to switches, has its own advantages
and therefore a proper use. Steve Laifman's illustration of how to use a
fused relay in a fan control circuit is a good example of how relays can
prevent excessive current draw through a switch, and the fuse protects
that circuit. But no fuse or relay can protect the connection between
the solenoid and the rest of the electrical system - there's just too
much current normally flowing through it, either for charging the
battery if the engine is running, or for operating the electrical
accessories if the engine isn't running. A fusible link, properly
installed in this circuit, will not blow unless there is an extended
short circuit, such as could happen if you are involved in a crash. In
that case the fusible link provides a predetermined weak spot in this
otherwise very beefy circuit, and allows the current path to be broken
in a place where it will do the least harm.
Best regards,
Theo Smit
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