Today's Message Index:
----------------------
1. 06:55 AM - Re: Shunt Questions (user9253)
2. 09:50 AM - Re: Shunt Questions (Eric M. Jones)
3. 10:20 AM - Re: Re: Shunt Questions (Robert L. Nuckolls, III)
4. 10:56 AM - Re: Re: Shunt Questions (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Shunt Questions |
See the attached picture from Dynon's installaiton manual. Is the shunt
located in position A or B or C? When the shunt is located in position "A",
which measures current into or out of the battery, as you know, very little
current flows through the shunt when the battery is fully charged.
The following pertains to a shunt in position "A". The engine needs to be
off while troubleshooting. If the electrical system operates normally, the
shunt must be good. If the shunt has a higher than normal resistance,
then there would be a larger voltage drop across the shunt. This would
show up as lower system voltage and a higher ammeter reading.
Since the electrical system operates normally, I suspect a bad connection
between the shunt and EFIS, or a problem within the EFIS.
Even though the EFIS displays amps, it actually measures millivolts.
I would test the EFIS by applying a millivolt signal to the ammeter input.
This millivolt signal could be provided by a resistor network or by an almost
dead flashlight battery.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=428262#428262
Attachments:
http://forums.matronics.com//files/shunt_location_143.jpg
Message 2
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| Subject: | Re: Shunt Questions |
Shunts and Hall Effect sensors.
Hall effect sensors seemed to be quite nice since they were small and could directly
interface with other electronics. But it is harder and harder to justify
their use over a simple shunt.
As for troubleshooting a shunt, cracks can be very hard to detect, so bolting another
in its place is a good approach. Shunts are ultra-reliable, but everything
fails sometime.
There is another way to measure current, and that is to measure the voltage at
each end of the wire in question. This is done by running a small (like AWG 26)
wire from each end of the high-current conductor (whose current you want to
know) to some voltmeter. The technique works because ESSENTIALLY NO CURRENT flows
in the tiny sense wires. The voltage sensor then measures the voltage drop
through the fatwire. Of course, you need to know something else, like the true
current/voltage drop, or true fatwire resistance. But this technique is the
preferred way for really big and reasonably long power conductors.
And there is no separate shunt or Hall Effect sensor.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=428270#428270
Message 3
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| Subject: | Re: Shunt Questions |
There is another way to measure current, and that is to measure the
voltage at each end of the wire in question. This is done by running
a small (like AWG 26) wire from each end of the high-current
conductor (whose current you want to know) to some voltmeter. The
technique works because ESSENTIALLY NO CURRENT flows in the tiny
sense wires. The voltage sensor then measures the voltage drop
through the fatwire. Of course, you need to know something else, like
the true current/voltage drop, or true fatwire resistance. But this
technique is the preferred way for really big and reasonably long
power conductors.
And there is no separate shunt or Hall Effect sensor.
I have used this technique in several applications
where calibration is not an issue. The ammeter shunt's
resistor is made from a special alloy7 called 'manganin'.
http://tinyurl.com/3f79fgy
This is an alloy crafted both for it's practical
resistance and very low temperature coefficient of
resistance. The ordinary conductors used to wire
things up tend to have higher temperature
coefficients. Which makes their use as an ammeter
rather squirrely. As the wire heats up, it's temperature
goes up, resistance goes up, power dissipated goes up,
temperature rises some more and so it goes. If one
plots current versus voltage drop in a wire in free-
air (waiting for temperature to stabilize after each
new current setting), you get a rather non-linear curve
that isn't even repeatable except when the ambient
conditions in which the wire operates are held constant.
One practical way to use wiring as a 'shunt' is to
detect lamp failure or perhaps failure in some other
load like pitot heat. In this case, it is sufficient
to simply know that current is flowing in the
wire . . . or not. It is not necessary to know how
much current flows.
Bob . . .
Message 4
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| Subject: | Re: Shunt Questions |
Even though the EFIS displays amps, it actually measures millivolts.
I would test the EFIS by applying a millivolt signal to the ammeter input.
This millivolt signal could be provided by a resistor network or by an almost
dead flashlight battery.
Even an 'almost dead' battery is probably too much . . .
Suggest you craft a millivolt test source from a couple
of resistors and a good flashlight cell. Any other combination
of resistors can be used as long as you hit the 'output
target' in the upper half of a 50mV full scale input
to the EFIS.
Emacs!
This is not an 'accurate' source, only a practical
source for carrying out a divide-and-conquer experiment
to isolate your fault.
Bob . . .
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