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Current limiter used as ammeter shunt

 
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user9253



Joined: 28 Mar 2008
Posts: 1907
Location: Riley TWP Michigan

PostPosted: Mon Feb 02, 2015 5:52 am    Post subject: Current limiter used as ammeter shunt Reply with quote

Considering that many modern EFIS ammeters can be calibrated to match any shunt, is it feasible to use the alternator current limiter (ANL fuse) as a load-meter shunt? If so, one device could serve a dual purpose.
The LittleFuse MEGA line of fuses are much less expensive than the ANL. Are the MEGA fuses just as good as the ANL?
http://tinyurl.com/LittleFuse-MEGA
Thanks, Joe


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PostPosted: Mon Feb 02, 2015 8:34 am    Post subject: Current limiter used as ammeter shunt Reply with quote

That's a really interesting question, Joe -

Shunts are designed to maintain their highly-calibrated resistance over a wide temperature range. I don't know if that is true for current limiters, since their mission is very different. Also, I wonder if:


1. a current limiter's response is linear
2. their resistance is consistent from manufacturing batch to batch
-Jeff


On Monday, February 2, 2015 6:01 AM, user9253 <fransew(at)gmail.com> wrote:



--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>

Considering that many modern EFIS ammeters can be calibrated to match any shunt, is it feasible to use the alternator current limiter (ANL fuse) as a load-meter shunt? If so, one device [quote][b]


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user9253



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PostPosted: Mon Feb 02, 2015 11:36 am    Post subject: Re: Current limiter used as ammeter shunt Reply with quote

Jeff, thanks for the input. Manufacturing variations would not matter because the EFIS ammeter would be calibrated to match the installed fuse. You are probably right that a current limiter resistance might not be linear as temperature and current vary. If the ammeter were an important instrument, then accuracy would be a concern. Others might not agree, but to me, the ammeter is not important. The voltmeter tells me that that alternator is working. Accurately measuring current and troubleshooting is best done on the ground. Even if the aircraft ammeter is inaccurate, the pilot will learn what is normal under certain conditions. If the voltage drops and the ammeter shows high current, then there is a short circuit some place. If the ammeter indicates low or no current, then the alternator has failed. If no dedicated shunt is installed, then there is one less thing to fail.
Joe


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PostPosted: Mon Feb 02, 2015 11:59 am    Post subject: Current limiter used as ammeter shunt Reply with quote

At 07:52 2015-02-02, you wrote:
Quote:


Considering that many modern EFIS ammeters can be calibrated to
match any shunt, is it feasible to use the alternator current
limiter (ANL fuse) as a load-meter shunt? If so, one device could
serve a dual purpose.
The LittleFuse MEGA line of fuses are much less expensive than
the ANL. Are the MEGA fuses just as good as the ANL?

To be a calibrated 'shunt' the temperature coefficient
of resistance would have to be very close to zero.
Just over a century ago, the grandfather of precision
measurements . . .

http://tinyurl.com/3f79fgy

. . . offered up an improvement on his earlier
low tempco wire (Constantan) dubbed Manganin

http://tinyurl.com/ln5q4at

This material is favored for use in ammeter
shunts and precision resistors. Fusing alloys
are modified for their I(squared)R melting
characteristics. fast blow fuses (like a
glass, AGC1) probably have a higher tempco
than say a current limiter with robustness
approaching copper wire.

Using the copper in a b-lead isn't a 'terrible'
idea . . . it's certainly no less useful
than the legacy -0+ battery ammeters used
in millions of vehicles.

Ammeters are generally not a flight operations
or diagnostics instrument. If the bus volts
are up, all is right with the universe. If
the bus volts are too low, then implementation
of plan-b is called for. The precise value
of ANY current in the system may be interesting
but it's of little use in comfortably
terminating your flight.

Bob . . .


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PostPosted: Mon Feb 02, 2015 12:39 pm    Post subject: Current limiter used as ammeter shunt Reply with quote

Joe,

1. "Manufacturing variations would not matter because the EFIS ammeter would be calibrated to match the installed fuse.
On Monday, February 2, 2015 11:54 AM, user9253 <fransew(at)gmail.com> wrote:
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>Jeff, thanks for the input. Manufacturing variations would not matter because the EFIS ammeter would be calibrated to match the installed fuse. You are probably right that a current limiter resistance might not be linear as temperature and current vary. If the ammeter were an important instrument, then accuracy would be a concern. Others might not agree, but to me, the ammeter is not important. The voltmeter tells me that that alternator is working. Accurately measuring current and troubleshooting is best done on the ground. Even if the aircraft ammeter is inaccurate, the pilot will learn what is normal under certain conditions. If the voltage drops and the ammeter shows high current, then there is a short circuit some place. If the ammeter a new current limiter. (granted, we hope that doesn't happen very often)Also - how would you calibrate the current limiter?

2. "Others might not agree, but to me, the ammeter is not important."
If the ammeter is not important and it is not displaying accurate info, then why bother with it? 3. "If no dedicated shunt is installed, then there is one less thing to fail."There's not much that can go wrong with a shunt (except maybe melting - and if that happens, you've got other problems) It's a solid-state hunk a calibrated metal and therefore very unlikely to fail. (it's probably one of the most reliable electrical components in the airframe)

4. For the scenarios you outline above, it seems that the voltmeter gives the appropriate indications.

I like ammeters for a few of reasons:
1. In a battery-only situation, they help the pilot confirm that electrical loads have been appropriately minimized so that you will achieve the designed endurance. In this case, you want fairly accurate information because a few amps might be critical to endurance calculations.
2. To the trained eye, they can provide hints to other problems
3. They can help make testing & trouble-shooting easier
-Jeff

[quote][b]


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PostPosted: Mon Feb 02, 2015 4:41 pm    Post subject: Re: Current limiter used as ammeter shunt Reply with quote

Quote:
how would you calibrate the current limiter?

I thought that could be done in the EFIS software.

Quote:
If the ammeter is not important and it is not displaying accurate info, then why bother with it?

If the EFIS has the ammeter feature but a builder does not want to install a shunt, then maybe the ammeter could be connected to the alternator fuse. I was not recommending that, only asking if it was feasible.

I did see an electric hi-low battery-charger shunt fail at work. The likely cause was a loose connection that generated heat.

Jeff, I can not argue with your reasoning. Everything you wrote makes sense.

Your response and Bob's indicate that using a current limiter as an ammeter shunt is a bad idea. Look at the good side. This topic filled a void on the AeroElectric list between more important questions. Smile
Joe


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