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  #1  
Old 05-21-2018, 01:15 AM
Yen Yen is offline
 
Join Date: May 2010
Location: Benaraby Queensland. Australia
Posts: 180
Default Fuses

I thought I was smart and put an LED across some of the fuses om my RV4. The idea being that if the fuse blows, there will be a very slight current flowing lighting the LED.
On Saturday I had what appeared to be a dead flat battery so I removed the battery, took it home and charged it. All good 13 plus Volts and reinstalled it on Sunday. Still appears dead flat.
I have a manual master switch at the battery and another master on the panel, which feeds several buses. I couildn't see a lit LED and I got a multimeter to read 13V at the input to the fuse and also at the output from the fuse and at the main bus, but to all intents the battery was dead.
3 hours of nutting it out and looking for a fault before I removed the fuse and would you believe it was blown, looked good but no continuity.
I was reading volts at the bus, but it was the miniscule current coming through the LED that I was reading.
I still think it is a good idea to parallel the fuse with a LED, but they are hard to see in bright sun and will allow current flow.
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  #2  
Old 05-21-2018, 06:59 AM
Mich48041 Mich48041 is offline
 
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,693
Default

Van's sells fuses with built in LEDs. Look in their "The List".
ES-00203 FUSE ATC 3 AMP LIT $1.25
ES-00205 FUSE ATC 5 AMP LIT $1.00
ES-00207 FUSE ATC 7.5 AMP LIT $1.00
ES-00210 FUSE ATC 10 AMP LIT $0.85
ES-00230 FUSE ATC 30 AMP LIT $1.20
Your post reminds me of another builder who was troubleshooting an electrical problem with a digital meter that showed voltage even though the circuit was not working right. I suggested that he buy and use a test light instead of the meter. The test light should be the kind that does not have a built-in battery, just a lamp. So he did and quickly found the problem. Digital meters are great but require understanding the difference between high and low impedance circuit testing.
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  #3  
Old 05-21-2018, 01:38 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 5,331
Default

Just so everyone is clear: the LEDs you are talking about must have built in resistors (or other current limiting devices).
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  #4  
Old 05-21-2018, 02:31 PM
Mich48041 Mich48041 is offline
 
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,693
Default

Yes, Van's "LIT" fuses have a built-in resistor in series with the LED. Like Bob said, if use a separate LED, a resistor is required. To get the maximum brightness without damaging the LED, the resistor value needs to be calculated. There are online calculators just for that purpose. You must enter the aircraft system voltage and the LED current and LED voltage drop. Or you can wing it and use a 680 ohm resistor.
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  #5  
Old 05-22-2018, 02:03 AM
Yen Yen is offline
 
Join Date: May 2010
Location: Benaraby Queensland. Australia
Posts: 180
Default

The LEDs I used are designed to go into a 12V circuit. the reason I posted was to alert others that a multimeter can give a reading that is not strictly correct.
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  #6  
Old 05-22-2018, 10:38 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,067
Default This thread's not really about fuses...

And there seem to be some misconceptions, too.

Note that if you place a '12volt LED' (LED+current limiting resistor in series) across a fuse holder, it will light if the load side shorts to ground and the fuse blows.

But.

You should measure little to no voltage on the load side of the fuse/LED, since that side is now shorted to ground. A partial short to ground (bad enough to blow the fuse, but not a dead short) *might* light the LED and still allow measurable voltage on the load side of the fuse/LED pair.

And.

Assuming a single load on the fuse: If you then remove the load side short and the load itself (as in, disconnect the bad component that shorted to ground), the LED will no longer light. There is no longer a complete current path back to the power source. In this case, you *will* measure very near the supply voltage on both sides of the LED.

So.

If, during troubleshooting, the fault is cleared by removing the defective component, you may get measurements you don't expect, if you're not well acquainted with electrical properties.

Charlie
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