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Breaker instead of shunt ? (Read 328 times)
Oct 3rd, 2017 at 1:23am

Grizzly   Offline
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France LFOP airport

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Bonjour,

I do not know if this is a good idea, instead of the shunt connected between Amps pins 24 and 25 of the EMS DB37 connector, I would like to use a main 60A thermal breaker (in C position of the schematic on page 7-62 of the Skyview System Installation Guide V15.0). This one has a slope of 1.6 mV / A which must be corrected in the file of definition of the sensors.
If the circuit-breaker is opened, the voltage between the Amps + and Amps- pins could reach 15V (12 V installation circuit).
Would the EMS inputs support these voltages?

Indeed the pull-up resistances of the Amps inputs will have to dissipate a power that I can not estimate without having knowledge of the electrical circuit.

What is the risk of re-supplying the circuit through these inputs with and without back-up battery ?

Note that the question also arises when opening the standard shunt.

Another questions:
Can I use 0.1 A Resettable PTC (like Littelfuse RXEF005) instead of the 1A fuse to protect the inputs Amps + & -?
Is there a bidirectional TVS (transient voltage suppression) component between the pins Amp + & - ?
If none, can we put a "PI" configuration consisting of 2 PTC and a bidirectionnal TVS?
 
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Reply #1 - Oct 3rd, 2017 at 10:16am

Dynon Support   Offline
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The challenge in using anything besides a shunt will be accuracy over temperature. The reason you don't just use a copper wire is because between -40C and +60C, Copper changes resistance almost 2X. So on a cold day 40A might read as 30A and on an hot day it will read 55A. A properly designed shunt on the other hand is about 0.1% over 100C.

Unless you have deep data on how that breaker's resistance over temperature, we would suggest not using it. It likely will change resistance quite a bit over temperature, and thus your ammeter will be inaccurate.

The EMS inputs are protected to +65V individually. So you won't hurt the EMS if the breaker opens.

You can use a PTC if you want as long as the on resistance is below 5 ohms. More than 5 ohms resistance will cause errors in the current reading. There is about 2Kohms differential resistance and 5Mohm common mode to ground.

There is no protection device between pins 24/25.. What is your goal with putting TVS here?
 

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Reply #2 - Oct 4th, 2017 at 1:05am

Grizzly   Offline
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France LFOP airport

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Dynon Support wrote on Oct 3rd, 2017 at 10:16am:
... What is your goal with putting TVS here?
Thank you for your answers. I plan to use a LifePo4 battery, in the event of an alternator or regulator fault this battery can be damaged. I have an emergency switch that isolates the battery through the negative terminal. With the absence of a battery the alternator/regulator may cause the voltage to rise to a very high value: this is the reason of the TVS to this place.
 
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Reply #3 - Oct 4th, 2017 at 9:44am

Dynon Support   Offline
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Most people with LiFePO4 batteries come up with a way to make sure the alternator goes offline if it fails. If you take the battery out of the circuit, then the alternator will go to hundreds of volts. In this case, the amps input on your EMS is the least of your worries since all your avionics will see this on the power inputs.

The standards for aircraft only require survival to 30V on a 12V airplane, so you really need to make sure your system stays within this at all times. Given our amps input can go to 65V, it really doesn't need additional protection.
 

Please do not use Private Messaging on forum to contact. For private support:
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Reply #4 - Oct 9th, 2017 at 5:54am

Grizzly   Offline
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France LFOP airport

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For fun and information, I made measurements on the circuit breaker I use. The accuracy is about +/-12% over a range of -10C to 50C. For me this is enough even if it is far from the accuracy of the dedicated shunt.
... ... ...
Note that the slope is 1.33mV/A.
« Last Edit: Oct 9th, 2017 at 5:56am by Grizzly »  
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