What does the Shunt do?

[Steve SR]

Trying to understand my electrical system better (I had it professionally done and helped), but can someone in simple airline pilot/dummy language explain what the Shunt in the Skyview HDX system does and is for?

I know it's not a fuse or a CB, we've got it wired Low off the + master solenoid and High from the Alternator CB Load (Rotax external alternator), starter is not in the sequence.

TIA

 

[Rhino]

Your aircraft produces much more current than your EMS can handle, and it would very likely fail if you put all that current through it. The shunt is wired in parallel, so it takes most of the current, and the EMS only receives a smaller amount.

 

[Steve SR]

Ok, so it's kinda like a reducer so the wires can go to pin 24/25 to read voltage or amperage?

 

[maartenversteeg]

Its the basic electricity that you can never measure either current or voltage completely separate, you always need a small current to measure voltage and you need some voltage to measure current. The simplest story is that the relative large current (number of amps) flows through the small shunt resistance (say 60 mOhm) and that flowing current created a small voltage drop over the shunt. Then the EMS (pins 24/25) measure that voltage created by the shunt resistance. 60 mOhm and a current of say 8 Amp creates a voltage drop of 480 mVolt (Ohms law) over the shunt and that voltage can be measured by the EMS. The EMS input is a relatively high impedance input and only such a small part of the current flowing through the shunt going into the EMS, that this can be neglected, the EMS mostly measures the voltage over the shunt.

 

[Rhino]

Ok, so it's kinda like a reducer so the wires can go to pin 24/25 to read voltage or amperage?

It acts that way, yes. As @maartenversteeg pointed out, the true story is more complicated, but you seemed to want a simplified interpretation.

 

[huntaero]

The shunt is calibrated and the voltage drop from one side to the other is used to show amps the more current that goes thru the shunt the higher the voltage drop or calibrated in amps

 

[cbretana]

It's analogous to measuring flow in a pipe by measuring the drop in pressure from one point in the pipe to another that is a fixed, known distance downstream from the first point. As the flow increases (imagine a valve on the output of the pipe), the pressure drop across a fixed segment of the pipe will increase, in a linear fashion.

 

[huntaero]

consider volts as pressure and amps as flow makes it easy to visualize.

 

[cbretana]

Yes, this is not just an Analogy. It is exactly what Voltage and Current are. Current is the number of electrons that move through the wire per unit of time, and voltage is the force that is pushing them. Ohms's law makes that clear. Current = Voltage [V] / Resistance[R], or V= IR

 

[Steve SR]

Thanks everyone, I'm getting it now.

30 years ago I missed 22 out of 25 questions on my first airline ground school open book take home exam on the electrical system, it's always been a weak spot for me and I find that if I go back to crawl-walk-run on things I don't understand I learn it faster.

 

[PhilzSkyHdx]

Thanks everyone, I'm getting it now.

30 years ago I missed 22 out of 25 questions on my first airline ground school open book take home exam on the electrical system, it's always been a weak spot for me and I find that if I go back to crawl-walk-run on things I don't understand I learn it faster.

Find yourself a Basic Electricity course and all this should become quite obvious.

here's a more detailed explanation:
(Warning: this is AI generated so check the installation manual for accuracy)
To indicate 13.6 volts on a general-purpose pin (GP input) of the Dynon SV-EMS-220, the sense voltage applied to the pin must be scaled down using a voltage divider to roughly 1.78 volts. Dynon’s general-purpose inputs are designed for a maximum of 5V, so they require an external resistor to safely measure higher voltages.

How to calculate and wire:
The Math: Dynon uses a piecewise linear calibration where the pin measures a fraction of the total voltage. For 13.6V, you are dropping it to a nominal value between 1.5V and 2V.

The Setup: Connect a 10 kΩ resistor in series with the positive voltage source to the pin to step it down.

Calibration: In the SkyView HDX setup menu (SETUP MENU > HARDWARE CALIBRATION > EMS CALIBRATION), you map the raw voltage input of the pin to read exact Volts (e.g., 13.6 V) by adding calibration points.

 

[Marc_J._Zeitlin]

here's a more detailed explanation:
(Warning: this is AI generated so check the installation manual for accuracy)

More detailed, sure. Accurate, no.

Page 7-44 of revision AT of the Skyview HDX installation manual very clearly indicates that all you have to do to measure VOLTAGE at one or two locations in your electrical system (I use the cold side of the master solenoids for my two buses) is hook up pins 1 and 2 on the D37 connector to those sources and call it good. No resistors, no calibration, nada. The system understands that it's measuring voltage on those pins.

All the AI stuff above is, to put it bluntly, innacurate and not useful. If you want to use OTHER D37 pins than Dynon says should be used for measuring voltage, sure - maybe some of that will apply, but a review of the installation manual will say for sure, and merely telling folks that they should vet what you're posting is inappropriate - don't post it if you haven't vetted it yourself.

This is just another data point indicating that using AI without vetting the output with the original source is GIGO. No one should be posting AI output without ensuring that they've vetted it for accuracy.

And with respect to the shunt and measuring current flow, @huntaero nailed it simply and accurately.

 

[PaulSS]