SDS CPI Tricks and Tips

[rv6ejguy]

We can configure for a remote LOP switch on the CPI-2.

[onebounce]

Originally Posted by rv6ejguy

We can configure for a remote LOP switch on the CPI-2.

Thanks Ross, much appreciated 👍

[akschu]

I'm looking at the wiring diagram for the CPI-2 and I don't understand why the coil and ECU power wires need to be switched, but not the backup battery.

The docs say that the backup battery draw is minimal, but how does the system know it's off?

In the documentation it says:

It is important to switch both purple and red wire for proper operation and to also keep the backup battery from slowly draining current when the system if shut off. Also allows proper power-off operation.

So, if in normal operation, both the purple and red wire loose their power, it doesn't go to battery backup? It just switches off?

Also, why does this system need 3 fuses (6 for a dual)? Can't I use one fuse to power it (red and purple) and one fuse for backup battery? I don't understand why the ECU and coil are powered separately when loosing either would make the ignition fail.

[David Carter]

Originally Posted by akschu

The docs say that the backup battery draw is minimal, but how does the system know it's off?

. . .

So, if in normal operation, both the purple and red wire loose their power, it doesn't go to battery backup? It just switches off?

It does not switch off if the primary power is lost in flight. The CPI has a tach pickup, so knows that the engine is turning and that its job is to keep it turning. Therefore, it goes to the backup battery. When shutting down on the ground, there is no RPM, so it does not automatically switch to backup power when the main power is cut.

[akschu]

Thanks for that. I see now in the documentation how it works:

Power down of the CPi2:
Due to the ecu having its own backup battery, power down is unique on the CPi2. When you turn off Mainbus 12 volt power to the CPi-2 it automatically switches over to its backup battery. If there is no engine rpms, and no keypad
keys are pressed, it will begin a power off countdown within 5 seconds, and after counting down from 10 to 0 it willshut off unless Mainbus power is switched back on. So total power off time is approximately 15 seconds.

Now I would like to know why it needs 3 fuses (7 if you include the built in ones). That seems pretty excessive:

If the documentation shows a double pole switch to power the coil and ECU at the same time, why not use a single switch and fuse to power both:

Power<-> fuse <-> switch <-> ECU and COIL

Why the extra wires, fuses, and larger switch?

[Ralph Inkster]

The 2 power wires have different draws, therefore they should be protected with separate fuses/breakers rated for each circuit. Do not tie the 2 feed wires together & try to protect a 20g wire with a 10 or 15A fuse!
You have a double poled switch (or 2 ganged switches) so you can switch both circuits off during power down. If one switch was left on by accident, your BU battery circuit would read it open & drain it down.
The mini fuses in the CPU housing on the other hand are protecting the internal components, not the wires.

[David Carter]

Originally Posted by Ralph Inkster

Do not tie the 2 feed wires together & try to protect a 20g wire with a 10 or 15A fuse!

That's a good point. Would there be anything wrong with replacing the purple wire with a wire with the same gauge as the red wire?

[Ralph Inkster]

Talk to Ross on that.
Kind of like feeding a Comm with a 6G wire?

[akschu]

20g can't carry 10 amps sustained over any real length, but it can carry 10amps for around 3 feet as the voltage drop is around 1/2 volt which is 6 watts. If the run between the red and purple wires to the switch was only 2 feet, there wouldn't be a problem from a protect the wire prospective.

Here is the actual answer:

https://vansairforce.net/community/s....php?p=1445172

Basically they are separating the ECU power from the coil because the coil pulls a bit of current and they are worried about noise on that line.

However, the counter point is that having two switches (even if they are in the same body), two fuses, two wires, and 10 connections that all need to work in order for the system to not fail over is not making it less likely to fail, rather more likely.

Another point is that this mitigation doesn't exist on the battery backup side. The coils and ECU will draw from the battery over the same 18 gauge wire when in fail over mode.

Now that I know what the actual issue is I can choose how I want to wire it, and I have a scope so I can see how noisy it actually is.

[F1 Rocket]

1 year/85 hrs since installing the CPi-2. Everything is wired, installed and set up per the manual. Everything works as it should. Great system with great support.