Alternative system diagram for SD8 back-up alternator?

Hello you all "sparkies" out there.

Maybe you can help me with the following alternative diagram that I made for the B&C SD8 back-up alternator (on vacuum pad). Unfortunately, B&C could not give me an answer on this, since their technician has left the company.


If the alternator is wired as proposed by B&C, the warning lite will only show if the Alternator is switched off or if the circuit breaker is tripped due to an over voltage situation. But I also want to know if the alternator quit charging ! Therefore I changed the diagram. I think this will work, but I need the expertise from one of the " Sparkies" out there to confirm this. If the alternator is not putting out any power, the relay should stay open and the light will be on. When the alternator makes power, the relay is pulled and the light goes off. The diode will avoid the battery to back-feed to the relay.

Please let me know if there is anything that I did not think of.

Regards, Tonny.

What warning do you really need?

The alternator could fail to provide enough voltage to charge the battery (brushes start to fail, regulator failure) but still provide greater than the pull-in voltage of the relay (10V?). This is not what you want. If you are relying on this circuit to warn of charging failure, you need to have a comparator circuit.
Most or all EMS provide this feature.
Of course, you could use a panel mount voltmeter.

I am installing a Dynon EMS, but it can only monitor one circuit (V + A), so that will be the main alternator.

I am also installing a Dynon EFIS and will connect the Aux. power supply to the aux. input so that I can monitor the voltage of the aux. system on the EFIS, but the EFIS will not put out an alarm if the voltage is low or the alternator is not charging! So that is why I am installing an alternator warning light. So, what you are saying is, that the alternator (actually it is a dynamo) may be failing, but still put out enough power to hold the relay in, so the light will not come on?

Regards, Tonny.

Assuming the Dynon EMS has a discrete V sense line, you should hook up the sense line to the battery bus (or essential bus). This is going to reflect the battery voltage, regardless of which alternator is powering it. This way, whichever alternator is enabled is checked for output voltage.

"So, what you are saying is, that the alternator (actually it is a dynamo) may be failing, but still put out enough power to hold the relay in, so the light will not come on?"
Yes, this is one possible failure mode. Sadly, alternators are not digital, they have many failure modes.

Ted,

Thanks for your input. Sorry, I did not mention this in my post before, but the SD8 will have its own battery. My plane is all electric (instruments and dual electronic ignition), that's why I am installing the SD8 as a back-up.

When I started designing my system, a Diamond Twin had an accident at take-off because the engines failed due to lack of electric power to the FADEC systems when they retracted the gear. That is when I thought I should have one of my ignitions on a completety separate system to avoid something like this. Also I wanted to be able to programm my GPS on the ground without draining the main (cranking) battery.

The only extra thing involved is a separate battery to make it a complete independent and separate systrem. I am planning on installing a small Lead-Acid-Gel-battery from a scooter (4 Ah) for this purpose. The Aux system (or call it system "2") will only run the second ignition and the GPS. For emergency situations there is a cross-feed switch.

One situation ,that would cause both ignitions to suddenly quit, would be an over voltage due to malfunction of the overvoltage protection of one of the alternators. I could not think of any other way to overcome this than two separate systems. Actually I think a single Alternator and dual batteries (maybe a slightly larger one than 4 Ah) would be enough for back-up, but there is no way to have separate systems that way.

But what is the use of a second system, if I do not know that it is working properly? That is why I want to monitor the second alternator separate from the main alternator. For the amperage I will install a second Shunt and a switch, to allow me to choose wich system I want to monitor (especially in critical situations).

Regards, Tonny.

Ahh, that makes more sense. Although, it's perhaps more common to have the second battery charging from the main bat bus through a schottky diode. In this case, only one voltage monitor is needed.

In your case of two completely separate systems, the only answer is to obtain another low voltage comparator circuit. Aircraft Spruce offers several, and the Aeroelectric Connection has one called the AEC9005.

I would recommend that you have a qualified "sparky" review your entire schematic, given the complexity you seem to be embracing.

My plane is set up almost the same, dual battery, dual alternator, second one an SD-8.

All my critical instruments are fed by the second batt/SD-8, but for even greater redundancy, I am using a Schottky diode to inter connect the two buses.

Either system operates my "E buss", seamlessly.

You might consider adding a diode to your setup,

Mike,

Is your SD8 connected directly to the second bat? If so, how are you monitoring the SD8 voltage (or current)?

Keep the systeme separated for more safety!

Mike, the Diode will allow an over-voltage situation to damage both ignitions instantaneously! That is why I keep the two systems separate.

If one of the alternators fails, I can decide, depending on: which alternator fails, how long I will have to fly to get to a safe Landing spot and what current I am using, to interconnect the two systems, immediately, later, or not at all.

Ted, I will check-out the low-voltage comparator. Thanks.

Any other ideas out there?

Regards, Tonny.

Ted Johns said:

Mike,

Is your SD8 connected directly to the second bat? If so, how are you monitoring the SD8 voltage (or current)?

Click to expand...

Yes, it is hooked directly------through a switch and fuse and regulator etc of course, to the aux battery (Odyssey 680).

The voltage monitoring is through the GRT efis's.

Tonny, I had not considered the over voltage possibility of this setup, however you have provided me some food for thought.

(sounds of mental chewing)

O.K., lunch now over, here is how I see it-----the regulator on the main alternator is a B and C unit, with the built in crowbar function. It is capable of shutting off the alt output in milliseconds, IIRC. The aux also has the crowbar, so I just dont think over voltage will be a problem (I may be a bit naive here) as I have things setup. I have had a number of experienced builders, EAA tech consolers, and even an electrical engineer look at my setup, and all have given it the green light.

As the plane is not flying yet I have no empirical data as to what will actually happen.

Mike

"The voltage monitoring is through the GRT efis's."

Ah, efis's, plural. The bigger hammer approach.

I wouldn't worry about over-voltage, given the B&C regulators. Assuming a good battery (i.e. very low impedance), the voltage rise is slow and easily handled by the crowbar circuit.

"Keep the systems separated for more safety!"

Umm, maybe. When chasing small increments of reliability improvement with increasing circuit complexity, one can easily obtain the opposite result. I can't say that's the case here, I'm just advocating careful analysis. If you are worried about instantaneous over-voltage damage, rest assured that any electrical event an alternator - battery combination can produce is dog slow compared to a crowbar transistor.

Ted Johns said:

"The voltage monitoring is through the GRT efis's."

Ah, efis's, plural. The bigger hammer approach.

Click to expand...

Well, here is the story----I bought a partially completed project, and it came with dual GRT efis's.

And, I am too cheap to not use them.

Same story for the SD-8 and dual battery.

I did, however have to figure out how to put it all together and make it work.

Ted Johns said:

I wouldn't worry about over-voltage, given the B&C regulators. Assuming a good battery (i.e. very low impedance), the voltage rise is slow and easily handled by the crowbar circuit. ---snip---

If you are worried about instantaneous over-voltage damage, rest assured that any electrical event an alternator - battery combination can produce is dog slow compared to a crowbar transistor.

Click to expand...

That is what I was thinking, thanks for the validation.

GPS screen as warning for low voltage!

I figured out an alarm for a low voltage situation with the alternator still putting out some power (enough to hold the relay in): The secondary system only powers the second ignition and the GPS (Avmap EKP-IV). The GPS will automatically switch to a lesser bright screen when the voltage gets low(er), to save power. That will probably happen around 10 Volts or so. The ignition however, will keep working on as low as 5 Volts. If I had not noticed the low voltage on the EFIS by than, the GPS screen will warn me and I can flick the cross-feed switch. Also I can use the Amp-monitor switch to see what either alternator is putting out.

One other thing that came to mind is that with this set-up, in case of smoke under the panel, I can switch everything off, with the mains switch, except the aux. system and the engine will keep running on the one ignition. If that does not solve the problem I can do it the other way around: have the mains back on and switch off the auxiliary system. That will have to be done by pulling the breakers though, because my master switch, that switches the battery, alternator and ignition on, is a 3 position keys witch (OFF / AUX / AUX +MAIN).

To be able to switch certain items off, I use a combination of fuses and pullable breakers. I try to keep things as simple as possible, but also safe and with options to do something about a possible failure in flight. If I could figure out a way to keep the two systems separate without the second alternator, I would actually prefer that and use a slightly larger battery (maybe an Odysey PC310).

I still have not made up my mind yet!

Regards, Tonny

Pilottonny said:

I figured out an alarm for a low voltage situation with the alternator still putting out some power (enough to hold the relay in): The secondary system only powers the second ignition and the GPS (Avmap EKP-IV). The GPS will automatically switch to a lesser bright screen when the voltage gets low(er), to save power. That will probably happen around 10 Volts or so. The ignition however, will keep working on as low as 5 Volts. If I had not noticed the low voltage on the EFIS by than, the GPS screen will warn me and I can flick the cross-feed switch. Also I can use the Amp-monitor switch to see what either alternator is putting out.

One other thing that came to mind is that with this set-up, in case of smoke under the panel, I can switch everything off, with the mains switch, except the aux. system and the engine will keep running on the one ignition. If that does not solve the problem I can do it the other way around: have the mains back on and switch off the auxiliary system. That will have to be done by pulling the breakers though, because my master switch, that switches the battery, alternator and ignition on, is a 3 position keys witch (OFF / AUX / AUX +MAIN).

To be able to switch certain items off, I use a combination of fuses and pullable breakers. I try to keep things as simple as possible, but also safe and with options to do something about a possible failure in flight. If I could figure out a way to keep the two systems separate without the second alternator, I would actually prefer that and use a slightly larger battery (maybe an Odysey PC310).

I still have not made up my mind yet!

Regards, Tonny

Click to expand...

Hi Tonny!

I want to do the same as you.
Do you have a wiring diagram ready to copy? I dont want to reinvent the wheel.

Best regards

Just a pencil drawing/sketch!

Detlef, no I am sorry, I only have a pencil drawing. As I go along wiring, I will add information to it (like pin-numbers, parts ref. etc.) . Ones I am finished I will heave to get someone to make a proper drawing with that information.

I have made the decision and ordered the 8 Amp Alternator. I am still looking for a small battery (Moped Battery 12 V, 4 AH) and was not sure if the 8 Amps will nor overcharge the little battery.

If one of the alternators fails, I have a cross-feed switch to connect the systems together. This will be a Toggle switch with a guard and breakers, that will only be used in case of an emergency (and only if it is a real emergency, because everything should keep running for several hours on the batteries only anyway).

I can post some more info when I get some of it wired up. The parcel with a lot of the stuff (breakers, fuses, etc.) is now in customs and I hope I get it delivered in the next couple of days so I can make a start with the wiring.

Regards, Tonny

Testing done!

Hello again,

I did some testing with the SD8. See my post here: http://www.vansairforce.com/community/showthread.php?t=31378

Happy reading (and picture).

Regards, Tonny.

The GRT EFIS (each one) supports up to 3 power inputs - each of which can be monitored for voltage with a low threshold that will put an alarm message on the screen. Consider using one of the extra power inputs for your voltage monitoring. If you really want redundancy, their AHRS also support 3 power inputs - you can supply the #2 power to the AHRS and one screen and get active low voltage notification as part of the package.

Bob
RV-10 N442PM
(3 screen GRT, dual batts & alts)

Dynon!

Bob, I am using a Dynon EFIS and a EMS. Only one voltage can be monitored on the EMS. On the EFIS it is possible to monitor the second voltage (aux. power supply) but it will not put out an alarm when the voltage is low (as per the Dynon rep.).

Regards, Tonny.