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electrical stuff I don't understand

RobertD

Well Known Member
My airplane electrical system is based closely on Bob Nuckoll's Z-11 plan. I have a GRT EIS 4000 that gives me voltage. This voltage reading has always been lower than I think it should be but I've never had a problem starting the plane (odyssey PC-625 battery) or running the full set of avionics and lights. My alternator is Van's 60 amp internally regulated.

The other day during a 2+ hour flight the EIS gave me a low voltage warning a couple of times, just briefly. So I decided to finally find out what's going on. The first thing I did was measure the resting battery voltage - 12.6, pretty good.

I then decided to test the 15+ year old alternator. Took it to O'Reilly and they said everything is fine.

Then I started measuring voltage at various points with the engine running. The battery was 14+ volts and so was the reading at the main buss, right at the terminal feeding the fuse block. With no load other than strobes, the EIS voltage matched the buss voltage within .1 or .2 volts.

When I started adding load the EIS voltage went steadily down to around 13.2 volts but the buss voltage remained steady at 14+ volts.

I then measured the voltage at the EIS panel switch. It matched the reading at the buss but also went down when I started adding load.

So what I don't understand is why the voltage drops so much for a relatively short run. couldn't be more than 18-20 inches. Is it something to do with the fuse block? How can the buss voltage remain steady but what the EIS is seeing drop so much?

I'm really not sure where to measure next but am sure open to suggestions. I guess I could measure out to other switches. Or maybe just to where the wire leaves the fuse block - just thought of that. That would tell me if the drop was across the block.

Sorry to make this so long but I'm just trying to anticipate questions.

thanks,

rd
 
Hi Robert, I think I remember your plane from several years ago. If it is the plane I am thinking of, you have a Radio Master switch. One side of the switch showed battery voltage, and the other side was 1.5 volts lower. I suggested that you replace the switch as the voltage drop across the switch was way out of line.

Is that you or am I confusing you with another plane at Siler City?
 
The large voltage drop is normal due to the diode between the main bus and the essential bus. The diode is needed when the alternate feed switch/relay is active to prevent the essential bus from feeding back into the main bus.

You can check this by activating the alternate feed switch/relay and verifying that the e-bus voltage increases by about one volt.
 
Hi Robert, I think I remember your plane from several years ago. If it is the plane I am thinking of, you have a Radio Master switch. One side of the switch showed battery voltage, and the other side was 1.5 volts lower. I suggested that you replace the switch as the voltage drop across the switch was way out of line.

Is that you or am I confusing you with another plane at Siler City?

Noel, must be some other plane 'cause I followed Nuckolls' preaching and don't have an avionics master. I did test both sides of the EIS switch and got identical readings, if that means anything.

thx

rd
 
The large voltage drop is normal due to the diode between the main bus and the essential bus. The diode is needed when the alternate feed switch/relay is active to prevent the essential bus from feeding back into the main bus.

You can check this by activating the alternate feed switch/relay and verifying that the e-bus voltage increases by about one volt.

thanks, this makes sense to me. The EIS is fed from the essential bus, so if I were to check the voltage drop from something fed from the main bus then I shouldn't get much drop, correct?

Sounds like I need to lower the setting at which the EIS gives the low voltage warning. Still learning how my plane works after all these years ...
 
so if I were to check the voltage drop from something fed from the main bus then I shouldn't get much drop, correct?

Yes, the exact difference between the essential bus and the main bus will vary depending on the specification of the diode used and the amount of current passing through it but I don't think you would ever see a drop of less than 0.4v or more than 1.5v.
 
Yes, the exact difference between the essential bus and the main bus will vary depending on the specification of the diode used and the amount of current passing through it but I don't think you would ever see a drop of less than 0.4v or more than 1.5v.

ok so one more question - why is there no voltage drop until I start adding load? If the drop is due to the diode shouldn't it be there with or without a load? I feel like I'm missing something ...

and thanks for your comments, they're very helpful.
 
A schottky on a good heat sink is a great idea. It could be that the diode you have is not rated for the power passing through it and heat is causing it to operate out of design specs. Power = voltage x current so 5a at 1.5v drop is 7.5W ideal add more for real world loss. So even if you don’t have a schottky a really good heat sink is wanted in this application.

The drop starts with load because there is no current draw with no load. V=IR so if I is 0 then V is zero too. As I goes up V goes up but R is a small value in a diode that goes down as I goes up until saturation for the diode is reached and the delta-V is established. It is this small resistance to saturate the diode that generates current dependent heat.
 
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Diode Manufacturer's data sheets have graphs that show voltage drop.
The data sheet for your diode will show that as current increases, so does voltage drop.
 
Many years ago (25) Bob Nuckolls was recommending a simple bridge rectifier for the essential bus feed. The part came in a 1.125" square package with a hole in the middle for mounting and had four .25" fast-on tabs. They were cheap, readily available and easy to install but not very efficient. Back then some people griped about the efficiency but Bob thought it was good enough.

I see that now his schematics call for a more efficient power schottky diode. It could be that he changed his mind but perhaps it reflects the fact that with modern avionics we tend to draw a lot more power from the essential bus than we used to.

If you have the older kind of diode and your drop is too high and/or the diode is running too hot then you might want to upgrade. A voltage drop of slightly over one volt used to be considered acceptable.
 
My experience is that the bridge rectifier results in a drop of 0.5 volts. I haven?t observed any reason to change to the Schottke diode.

Erich
 
Many years ago (25) Bob Nuckolls was recommending a simple bridge rectifier for the essential bus feed. The part came in a 1.125" square package with a hole in the middle for mounting and had four .25" fast-on tabs. They were cheap, readily available and easy to install but not very efficient. Back then some people griped about the efficiency but Bob thought it was good enough.

I see that now his schematics call for a more efficient power schottky diode. It could be that he changed his mind but perhaps it reflects the fact that with modern avionics we tend to draw a lot more power from the essential bus than we used to.

If you have the older kind of diode and your drop is too high and/or the diode is running too hot then you might want to upgrade. A voltage drop of slightly over one volt used to be considered acceptable.

yep, I've got that exact diode. For $59 and a little bit of work it sure looks worthwhile upgrading to the Schottky.

thanks again for all the help in understanding what's going on.
 
A schottky on a good heat sink is a great idea. It could be that the diode you have is not rated for the power passing through it and heat is causing it to operate out of design specs. Power = voltage x current so 5a at 1.5v drop is 7.5W ideal add more for real world loss. So even if you don’t have a schottky a really good heat sink is wanted in this application.

The drop starts with load because there is no current draw with no load. V=IR so if I is 0 then V is zero too. As I goes up V goes up but R is a small value in a diode that goes down as I goes up until saturation for the diode is reached and the delta-V is established. It is this small resistance to saturate the diode that generates current dependent heat.

The current draw in question is that from the buss to the EIS. This does not change with general system load. If the buss is 14 and the EIS 13.2, the only draw/load that is relevant is that between the Buss and EIS. I don't see how the current or load on the EIS feed could or should change based upon other loads on the buss.

I would keep checking the voltage further out from the buss to the EIS. As you go, whenever you hit a lower voltage, address the connections/wire/component between the good point and low voltage point.
 
yep, I've got that exact diode. For $59 and a little bit of work it sure looks worthwhile upgrading to the Schottky.

thanks again for all the help in understanding what's going on.

Still not seeing how a diode has a voltage drop of 0 sometimes and .8 at others, if the load is not changing and I don't see the load from the EIS changing much. It has a pretty stable draw.

Larry
 
The current draw in question is that from the buss to the EIS. This does not change with general system load. If the buss is 14 and the EIS 13.2, the only draw/load that is relevant is that between the Buss and EIS. I don't see how the current or load on the EIS feed could or should change based upon other loads on the buss.

I would keep checking the voltage further out from the buss to the EIS. As you go, whenever you hit a lower voltage, address the connections/wire/component between the good point and low voltage point.

my essential bus and main bus are right next to each other so I'll swap the EIS to the main bus and see what voltage drop I get.
 
what I learned today

The large voltage drop is normal due to the diode between the main bus and the essential bus. The diode is needed when the alternate feed switch/relay is active to prevent the essential bus from feeding back into the main bus.

You can check this by activating the alternate feed switch/relay and verifying that the e-bus voltage increases by about one volt.

today I swapped the EIS feed from the essential bus to the main bus and, as predicted, the EIS voltage was the same as I was reading at the main bus, even as I added load.

so my next, and hopefully last, question on this topic is am I hurting anything by continuing to use my old cheap Radio Shack diode? The stuff being fed from the E-bus seems happy with what it's getting voltage-wise, and it's easy enough (already done) to lower the EIS low voltage warning point.
So is there any specific reason to change to the Schottky? I sure don't mind but I'd like to know if I'm gaining anything by making the change. Surely there are a bunch of planes of my RV's vintage flying around with the old diode, blissfully unaware of the voltage drop ...

thanks for all the help.
 
All diodes will have a forward voltage drop, even at very low currents. Where this is important is in keeping the standby battery charged. For example, Odyesse recommends 14.4 volts (or more) to properly charge its batteries. If you have the typical (standard silicon diode) forward drop of 0.7 volts, your backup battery will see 13.7 volts. This is too low, and will result in a shorter than expected lifetime. Shottkeys have a smaller drop, which helps to fix this problem.
 
All diodes will have a forward voltage drop, even at very low currents. Where this is important is in keeping the standby battery charged. For example, Odyesse recommends 14.4 volts (or more) to properly charge its batteries. If you have the typical (standard silicon diode) forward drop of 0.7 volts, your backup battery will see 13.7 volts. This is too low, and will result in a shorter than expected lifetime. Shottkeys have a smaller drop, which helps to fix this problem.

you're talking about backup batteries in avionics such as a 430, right? That's a good point and I'm pretty sure anything downstream of that diode isn't seeing much over 13 volts.
The odyssey I use has always stayed nicely charged so I assume my alternator is giving it what it needs. I do (not very often) plug it into a battery tender rated for AGM batteries just to give it the proper charging profile.
 
No, I meant a second battery that was powering a backup buss, which is diode isolated from the primary.
Those internal batteries generally have their own regulated power supplies to keep them charged.
 
So is there any specific reason to change to the Schottky?

I would only change it if the diode you are currently using is getting too hot under normal full load. How hot is too hot? If you can hold your finger on it then it is probably fine.
 
am I hurting anything by continuing to use my old cheap Radio Shack diode?
No, nothing is suffering at 13 VDC. Many avionics are rated at 10 to 30 volts. Alternators have failed many times.
The pilot keeps flying as the voltage slowly drops to 13, 12, maybe down to 11. The avionics are not destroyed.
There may be exceptions. But at 13 volts, everything should be happy except batteries that want 14 VDC or more to be charged.
 
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