How much voltage loss is normal thru a diode?

kbehrent · #1 11-28-2011, 11:11 AM

A friend and I were doing some voltage level checks at various points in my electrical system and we found that the biggest loss came from the diode that connects my Master buss to the e-Buss. There was 12.5v on the Master buss side of the diode and 11.8v on the other side of the diode to the e-Buss. There was no load on the e-Buss side as everything was turned off. Wow, wasn't expecting that much loss!

It made us curious whether the loss would be higher/lower once the engine/alt is running (14.2v) and load is applied? We plan on running the engine/alt and checking the voltage again weather permitting. I bought the diode from BNC and it does have the big heat sink to keep it cool.

DCat22 · #2 11-28-2011, 11:14 AM

I recollect silicon diodes would drop by about 0.7V... So, sounds about right. [deleted stuff -- wasn't right]

(I'm no expert in this application....not sure if you could use a Schottky diode in this case...voltage drop should be closer to 0.3V?)

vlittle · #3 11-28-2011, 11:27 AM

Quote:

Originally Posted by kbehrent

A friend and I were doing some voltage level checks at various points in my electrical system and we found that the biggest loss came from the diode that connects my Master buss to the e-Buss. There was 12.5v on the Master buss side of the diode and 11.8v on the other side of the diode to the e-Buss. There was no load on the e-Buss side as everything was turned off. Wow, wasn't expecting that much loss!

It made us curious whether the loss would be higher/lower once the engine/alt is running (14.2v) and load is applied? We plan on running the engine/alt and checking the voltage again weather permitting. I bought the diode from BNC and it does have the big heat sink to keep it cool.

Nominal voltage drop for a regular silicon power diode is about 0.7 volts, but can increase substantially with load. You should budget about 1 to 1.5 volts under load to be safe.

Schottky diodes have a lower voltage drop, but may still have substantial load effects, increasing the drop.

Stick with what you have, it's working properly.

kbehrent · #4 11-28-2011, 01:23 PM

Quote:

Originally Posted by vlittle

....... You should budget about 1 to 1.5 volts under load to be safe.

Thanks for the info and I believe the 1 to 1.5 is probably correct under load. I have a Eagle EMS system that was connected to my E-Buss that was complaining of low voltage (10.2v) when it sampled the voltage under load. My avionics buss is connected to the E-Buss so there is probably a good load running thru it. I didn't think it would be a problem since I have a backup ALT and I can easily turn off avionics that I don't need to get me back on the ground.

I've removed the Eagle EMS system from the E-Buss and connected it directly to the Batt buss. I'll do a test run when weather permits to see if that resolved the low voltage issue. I'm also going to test the voltage on the E-Buss with the Engine/Alt running to see what the voltage really is with everything running.

rocketbob · #5 11-28-2011, 02:19 PM

This shottky has better Vf characteristics than what is recommended in the Aeroelectric book:

http://www.mouser.com/ProductDetail/...52by2F6w%3d%3d

If you look at the chart in the datasheet the forward voltage drop under typical loads/operating temps is 0.3-0.4V.

Rick_A · #6 11-28-2011, 03:07 PM

Perihelion (sp?) Designed sells a nice Shottky power diode that's easy to install, (if you're not into roll your own) and his price is reasonable.

vlittle · #7 11-28-2011, 06:10 PM

Quote:

Originally Posted by kbehrent

Thanks for the info and I believe the 1 to 1.5 is probably correct under load. I have a Eagle EMS system that was connected to my E-Buss that was complaining of low voltage (10.2v) when it sampled the voltage under load. My avionics buss is connected to the E-Buss so there is probably a good load running thru it. I didn't think it would be a problem since I have a backup ALT and I can easily turn off avionics that I don't need to get me back on the ground.

I've removed the Eagle EMS system from the E-Buss and connected it directly to the Batt buss. I'll do a test run when weather permits to see if that resolved the low voltage issue. I'm also going to test the voltage on the E-Buss with the Engine/Alt running to see what the voltage really is with everything running.

Normally, an ignition should be directly connected to a battery. This can be a hold-over battery (smaller battery diode isolated to the main battery). In this case, the secondary battery provides the ignition power during cranking, and the diode provides charging and load current when the main battery is on-line at full voltage.

In this case, a schottky diode is recommended for the low voltage drop and to ensure the secondary battery is properly charged.

I would not put any other devices on this bus other than the ignition.

V