Diode Question

[Brambo]

I have the square diode from B&C betwen my main buss and Ess Buss and I notice there is a drop in voltage of almost 1 volt between them. Is this normal?

Bill Rambo
RV-7A

[JonJay]

Originally Posted by Brambo

I have the square diode from B&C betwen my main buss and Ess Buss and I notice there is a drop in voltage of almost 1 volt between them. Is this normal?

Bill Rambo
RV-7A

.7 volts is the drop across a diode.

[Rick_A]

Try one of these:
http://www.periheliondesign.com/powerschottkydiodes.htm

I've got the same diode you have sitting in my "stuff I bought but won't use box". You could buy the basic diode for less and fabricate your own mount but I got mine from Eric.

[kcameron]

When I get to the power system on my RV-7, I plan to use a part in this family:
http://ixdev.ixys.com/DataSheet/L227.pdf

This will reduce the voltage drop and it also has higher current and power dissipation capability. I don't think it would need a separate heatsink: just bolt it to the firewall or some other metal part.

[Dean_aeroleds]

Originally Posted by Brambo

I have the square diode from B&C betwen my main buss and Ess Buss and I notice there is a drop in voltage of almost 1 volt between them. Is this normal?

Bill Rambo
RV-7A

The forward voltage drop across a diode is a function of the type of diode, and the amount of current flowing through it. Standard silicon diodes range around 0.7V, but the actual voltage will depend on the current.

The best diode for low drop and power loss is a Schottky diode. Typically the higher the current rating of a Schottky diode, the lower its drop will be. Most Schottky diodes will drop between 0.1 and 0.5V depending on their current rating and the actual current flow.

A lower forward voltage means that the diode will burn less power and run cooler.

You need to also consider what the max reverse voltage is that you need to hold off when selecting a diode.

Dean Wilkinson
AeroLEDs LLC
www.aeroleds.com

[DGlaeser]

Originally Posted by kcameron

When I get to the power system on my RV-7, I plan to use a part in this family:
http://ixdev.ixys.com/DataSheet/L227.pdf

This will reduce the voltage drop and it also has higher current and power dissipation capability. I don't think it would need a separate heatsink: just bolt it to the firewall or some other metal part.

That one is pricey! I went with a 50HQ035 which I got for about $9 (but I just checked and Digikey no longer has them in stock).
For the E-bus diode, a Schottky is arguably overkill. While the alternator is working, the E-bus sees 13+ volts even with a regular diode - and a Schottky will give you about a half a volt more. In battery only mode, the E-bus is powered directly from the battery (no diode) and sees less than 13V. So the 'loss' of a volt for the E-bus for normal operations is no big deal.
I went with the Schottky because I got one for about the same price as the regular square one - and it looks cooler Spending a lot of money for that half a volt is not warranted IMHO.

[Jeff R]

Those square diodes are typically bridge rectifiers with 4 diodes in them, so maybe you are actually reading across two diodes in series? If the current is very low, you might see as little as .5 volts across an individual silicon diode.

[kcameron]

Originally Posted by DGlaeser

That one is pricey! I went with a 50HQ035 which I got for about $9 (but I just checked and Digikey no longer has them in stock).
For the E-bus diode, a Schottky is arguably overkill. While the alternator is working, the E-bus sees 13+ volts even with a regular diode - and a Schottky will give you about a half a volt more. In battery only mode, the E-bus is powered directly from the battery (no diode) and sees less than 13V. So the 'loss' of a volt for the E-bus for normal operations is no big deal.
I went with the Schottky because I got one for about the same price as the regular square one - and it looks cooler Spending a lot of money for that half a volt is not warranted IMHO.

Ixys has many parts in that family. I couldn't find an online price for the 120A version but see that the DSS2X81-0045B is about $20. I think that makes it a win over the 50HQ035 since you only need one and mounting is much simpler. The Ixys DSS 2x series is isolated from the case so you basically only need to bolt it down. I've seen plenty of failed chassis insulators for parts like the 50HQ035. There's not much worry with an isolated part like the DSS.

[Brambo]

Thanks for the replies. I will look into these other diodes.

Bill Rambo
RV-7A

[gmcjetpilot]

Originally Posted by DGlaeser

Schottky is arguably overkill. Spending a lot of money for that half a volt is not warranted IMHO.

I would argue. . Voltage drop is about 1.1 volts for the power P-N diodes (0.70 volt is drop for signal diode). The Schottky is about 0.20 volt, so the difference is almost a full volt, worst case.

An extra $20 going to kill you? Aeroelectric recommend the cheap rectifier p-n diode block, which Radio Shack use to carry in store. Having 0.20 volt drop (Schottky) verse 1.1 volt drop (p-n junction rectifier diode) is worth extra nickles to me. Again its arguable. (The Radio Shack bridge in my junk drawer says 1.7 volt fwd volt drop! That's fine if you are running at 100 volts or 200 volts, 1.7 volts is not much.)

On a good day say your alternator supplies 14.3 volts at the main buss, so you will only have 13.2 volt for your essential buss? Now the scenario is your alternator dies, you're down to battery power (12-12.6 volts). Do you want another 1 volt drop? Some radios will not run on 11 volts. Aeroelectric says add a second switch & more wire for an aux circuit to power the essential buss directly from the battery, by-passing the main contactor/relay and diode. That will get the voltage back up but it's more complication.

How about not having a diode or switch bypass for the essential buss at all! If you want to "shed load", manually turn items off or pull CB's (yes I have pull-able CB's). Aeroelectric loves fuses of course, and if you use CB's the terrorist will win, I'm told. (its a joke)

If you want an isolated and have alternative power to the essential buss, you could use two switches, one to isolate and one by-pass direct battery power, no diode. You just need to remember the sequince of switch thowing, MAKE before you BREAK the connection.

The claim to fame of the "25A, 50V Full-Wave Bridge Rectifiers" was you could walk down to Radio Shack and buy one for $4. I was told they don't carry them in store any more, avaiable only on-line now.

I can see where $4 does sound better than $30. I totally understand. I have the cheapskate gene as well. Either diode is OK and will work, but check out the link for Perihelion Design. Here is their propaganda sheet of why its better. I know aeroelectric poopoo's the idea; probably because he didn't think of it first. Why waste energy? Periheliondesign has a good price; wholesale they cost $25-$29 anyway. They really are better than any diode you can get for what we are using it for.