noob question about diodes

I’m working on a robot that has a few onboard power supplies because the Roomba (host platform) voltage is not otherwise compatible with the electronics I’m using.

I’ve heard that diodes inherently induce a voltage loss, and I read somewhere (no link) plan on 1V loss. That seems like a lot in the low voltage DC world. I found another article (link below) that says all diodes induce a .6V loss. I think absolute numbers in general like this don’t fit, there can’t be a blanket statement that all diodes have a X volt loss when used.

http://www.bcae1.com/diodes.htm

Am I wrong? How do I tell what kind of voltage loss a given diode might create?

On a related note, how do I know what diode to select current wise? Are there markings like with resistors? I have a pack from Microcenter and need to figure out which ones to use and what kind of effect they will have on my supply voltage and if it would be OK to use one after the step-down power supplies as well.

I think absolute numbers in general like this don't fit, there can't be a blanket statement that all diodes have a X volt loss when used.

No. Different types of diodes have different voltage drops. A typical radio shack diode has ~.7v. if you want the details, data sheet for the diode or simple testing are your friends.

I think I found the datasheet for one of the diodes I think I might want to use: http://www.datasheetcatalog.org/datasheet/wte/1N5404-TB.pdf

Diode 1N5408rated for 1000V and 3A.

The amp rating, is that a working rating or a maximum? The diode is pretty big, and I'm not so much wanting to limit current as just prevent feedback on my power supplies.

For example, I'd like to put a diode on the leads that go to the battery so that any feedback from the power supplies doesn't go to the battery, and then after the power supplies before the leads go to the boards that will be using the power. I'm not sure if I need them all, but I'm pretty sure I need one on the voltage in to the motor controller board so I can solve this problem:

http://forums.adafruit.com/viewtopic.php?f=31&t=35450&sid=a5ade69323605a2ff26a1f41cb8cacd2&p=175276#p175276

http://arduino.cc/forum/index.php?topic=139091.new;topicseen#new

The output of the power supply is 5V 2A, and in that kit I have 1A and 3A rated diodes. I think I would need to use the 3A one, right? I think the diode lead is as thick or thicker than the wire I am using though. Is it too big for what I am doing?

Forgetting about zener diodes, which work in reverse-bias mode to provide a more or less constant voltage drop, there are basically silicon, Schottky, and older style germanium diodes [which aren't found much anymore].

These are all analog devices, so their ratings vary somewhat, with no exact values for anything. Silicon diodes have a nominal 0.7V forward drop, but this varies from much lower to over 1V, depending upon the current flowing through them.

Some of these come in small sizes and low currents, such as so-called signal diodes like 1N914 and 1N4148. The larger 1N4000..1N4005 diodes are used as "rectifiers" and have a typical 1A rating. Then you have higher current diodes, like your 1N5400 series.

http://www.vishay.com/docs/88503/1n4001.pdf

The other major type are Schottky diodes, like 1N5819. These have a smaller forward drop than the regular silicon ones, eg 0.2-0.4V, for the same current ratings.

http://www.vishay.com/docs/94614/1n5819.pdf

If you look at the spec sheet curves, you see every parameter varies with current, temperature, DC or AC operation, etc. They're analog devices. You choose the diode based upon how much current needs to flow through it.

For the diode you are considering and looking at the data sheet you refer to, you will see that the forward voltage is 1 volt with a current of 3 amps. That equates to 3 watts of power being generated by the diode junction. That's a lot of heat that needs dissipating and hence they use thick copper leads in an endeavour to conduct the heat away from the junction. If you cut the leads too short and then solder the diode to a thin pcb track you are going to burn up the diode even if you do not exceed its rated electrical parameters. There's more to this game than meets the eye.