diodes rotary switch ajustable power supply question

[demether]

Hi,

I ordered some electronic parts to build a rotary switch to adjust my power supply (13.5v, 8A), following P.Marchand tutorial here :

http://www.philippejmarchand.com/Slots/ ... age58.html

But I made a mistake in my order list, I bought 3A diodes instead of 8A diodes. SInce I want just one general rotary switch just after my power supply, and I'm using a 4 lanes track wired on it, I'm afraid that 3A will not be suficient/dangerous.

Am I right, or does the Ampere value not important in this thing ?

thanks !

[Florida_Slotter]

demether,

The answer depends on many factors.

What type of cars are you running? Does your track allow for lots of wide open throttle racing? How much voltage control will you be using?

Since you are using the diodes as a voltage control, all the lanes will draw current through the 3 amp diodes.

Ohms Law states that P = I x E, so the lower the voltage, the less power.

If you are running the Carrera type of car, it will be close, but I do think you will be fine. Run some cars with hotter motors in them, and I do not think you will make it with the 3 amp diodes.

Just my opinion of course.

[4424ever]

If you want to use what you have just double up your diodes in parallel, two 3 amp diodes will deliver 6 amps total. This will get you closer to what you were after!

[demether]

Thanks for the advises !

I'm not sure I ordered enough spare diodes to double them I'm afraid. Since I want something really sturdy and safe, I will probably order some new diodes instead.

[dr fabio]

There is this myth in the slotcar world that diodes drop 0.6V. That is their nominal no load voltage drop. Under load diodes will drop a lot more and this will change as the load changes. Really comes down to how well you want to control the voltage. Plus minus 1V of the nominal selected voltage drop? This may happen if one or more cars come off, or if one car has a hot motor as it accelerates it will sag the voltage. If thats OK then diodes will do the job.

Also paralleling up diodes is not the same as resistors. The diode with the lower voltage drop will avalanche first and take most of the load. The other may take say 10%. So to 3A diodes in parallel does not equal 6A capacity.

[demether]

thanks for your answer,

I thought that the load on slot car motors was more related to amperes than voltage ? I was using this rotary switch assembly on my first track years ago, it worked pretty well, I didn't feel noticeable variations. But I'm far from being an experienced competitor or tech specialist, so maybe I missed it.

[dr fabio]

Yes, as the load changes the current will change. The change in current will affect the voltage drop across the diode. The 0.6V drop is a nominal (no load) value. This diode voltage drop will change as the load (current) changes. For low powered cars its probably not too much of a problem.

[demether]

Thanks for the explanation. If I understand correctly, logically using a diode with a high nominal value, with low powered cars, the voltage drop should be minimal.

Anyway, 3A is still too low for 4lanes 1/32 cars I'm afraid. I don't know why I selected them instead of the 8A I wanted !

[demether]

Hi,
A last question, I received my electronics order, and now I'm looking for new diodes for my voltage assembly.

Remember that I want to use a main voltage regulator, directly after the power supply (8A). So the 4lanes will take "juice" thru this assembly (note my technical knowledge).


I found diferent 8A or 10A diodes. And I noticed that some of them are rated for 50V, other for 100V, or even 400V. Some of them are called "power diodes" , "zener diodes" or "Schottky diodes".

I made my own research apparently the "Schottky" diodes voltage drop is half lower than the "regular" diodes. SO the diode name is not unsignificant.

Before spending new money on this (not speaking of the prices of the components, but the shipping costs), I'd like to not make a new mistake. Any help would be appreciated.

thanks

[dr fabio]

You need a power diode. Schotty diodes are designed for very low voltage drop, of the order of 0.16 to 0.25V depending on the metallic contact used in the diode. They are usually not a power diode. I use a schotty diode and then resistors for my brake dial on my controller. Zener diode are usually connected in "reverse" and will avalanche at a particular voltage, say 3.3 or 5.1V etc. This would be very unsuitable for what you want to do.

Go for the bread and butter power diode, either 8A or 10A.

Hope that helps.

[demether]

Really helps, thanks. These are not the most expensive components, but error after error, it can be expensive !