Which Diode do I need?

Mash · Nov 17, 2013

Hi all!
I am doing a lighting system for the cupboards in my house, and I think I need a diode in there.
Here is the situation:

I have a cupboard with two doars, one LED strip on each door, being fed by two 12V 15W constant voltage driver( one for each door), controlled by door switches. I have one central strip in the middle of the cupboard, fed by BOTH drivers, so it comes on whichever door I open. Imagine a big H, in which I need to power each side and the middle individually, without power going from one side to the other.
For this I think I need two diodes on the central strip, so when I open one door, it doesnt feed the strip on the OTHER door through the central stip.
So I started looking into diodes and got confused with the types etc.
So my questions:

1- which type of diode do I need ( are they going to cause voltage loss, on the voltage being fed to the strips?)
2- do I need to use one with a specific power rating? ( 12v 15W, circuits, so the back voltage will be 12v as well.)

Thanks in advance for your help guys!

electronupdate · Nov 18, 2013

Mash said:
Hi all!
I am doing a lighting system for the cupboards in my house, and I think I need a diode in there.
Here is the situation:

I have a cupboard with two doars, one LED strip on each door, being fed by two 12V 15W constant voltage driver( one for each door), controlled by door switches. I have one central strip in the middle of the cupboard, fed by BOTH drivers, so it comes on whichever door I open. Imagine a big H, in which I need to power each side and the middle individually, without power going from one side to the other.
For this I think I need two diodes on the central strip, so when I open one door, it doesnt feed the strip on the OTHER door through the central stip.
So I started looking into diodes and got confused with the types etc.
So my questions:

1- which type of diode do I need ( are they going to cause voltage loss, on the voltage being fed to the strips?)
2- do I need to use one with a specific power rating? ( 12v 15W, circuits, so the back voltage will be 12v as well.)

Thanks in advance for your help guys!

This appears to be a diode-or'ing application. You did not state the power of the light strips, only that the power supplies are rated to 15W. There is not enough details here on which to select a component.

Mash · Nov 18, 2013

Thanks for your reply!
My strip is 5W a metre, I will have 2m in the doors, and 1m in the horizontal section.
So my circuit will be 12v, and 15W of consumption. Hope thats enough information.

electronupdate · Nov 18, 2013

Therefore the center strip will consume 5W. The current through the diode will be 5W/12V = 417 mA. That's the minimum forward current the diode needs to be rated for. When blocking current the reverse withstand voltage needs to be 12V.

The other consideration is picking a diode which will not get too hot when the door is open and current is flowing through it. If the diode is silicon the power dissipation will be approx 417mA *.7V = 291 mW.

These are all pretty modest numbers and many solutions exist. Power Diodes are often listed as rectifiers in many distributors catalogs.

You need two power diodes.. This diode or'ing function is so common some parts are made which contain two diodes. You will probably want a diode in a TO-220 which allows it to be easily mounted to a heat sink

These would be typical and may be a good place to start:

http://www.digikey.ca/product-searc...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

p.s. You mentioned 15W of consumption... however when both doors are open you will have 5 meters of lighting on which will consume 25W.

Mash · Nov 18, 2013

Thank you very much for your detailed answer!
re your last point, as mentioned before I have two power supplies, one for each door, plus both feeding the center rail (therefore centre rail comes on if any door is opened).

Now, having looked at digikey, i see the rectifiers and their packaging design (called mosfet???), which Im not familiar with wiring etc...
In my mind I was thinking of the diodes that look like circuitboard resistors, to be wired in line with the power supply cables.
eg http://cpc.farnell.com/jsp/search/b...etricAttributeId=&prevNValues=411+2005+205707

That site is probably where i will buy my part from, would any of the above two parts be good? Can you see a better suited model (by playing with selection criteria on top of the page?)

Thanks again for your usefull advice! Learning as Im going along here!!!!

electronupdate · Nov 19, 2013

those are small signal diodes. note how they state "Forward Current If(AV):350mA". That is far less than the current required and they are not suitable. You need a power diode. A 1N4001, for example, is rated to 1A. However...you must also consider electrical safety and thermal safety. You might be thinking of soldering the diode in line and then wrapping with shrink wrap? That's a weak solution. Make sure that the diode is mounted in a way so it sees no mechanical stress on it's leads when the wires are pulled, make sure that when the power supply is shorted that the diode surface does not become too hot and a fire hazard . The TO-220 package that I had suggested above, inside a enclosure with a heat sink and all wires with strain relief would be one such way of addressing these items.

billy_gr · Nov 19, 2013

Get an 1N4001 and you will be ok

Mash · Nov 19, 2013

Perfect! Thank you so much, your help is truly appreciated!
Checking the spec, it says Forward Voltage VF Max: 1.1V. Does this mean my LED strip will receive 11v?

Forager · Nov 19, 2013

Mash said:
Perfect! Thank you so much, your help is truly appreciated!
Checking the spec, it says Forward Voltage VF Max: 1.1V. Does this mean my LED strip will receive 11v?

Yes, you will have to subtract the forward voltage from the voltage delivered to your center strip.
I would use a Schottky diode instead like the 1N5817, since your current is only 5/12 amps, and it is an axial package like you wanted.
It has a forward voltage of .45V, has a sufficient reverse voltage rating, and will only dissipate 3/16W in your setup.
That would give your center string 11.55V, which will not be noticeable difference from 12V for an LED string with integrated resistors.

As for the comments about mounting, you will want to J-hook the wire to the diode leads, solder, and heat shrink tube for sure.

Mash · Nov 20, 2013

Thanks forager for your suggestion, they are cheap (the diodes, not your suggestion!

), so will try both of them and see how it goes...

Which Diode do I need?

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