Simple Diode question

CharlieII · 2009-04-07 17:37

I spent some time on both Newark and Digi-key's websites trying to locate a suitable diode for my application, but quickly realized I don't know enough about diodes to do that.

So.... I figured it would be more effective to just list my needs, and let you fine gentlemen suggest some if you will.

I have a DC circuit that I need to protect from potential reverse polarity. It will be running between 10 and 15 volts, and a max of 2A. So, I figured a max of 30w. I need one that is dimensionally similar to a resistor with the little leads leaving each end to be bent down and run into a breadboard/pcb. The spacing I have allotted is .4" lead spacing. I had assumed I could find a diode that could be placed in the same location as a resistor.

However, simply looking for a 30w diode proved a bit harder than I expected, as there are apparently a multitude of diode types. I am moderately familiar with the Zener, the rest however, I have no idea on.

Can someone point me toward a good diode to use here? ~30w with .4" lead spacing. And I suppose these are termed "Axial leads". I plan to place it inline on the main power trace.

Thanks,

Charlie

mctrivia · 2009-04-07 17:56

well surface mount is my specialty.

1n4001 will take 6 amp spike 1 amp normal. any diod with leads can be stood on end if needed to be closer together.

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Philldapill · 2009-04-07 18:15

Charlie, one thing you need to understand about power, is that 30W is not "passing through" the diode. The formula for power dissipation in any device, is current through, times the voltage across it. Typically, a normal diode will have a voltage of 0.7V(or around there). This means, that if you have 2A going through the diode, it will dissipate 1.4W. The 1n4001 that mctrivia refers to will not allow 2A continuous current. The diode is rather small, and can't dissipate enough heat(power) to be able to allow 2A to flow through it.

What is your application? What is the diode for? Sometimes, a diode doesn't always have a continuous current through it. If you are using it as a protection diode, i.e. current flowing into it, but if someone accidentally plugs the power in wrong, the diode will prevent that reverse polarity from frying your circuit. If this is the case, then yes, you will need a diode rated for at least 2A. If, however, the diode is merely to "catch" any inductive EMF from, say, a DC motor, then it's a little more complicated, but you probably won't need a 2A diode.

You say you want to put it inline on the main power trace, which suggests you will probably have a continuous 2A current(worst case scenario). If that's so, yes, you will need a diode rated for 2A.

Again, you need to have a better grasp on power. The 30W has nothing to do with the diode itself. If your supply is 100V, and you have 2A flowing through the diode, you aren't dissipating 200W... again, only a 0.7V drop across it, 2A through it, 1.4W. Your LOAD, however, will be dissipating about 200W if it has about 100V across it(not 100V due to the 0.7V diode drop).

If you want to go the cheap route, you COULD use some 1n4001 diodes. It's not a very good practice to do this, but you COULD put 3 or 4 diodes in parallel, acting as a single diode. The reason this isn't good practice is a little complicated, but basically, one diode may be allowing more current than the others to pass through, resulting in more power dissipate in that ONE diode... You should be good if you double the current handling ability though by using 4 diodes(or more). I have a huge roll of these diodes so I sometimes string them together in parallel for a quick-and-dirty solution.

mctrivia · 2009-04-07 18:28

I have switched to mostly shotkey diode because they are only 0.3v drop. I have some rated at 30 amp in sot123 pack sure you could find in through whole easily

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Carl Hayes · 2009-04-07 19:57

Phil, you beat me to it, and with admirable clarity too.

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CharlieII · 2009-04-07 21:37

Thanks Phil. Although it seems my error was in assuming the diode would be rated for the amount of power it should be expected to transfer, not how much it could waste away without failure. If that had been the case, then a 15v source, and a 2A load, it would have been transferring 30w. Although I can't see that this is not the case, it appears that the rated power for a diode is that relative to the .7 or so volts of forward bias needed to cause it to conduct. And if that is the case, then yes, a 30w diode is far from what I need, even though that's precisely how much power would be traveling through it worst case.

Since I'm not going to catch on to the terminology enough to get this solved anytime soon...

I just need a diode that will function well when subjected to anything from ~10vdc to ~15vdc, with a worst case load of ~2A, and it needs to go in place of one of these...

I need it to protect my circuit from accidental reverse polarity that will take the place of a resistor that looks exactly like that. If someone already gave a valid PN for that, I applogize, as my lack of understanding caused me to miss it.

Thanks guys.

Post Edited (CharlieII) : 4/7/2009 9:44:57 PM GMT

mctrivia · 2009-04-07 21:40

w =v * I

since v=0.7 then I is all that matters

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CharlieII · 2009-04-07 21:44

Somebody said...

w =v * I

since v=0.7 then I is all that matters

My mistake was in expecting the diode to be rated for the amount of power it would actually be transferring. As in... 15v * 2A = 30W...

This isn't the first thing that seems horribly counter-intuitive to me about electronics either. For instance, the diode itself is based on the same semiconductor conception where electron flow is the premise for everything. Yet.... the symbol for a diode is pointing completely in the opposite direction of electron flow. It might seem obvious to the casual observer that you all may have acclimated yourselves to these conventions that are not immediately obvious, or even apparently logical for someone trying to think through things without any education on the subject.

Post Edited (CharlieII) : 4/7/2009 9:59:51 PM GMT

mctrivia · 2009-04-07 21:49

the mistake is thinking power is transfer that way.

in a series circuit current is the same through. voltage is dropped across each device

power or w is the product of the 2

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CharlieII · 2009-04-07 21:54

Somebody said...

the mistake is thinking power is transfer that way.

in a series circuit current is the same through. voltage is dropped across each device

power or w is the product of the 2

I was only using wattage because I had seen resistors rated in wattage. Larger wattage, larger resistors.

Fwiw, I still have no idea how the diode would not be passing 30W if the voltage was 15 and the amperage was 2. It merely seems they are not rated based on power transfer, but on power consumed.

Philldapill · 2009-04-07 22:02

Charlie, there really isn't any such thing as "power transfer". I can understand how it seems that way, however. The traditional analogy for electricty is water flowing through a pipe. This analogy works very well when describing voltage/pressure and current/flow, but isn't really all that correct in much else. "Power" is energy per second. You can't "move" power, but you can "cause" power - if that makes any sense. Your voltage source, whether it's a battery, a wall outlet, or the clouds in the sky(lightning), all of which "cause" work to be done.

Just remember that in electricty, power is current THROUGH something, times the voltage ACROSS something. I really get bugged by advertisements boasting some product that can "handle" X amount of watts... The device doesn't care what the load is doing. It only cares about how -IT- is affected, i.e. how much power -IT- dissipates. That's another topic altogether though...

CharlieII · 2009-04-07 22:07

I see.

So it seems that I should go back to DigiKey or similar and search for 1.5 to ~2W diodes then.

Is "Axial Leads" the correct term for what looks like the resistor pictured? And I recall there being 5 to 10 subcategories under the heading "diode" each with hundreds if not thousands of choices.

What "type" of diode should I be looking for?

Thanks guys.

Philldapill · 2009-04-07 22:11

Yes, "axial" lead is the one you want that has wires on the ends that can bend like that resistor. A 2W diode should work, but you could also get another type of diode called a "schottky" diode. These have a voltage drop of around 0.35V, so they dissipate about HALF the power as a regular diode. This is a good thing, because the diode can handle more current. Also, it's just a good practice to use parts that can handle about twice the amount of power(if not more) than you expect for your worst case design.

i.e., if you think you'll only be pulling 2A, get a diode that can handle 4A or so. This is just to ensure a little wiggle room in case something goes wrong that you didn't expect. Just good practice.

CharlieII · 2009-04-07 22:26

So I should use a Schottky then?

I have to ask now.... if a device exists that only produces half the voltage drop..... why is it not used in every application requiring a diode, and why are the ones producing a .7 volt drop not discontinued?

What is the benefit?

CharlieII · 2009-04-07 22:55

Is this any good?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MUR405GOS-ND

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N5820DITR-ND

Post Edited (CharlieII) : 4/7/2009 11:52:13 PM GMT

mctrivia · 2009-04-07 22:55

some times you may want a reference voltage 1.4v below the input 2 diode can give you that. but generally shotkey are better in every way except price. faster response time and lower voltage drop.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

CharlieII · 2009-04-07 23:20

I suppose this one should work well?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N5820DITR-ND

One thing I can't seem to find on in the specs nor on the datasheets is the size. I don't see any dimensional information. How am I to know the diameter of the thing isn't 3 feet with 1 foot diameter leads?

What do you use for scale when ordering something? I have to know it will fit.

(confused)

Carl Hayes · 2009-04-07 23:46

(1)· The power rating for any device is the power it can dissipate as wasted heat.

(2)· Physical dimensions of devices are given as standard JEDEC package types, as DO-1, TO-3, etc.··Nominally, the DO stands for "diode outline", TO stands for "transistor outline" and so forth.· These are standard, and listed in many places.· Probably you can find them in a Google search.

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mctrivia · 2009-04-07 23:46

usually dimensions are at the bottom of data sheet will check when I get home

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

mctrivia · 2009-04-07 23:51

5.3mm diamiter 9.5mm length

leads are 25.4mm long 1.3mm wide

first page of datasheet

P.S. You should read the datasheet for any part you want to use. It has all the important information.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Leon · 2009-04-07 23:52

CharlieII said...
So I should use a Schottky then?

I have to ask now.... if a device exists that only produces half the voltage drop..... why is it not used in every application requiring a diode, and why are the ones producing a .7 volt drop not discontinued?

What is the benefit?

Ordinary diodes are much cheaper, the greater voltage drop often doesn't matter.

Lepn

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CharlieII · 2009-04-07 23:53

Haha....

I looked all over the first data sheet, and just ASSumed that the other would be the same.... yet right on the very first page the second one shows the dimensional data.

(smacks forehead)

[noparse]:)[/noparse]

Mike Green · 2009-04-07 23:57

Schottky diodes have more leakage than ordinary diodes and I believe ordinary diodes can be made with higher reverse voltages than Schottky diodes.

Philldapill · 2009-04-08 05:37

Mike/Leon, you beat me to it.

Here are the Pro's of a Schottky:
-Lower capacitance, which translates to faster response times.
-Lower voltage drop, which makes them more efficient and closer to an "ideal" diode by only blocking, but having minimal drop when forward biased
-That's about it...

Con's of a schottky:
-Generally, lower reverse bias voltage. This is the voltage that the diode can withstand if you plug your power in wrong. If it's rated for 35V(common voltage for schottky's), and you apply more than 35V, chances are, the diode will "break" electrically - bad news.
-A little more expensive than normal diodes
-Can't think of anything else off the top of my head...

For the Pro's and cons of normal diodes, just invert all that. [noparse]:)[/noparse]

CharlieII · 2009-04-09 21:40

Thanks Guys.

Simple Diode question

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