Electronics > Projects, Designs, and Technical Stuff
High Forward Voltage Diodes
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tggzzz:

--- Quote from: Glenn0010 on November 08, 2019, 08:27:57 am ---That is far too expensive for the purposes of this project unfortunately. But I do agree it would be a better solution

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You have described one "solution" and other people have described other "solutions". I use quotation marks because we've no way of knowing if they are solutions!

Tell us your problem/goal, as well your solution. In most walks of life, technical or non-technical, if we know the reason for the question you will probably get a better answer. Either the answer will be more pertinent to your needs, or perhaps it can suggest a better alternative that you haven’t even considered. Don’t ask “Can you give me a lift into town?” Do ask “Can you give me a lift into town, so I can replace my broken frobnitz?”. The answer might be “There’s a spare frobnitz in the attic”, thus saving time, money, the environment – as well as making some space in the attic.

More hints and tips here.
Glenn0010:

--- Quote from: forrestc on November 08, 2019, 08:45:57 am ---
--- Quote from: Glenn0010 on November 08, 2019, 08:33:52 am ---Take the BZX84C5V1-7-F, for example. It has a Vz of 5.1 V multiply that by 0.03A = 0.153W, which is well below its 0.300W value.

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A bit of thermal engineering:

A random datasheet I pulled shows a 465 C/W Junction to ambient   .153W*465C/W= 77C rise.    77C rise + 25C Ambient = 102.        It will probably be a bit lower than this due to the thermal conductivity to the traces (220 C/W junction to leads), assuming the traces on the board can carry away some heat (will help to add surface area to the board, probably on pin 3).

With Tj = -55 to +150C, it should be ok, as long as ambient doesn't get way hot.  Or you run it near another heat-producing component.   Just realize it's going to be *HOT*, aka possibly boiling water, burning skin, hot.

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Great point! I'll have to look at it in more detail!
thinkfat:

--- Quote from: forrestc on November 08, 2019, 08:45:57 am ---
--- Quote from: Glenn0010 on November 08, 2019, 08:33:52 am ---Take the BZX84C5V1-7-F, for example. It has a Vz of 5.1 V multiply that by 0.03A = 0.153W, which is well below its 0.300W value.

--- End quote ---
With Tj = -55 to +150C, it should be ok, as long as ambient doesn't get way hot.  Or you run it near another heat-producing component.   Just realize it's going to be *HOT*, aka possibly boiling water, burning skin, hot.

--- End quote ---

Also realize that there will be an impact on the lifetime of the diode if you run it at 100°C constantly.
filssavi:
Actually that is a misconception, most of the failures of silicon devices (this mostly affects power devices, for obvious reasons) are not due to the temperature in itself, but to thermal cycling, as they are caused by mechanical failure of the connections between the various materials in the package, in this case it would be the bonding between leads and the die itself.

The main cause of failure is mechanical fatigue due to differing CTEs of the materials (coefficients of thermal expansion), this is not due by temperature in itself but by thermal cycling, that is going from hot to cold to hot again.

in this case if the zener is kept at a constant temperature throughout the design life i wouldn't worry.

if you want proof of this you have to look no further than ovenized quartz oscillators, despite working at a quite toasty 75 degrees C, the ones inside test equipment that are kept constantly at the same temperature can last for twenty years or more
Psi:
Why has no one suggested LEDs yet?
They have forward voltage in the 2-4V range. and 30mA is well within spec for lots of cheap power leds.

You can get SMT chip leds for super cheap. $1 for 100 etc..

1 White and 1 Red in series should get you pretty close to 5V
2 Red would be 4.5V or so
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