High Forward Voltage Diodes - Page 1
Forum section: electronics / projects

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 07, 2019, 11:32:06 am
content: Hi,I'm looking after diodes with a high forward voltage drop even at around 30mA. I want to have a string of series diodes that generate around 4-5V of drop. I have seen some high voltage diodes with a drop of around 1V. I only need a booking voltage of around 20V so this is not a big issue, also reverse recovery and leakage and so on are not important.My main driver is cost so ideally cheap as possible with a high forward voltage drop. Smaller size and surface mount would also be an advantage.Does anyone have any part numbers in mind?Cheers!

author: Berni
author_rank: Super Contributor
timestamp: November 07, 2019, 12:39:04 pm
content: Or you could use a 4.7V zenner diode. If it still needs to work like a diode in the reverse direction then you can add a normal diode in series.Keep in mind that the voltage drop of a diode is fairly unstable and will change a lot with the current and temperature.

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 07, 2019, 12:43:17 pm
content: I was going to suggest a zener diode, too, but with a warning that zener diodes with less than about 5.1V to 5.6V breakdown voltage tend to have a high slope resistance; that is, the breakdown voltage varies quite a bit with current. If the current is fixed and the breakdown voltage doesn't need to be precisely specified then this won't be a problem.

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 07, 2019, 12:51:31 pm
content: Fantastic, I was considering zener diodes as well so it's good that it is confirmed by other people as well.I am not looking for a lot of accuracy here so I it will do. A voltage drop in the range of 5 V is what I am after. Thanks for your help guys

author: graybeard
author_rank: Frequent Contributor
timestamp: November 07, 2019, 03:29:16 pm
content: why don’t you just use an adjustable shunt band gap voltage regulator? They are inexpensive and far more temperature stable.

author: magic
author_rank: Super Contributor
timestamp: November 07, 2019, 05:15:30 pm
content: Perhaps that could work. Texas Instruments TL431 datasheet permits 50µA of reverse current out of the input, so make the divider with high enough impedance and input protection under reverse bias is solved. Then place a diode in series with the cathode within the feedback loop and you have a diode with perfect 5V forward voltage and <50µA reverse leakage.Probably overkill for the OP.

author: thinkfat
author_rank: Unknown
timestamp: November 08, 2019, 06:50:51 am
content: 30mA, that is some serious current for a zener. You want to be careful about the power rating of those diodes.

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 08, 2019, 08:27:57 am
content: That is far too expensive for the purposes of this project unfortunately. But I do agree it would be a better solution

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 08, 2019, 08:33:52 am
content: Quote from: thinkfat on November 08, 2019, 06:50:51 amTake 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.I am not missing anything am I?

author: forrestc
author_rank: Unknown
timestamp: November 08, 2019, 08:45:57 am
content: Quote from: Glenn0010 on November 08, 2019, 08:33:52 amA 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 08, 2019, 09:22:57 am
content: Quote from: Glenn0010 on November 08, 2019, 08:27:57 amYou have described one "solution" and other people have described other "solutions". I use quotation marks because we'veno wayof 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 tipshere.

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 08, 2019, 09:24:17 am
content: Quote from: forrestc on November 08, 2019, 08:45:57 amGreat point! I'll have to look at it in more detail!

author: thinkfat
author_rank: Unknown
timestamp: November 08, 2019, 09:53:25 am
content: Quote from: forrestc on November 08, 2019, 08:45:57 amAlso realize that there will be an impact on the lifetime of the diode if you run it at 100°C constantly.

author: filssavi
author_rank: Frequent Contributor
timestamp: November 08, 2019, 10:29:02 am
content: 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

author: Psi
author_rank: Super Contributor
timestamp: November 08, 2019, 11:49:50 am
content: 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 5V2 Red would be 4.5V or so

author: ezeisel
author_rank: Newbie
timestamp: November 09, 2019, 03:08:36 am
content: maybe consider a Vbe multiplier?https://en.wikipedia.org/wiki/Rubber_diode