"Digital Transistors" but no Digital LEDs? - Page 1
Forum section: electronics / general

author: JoeN
author_rank: Frequent Contributor
timestamp: January 11, 2017, 09:32:27 am
content: I call discrimination.Digital transistors seem like they might be useful once in a while, but probably not all that often. There sure seems to be a lot of makes and models of them, though.Why no pre-biased LEDs for the popular voltages - 5V, 3.3V? You would think it would be useful and easy to manufacture. But nope, no one ever did it.Why not do you think?

author: Kjelt
author_rank: Super Contributor
timestamp: January 11, 2017, 09:37:44 am
content: What do you mean with a "digital" transistor? Even a fet can be operated in a linear (analog) mode.Do you hint on digital buffers like the older TTL 74 or CMOS 4000 series? You can get leddisplays with integrated ic that operate on 5V.Why would you want a led with a built in resistor to limit the current and operate on a prefixed voltage, the manufacturer would limit its usefullness and so its target audience.So I do not understand your point or problem

author: ebclr
author_rank: Super Contributor
timestamp: January 11, 2017, 09:43:11 am
content: Digital transistors are transistor with resistor inside

author: Kjelt
author_rank: Super Contributor
timestamp: January 11, 2017, 09:49:05 am
content: Quote from: ebclr on January 11, 2017, 09:43:11 amOk only saw those myself in arrays like the well known ULN2803 and then you need a different version per operating voltage which is probably why they are scarce.For leds it is just one resistor you need, the only application I can think of this could be worth while is in the ledstrips business, saves some steps.

author: MK14
author_rank: Super Contributor
timestamp: January 11, 2017, 09:50:41 am
content: Quote from: JoeN on January 11, 2017, 09:32:27 amTwo problems, come to mind.One is that it will potentially (ignoring some smart, inbuilt PWM or something), make the LED run hotter inside, for the same current. that would be a bad thing. As they don't seem to be able to (or like) dissipating heat.The other thing, is that people (in their specific application), want to also choose a brightness (range) for the LED. A pre-biased one (e.g. a built in resistor) would only have one brightness level. Unless PWM brightness modulation or something else was done.tl;drThe external resistor, also allows the designer to choose a suitable brightness level and/or have it automatically change with ambient light level via a light sensor.

author: Jorpy
author_rank: Contributor
timestamp: January 11, 2017, 09:57:02 am
content: Optosupply makes LEDs with an inbuilt resistorThis is the datasheet of the 5V line:http://www.optosupply.com/uppic/2016813432264.pdf

author: Kjelt
author_rank: Super Contributor
timestamp: January 11, 2017, 10:01:04 am
content: Quote from: MK14 on January 11, 2017, 09:50:41 amGood point, for instance leds to signal a devices heartbeat (alive) are often operated at way higher than the nominal current to make it look very bright but since it flashes only a few % of the time it will survive.

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2017, 10:40:41 am
content: Kingbright also make them:http://sg.element14.com/kingbright/l-53id-5v/led-5mm-he-red/dp/1142556

author: CJay
author_rank: Super Contributor
timestamp: January 11, 2017, 10:45:51 am
content: Quote from: MK14 on January 11, 2017, 09:50:41 amCertainly for a given voltage you'd have a limit to maximum brightness but you could dim down from that though not being able to pulse it with higher current at that VCC could be a problem.I don't remember ever having seem a LED with inbuilt resistor used by anyone other than car 'customisers' who seem unable/unwilling to put the effort in and calculate their own resistor values.I much prefer to be able to blow up my own LEDs by selecting the current limit for myself.And, I suppose, LEDs with chips like the WS2812 embedded in the package are 'digital' if we wanted to stretch a point.

author: rdl
author_rank: Super Contributor
timestamp: January 11, 2017, 08:40:26 pm
content: "Digital LEDs" are not hard to find if you know how to look. I have a bunch of them and they're very useful for breadboarding (less stuff to wire up and connect). They are usually called resistor LEDs, or sometimes by the voltage, such as 5 volt LED. If you go to Mouser and search, just specify 5 as the voltage.

author: CJay
author_rank: Super Contributor
timestamp: January 11, 2017, 08:42:08 pm
content: Quote from: rdl on January 11, 2017, 08:40:26 pmWell now you mention that, it's obvious how useful they'd be for breadboarding

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 11, 2017, 09:02:16 pm
content: They definitely did exist many years ago, but can't recall last time I saw them - I think some had constant-current regulation over a reasonable voltage range, but I think the vast drop in cost of LEDs means it's not viable to make specials for a very limited market.I have a feeling they came out around the same time as flashing LEDs, so maybe they used the same leadframe with a current reg instead of the flasher IC

author: CJay
author_rank: Super Contributor
timestamp: January 11, 2017, 09:07:10 pm
content: Definitely still out there:https://www.rapidonline.com/kingbright-led-5mm-5v-and-12v-60113http://www.maplin.co.uk/p/5mm-red-led-ck46aAnd many others.

author: rdl
author_rank: Super Contributor
timestamp: January 11, 2017, 10:52:11 pm
content: The ones I got from Mouser weren't real cheap, but worth it to me for the easy breadboarding. I have seen on eBay some 12 volt versions listed as "automotive" LEDs, but I don't remember the price. The 12 volt type should light up pretty well even at 5 volts since modern LEDs are so efficient.http://www.mouser.com/Optoelectronics/LED-Indication/_/N-b1d1kZscv7?P=1yox9hx&Ns=Pricing|0

author: mmagin
author_rank: Frequent Contributor
timestamp: January 11, 2017, 11:12:39 pm
content: I suspect it's easy to make a 'digital transistor' using standard analog IC processes, but it's harder to make a resistor on the same die as a LED, and thus they have to stick a seperate resistor in the same package for significantly more cost.

author: Brumby
author_rank: Unknown
timestamp: January 12, 2017, 02:19:22 am
content: Quote from: CJay on January 11, 2017, 10:45:51 am

author: JoeN
author_rank: Frequent Contributor
timestamp: January 12, 2017, 03:50:56 am
content: Thanks, guys. Newark/Element 14 has the Kingbrights in stock, all varieties, so I am going to order a few with my next order. I'll put it under the scope and see if I can pick out the resistor or see what is going on there. Too bad none of them use a clear plastic lens.

author: amyk
author_rank: Super Contributor
timestamp: January 12, 2017, 12:02:38 pm
content: I think bigclivedotcom has a video of an LED lamp that puzzled him briefly because it seemingly connected the LED directly across the mains. The LED had a constant current driver integrated into the package along with a string of regular die.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 12, 2017, 02:56:22 pm
content: Quote from: JoeN on January 12, 2017, 03:50:56 amIf you have some trichloromethane, that will dissolve the plastic off.

author: JoeN
author_rank: Frequent Contributor
timestamp: January 12, 2017, 03:27:24 pm
content: Quote from: mikeselectricstuff on January 12, 2017, 02:56:22 pmI don't, but I never need much of an excuse to order dangerous chlorinated hydrocarbon chemicals. I will add that to the list too.Actually, though it looks like I can acquire this, it would be more difficult and expensive than I had expected. This is also known as chloroform and many sellers treat it as a chemical with nefarious purposes. Are there any other options?