I've had a look round both the forum and the internet (via google) and cannot find a definitive answer to my question. I have an arbitrary number of RGB Piranha LEDs (data sheet linked to above) and want to answer 2 questions:
What is the best/correct way to wire these of the 2 methods below/attached?
Why? What are the advantages/disadvantages of each? For example does the second method risk thermal runaway?
The LEDs are being driven using the +5v pin from an Arduino UNO via a shift register (actually there are several chained together) but I don't think that's an issue right now. The objective of the project is to be able to build/create a number of small light panels with the piranhas (10 LEDs per panel) and make each one addressable via the arduino and I just want to be sure I'm getting the most out of these tiny but capable LEDs without burning them out.
The top circuit but this does not show them being driven by a shift register it shows them all being on at the same time.
The bottom circuit has the raw LEDs wired in parallel and with an LED not being a linear device they don’t shair the current evenly so they will be different brightnesses.
I've had a look round both the forum and the internet (via google) and cannot find a definitive answer to my question.
Anyone using the bottom circuit is an idiot. There are a lot of idiots on the internet.
The top circuit for sure, with a suitable high current sink shift register output connected to each resistor.
TPIC6C595 or TPIC6B595 would be great.
Limit current to 20mA per the datasheet for long life of the LEDs.
The top circuit but this does not show them being driven by a shift register it shows them all being on at the same time.
Correct, it doesn't. This was just a sample to ask the question about hooking up the LEDs. In terms of them all being on at the same time, there will be multiple LEDs off each shift register, the example is just a small part of a wider circuit which felt irrelevant to the question.
There are a lot of idiots on the internet.
Yes, yes there are.
Limit current to 20mA per the datasheet for long life of the LEDs.
Good point, will make sure this is done in the final project.
In terms of shift register I'm using the staple 595n (https://www.switchelectronics.co.uk/sn74hc595n). Thank you for your input and advice.
Well now, the datasheet cited in one place (only) specifies these as common anode RGB LEDs.
So neither wiring diagram is actually realistic. It depends on how you propose to control them.
The common anodes all connect directly to 5 V. Each LED requires its own resistor, that wodul be a minimum of 100 Ohms for green and blue, and 150 Ohms for red but you are unlikely to need full brightness.
If you propose to drive them from separate outputs of a 74HC595 (not such a good idea) then you should use 220/ 330 Ohm resistors to limit the total current per chip. If you use a TPIC6B595 you can common the resitstors for several (up to seven) LEDs to each output.
do-digital:
the example is just a small part of a wider circuit which felt irrelevant to the question.
Generally considered a very bad approach to posing questions.
Well now, the datasheet cited in one place (only) specifies these as common anode RGB LEDs.
So neither wiring diagram is actually realistic.
Fair point, the result of me rushing.
you are unlikely to need full brightness
I would like full brightness as these will be used in a large space.
If you propose to drive them from separate outputs of a 74HC595 (not such a good idea)
Why isn't it such a good idea? This is quite common in a number of guides/tutorials online though I'm concious that these may not all follow best practice so I'm keen to understand why.
If you use a TPIC6B595 you can common the resitstors for several (up to seven) LEDs to each output.
By "common the resistors" I presume you mean using one resistor for several cathodes? Does this risk thermal runaway if one fails?
Generally considered a very bad approach to posing questions.
I genuinely didn't think the whole circuit was relevant to a question about how to use resistors with a piranha LED, but will take the feedback on board.
do-digital:
This is quite common in a number of guides/tutorials online though I'm conscious that these may not all follow best practice so I'm keen to understand why.
Sadly, the number of copycat fervent febrile imaginations on the Internet has no actual connection with Reality.
Do you happen to use Facebook?
Mike has explained about the 74HC595. (I gave the datasheet link as a hint. )
Brightness is somewhat relative, by which I mean to say that a half reduction in power is not as dramatic as you might think. If you really want to drive the LEDs to the full 20 mA each, you will need the TPIC6B595s.
Paul, while I appreciate your advice and knowledge I'm not sure I appreciate the passive aggressive undertone in your comment about Facebook. You may have intended it to be a joke but please remember that things read differently in plain text.
I will have a look into the TPIC6B595s, thanks for the suggestion.
This is completely off-topic, but as it is clearly something which bothers you... Yes, I use Facebook as a communications platform to share information with friends & family who are spread far and wide. I do not use it as a source of information pertaining to news or research, having worked with the internet for most of my adult life I am well aware that people have mis-informed opinions and share them as they see fit.
I'm just not so great at electronics and trying to learn.
I just wanted to close off this thread by saying thank you again for the advice and input to my situation.
Going forwards I will be replacing the 74HC595s with one of the TPIC ICs mentioned but will be doing more research on the current requirements of the LEDs in my project before deciding which to use. I have learned more about current requirements of LEDs and the ability to draw this current from ICs which isn't really covered in many of the tutorials online.
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