T8 LED questions - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 23, 2016, 05:59:12 pm
content: I am baffled by these LED T8 replacement tubes they are making now. What possible use does the old fluorescent ballast have for LED lights?I can't seem to find any answers and in true info overload style, all I get is more confused.http://www.lighting.philips.com/main/support/support/faqs.htmlIt's like Cadence wrote this documentation, where you just read more and more and understand less and less.What type of power supply is inside the tube? What kind of power does an "instant start" ballast provide that a "rapid start" doesn't? Can't I just plug the T8 directly to mains?My parents have an old GE low voltage switching system, I have to replace several expensive relays. I want to put LEDs everywhere in the basement as it is quite gloomy these days, with fewer than half of the lights working. There are 8 fluorescent tubes to be replaced, for a total of 4 rapid start ballasts.So on top of the 40$ relays and the 20$ tubes, now I need to rip out the old rapid start ballasts and buy instant start ballasts as well? Makes little to no sense to me.

author: jitter
author_rank: Frequent Contributor
timestamp: September 15, 2016, 06:47:10 pm
content: Better late than never...I don't pretend to know all the ballast types sold in North America, but here in 220-240 V countries by far the most common are the magnetic ballast + starter ("preheat start") or the electronic ballast (HF).Because of their simplicity, the magnetic ballast will be around in sheer numbers.When installing LED-tubes, for best efficiency, it should be removed or bypassed, so that the LED tube runs straight from the mains. So yes, an LED-tube is just like any other LED lamp operating directly on mains voltages in that it has an LED-driver converting mains to a suitable DC voltage.But the fluorescent ballast may not be easy to reach inside most luminaires making conversion time consuming. So for ease of conversion (especially for non techies or big jobs) they can be left in place and a dummy starter takes the place of the fluorescent starter. That dummy starter is just a short circuit. The ballast basically is degraded to being a conductor.E.g. this Philips tube's name implies the use on mains or magnetic ballast:MASTER LEDtube EM/Mains.I'm not familiar with "instant start" or "rapid start" ballasts. But what I read in thisWikipedia articleis that the former forces a tube on by sheer high voltage (the tubes have only one pin on either side) and the latter by separate windings and an grouding strip applied on the outside of the tube to aid strike an arc. I guess neither of these two types would be really suitable to be left in place when an LED tube is installed.There are also LED tubes that are supposed to work off some of the electronic ballasts as well (the Philips tubes suitable for that seem to have "HF" in the name (e.g.MASTER LEDtube InstantFit HF T8), for the same reason as above, however, compatibility may be more of an issue.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 15, 2016, 11:59:30 pm
content: Hey thanks. I was just stymied by the incredible amount of printed information on the tube packaging, but none of it was useful.Turns out the T8 LEDs at Home Depot work only with instant start ballasts, presumably the tube then bucks that back down to whatever it needs.I just found this so non obvious and silly I couldn't quite get it.I found a store that sells direct connect T8 LEDs, this is much better because I don't have to buy 4 instant start ballasts to replace the 4 rapid start ballasts already installed.I'm going to open up the light fixtures and simply disconnect and bypass the existing old ballasts, then I don't even need to dispose of them.My parents are happy that I fixed those broken relays, replaced the incandescent lights with LEDs, now I just need to finish the T8 job.

author: amyk
author_rank: Super Contributor
timestamp: September 16, 2016, 12:57:51 am
content: Quote from: Alex Eisenhut on July 23, 2016, 05:59:12 pmFluorescent tubes are driven with constant current. LEDs are driven with constant current. You can figure out the rest...and the Chinese figured it out long ago, that by simply putting enough LEDs in series and parallel to approximate the tube's operating voltage and current, you could run LEDs in an unmodified fluorescent fixture. The starter usually relies on the voltage across the tube initially being high enough that it means no discharge across it, so with an LED string in its place, it never actually attempts to "start" the "pseudo-tube".http://m.eet.com/media/1202078/F1large.jpg

author: jitter
author_rank: Frequent Contributor
timestamp: September 17, 2016, 06:09:30 am
content: Quote from: Alex Eisenhut on September 15, 2016, 11:59:30 pmThat kind of text usually is only there for arse covering of the manufacturer. As a techie I understand what they're saying, but the wording is primarily chosen to be concise, not to be obvious.It usually only says what it is or is not suitable for but never why.QuoteSo you can't operate those directly off the mains?QuoteGood choice and good job.Direct connect implies it has a proper LED-driver on board, and IMHO is the only way to go. Out with the antiquated stuff...

author: jitter
author_rank: Frequent Contributor
timestamp: September 17, 2016, 06:32:13 am
content: Quote from: amyk on September 16, 2016, 12:57:51 amI kinda agree with what you're saying and kinda not. Once the arc has been struck inside the tube, which is one of the functions of the ballast + starter, the other function of the ballast is to act as a current limiter. It is not a current source, variations in the mains are not regulated and a dipping mains is easily seen in dipping light output.QuoteWith our higher mains voltage, I don't think they are common over here, and possibly not allowed for safety reasons.The commonly found LED tube is supplied with a dummy starter to close the circuit. This way, there's no need to open a fixture and rewire stuff.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2016, 06:48:20 pm
content: Quote from: jitter on September 17, 2016, 06:32:13 amIn an ideal world, the fluorescent tube should be powered from constant current source and the same goes for an LED. A magnetic or capacitive ballast provides a fairly close approximation to a constant current source.