Do any LEDs require ballasts? - Page 1
Forum section: electronics / beginners

author: Sam Hobbs
author_rank: Contributor
timestamp: August 01, 2019, 07:40:07 am
content: I say no, that no LED requires a ballast. Truly, the idea sounds preposterous to me. Yet in another website's forum someone is saying that some LEDs require ballasts. The following is a summary of that discussion.The other person says to install LED lights made to work with a ballast. Someone else said that their understanding is that some LED lights still require a ballast.So I said:For LED lights a ballast just wastes electricity. I am not sure why a ballast is needed for fluorescent lights but they have mercury-vapor gas in them.Electrical ballastslimit the amount of current, I assume due to the gas.Then someone says something that makes no sense to me. They essentially say that the reason why a ballast is needed for fluorescent lights is so that you do not have to rewire a LED light to bypass a working ballast. Huh?So I said:The important thing is that no LED light requires a ballast. Perhaps there is a need to make LED lights such that they work in spite of ballasts but they do not require a ballast.A Light Emitting Diode (LED) is a diode. I am not an electronics expert but I learned about diodes nearly half a century ago. Diodes are solid-state and can work totally on their own with nothing more than a wire with electricity.So then someone said that LED lights have a driver of some sort that are analogous to ballasts. So my reply (among other things) was thatThey do require conversion from AC to DC (but actually that is exactly what a diode does) but that is called other things, such as regulated power supply.So then I left it at that. I said I hope that others are not mislead but I was not going to dwell on it. Perhaps I should agree with them that LED lights require something like a ballast but that seems too misleading.If anything I said is wrong then I want to know that. I can link to the entire discussion if the moderators will not complain.Related to this, the Wikipedia article about fluorescent lights says very much about ballasts but does not explain why a ballast is needed other to say that a ballast regulates current. Perhaps someone could enhance the Wikipedia article.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 01, 2019, 07:51:18 am
content: LEDs are current driven devices. They require extra circuitry, whether that be a simple resistor, an active regulator or DC/DC converter, to ensure they are driven by the correct amount of current.LEDs cannot regulate their own current. So yes, LEDs require a ballast.

author: oPossum
author_rank: Super Contributor
timestamp: August 01, 2019, 07:57:04 am
content: There has to be something to regulate the current. Calling that a ballast, a transformer, a power supply, or an inverter are all reasonable terms within certain contexts.

author: Brumby
author_rank: Unknown
timestamp: August 01, 2019, 08:25:42 am
content: Quote from: oPossum on August 01, 2019, 07:57:04 amWell said.It should be pointed out that the word "ballast" has more than one meaning. The key meaning here is a functional one - being any device that limits current.Another meaning is the specific component used, for example, in old fashioned fluorescent lights. This is what you would get if you walked into an electrical wholesaler and asked for a ballast. It is this product that limits the current flowing through a fluorescent tube.

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2019, 10:08:33 am
content: Quote from: Sam Hobbs on August 01, 2019, 07:40:07 amYou seem to be confused about about ballasts and why they're needed, both with LEDs and fluorescent tubes.Fluorescent lamps need a ballast because their resistance drops, as the current increases, i.e. they have a negative voltage vs current coefficient. When power is first applied, the tube will be an open circuit, the starter connects the heaters to the mains, which reduces the starting voltage, by increasing the energy level of electrons in the electrons in the electrodes. Once the tube is lit, the resistance the tube drops, as the current increases. Without a ballast, the current would increase to the point where only the wiring resistance limits it, causing the breaker to trip.LEDs also need a ballast to limit the current. They have a logarithmic current vs voltage coefficient. This isn't as bad, as a fluorescent tube, because the voltage does increase, with current, but only slightly. A small increase in voltage, will cause the current to increase several times. Another issue is the resistance of an LED has a negative temperature coefficient. If the voltage is kept constant, the current will increase, as the LED warms up, causing further heating, which could result in a runaway situation which will destroy the LED.There also seems to be some confusion between rectifiers and LEDs: why not use the LED to convert AC to DC, as well as light? Because an LED only illuminates when it's forward biased. Connecting an LED straight to AC would result in it only lightly half of the time. Another issues is the maximum reverse voltage an LED can take is typically 5V, where as rectifier diodes can withstand hundreds of volts. To light continuously two LEDs would be required: one to light on the positive cycle and another for negative. Some small indicator LEDs are run directly off AC, but efficiency isn't the main goal.Bare LEDs don't have a built-in ballast and will need an external one. LED bulbs/lamps designed to be connected directly to the mains will have a built-in rectifier and ballast.Are you talking about retrofit LEDs for fluorescent tube fittings? I've not used them before but you should check the instructions. It wouldn't surprise me if some will work with the fluorescent fitting's internal ballast, whilst others have an in-built ballast and need to be wired directly to the mains, with the fluorescent lamp ballast bypassed.

author: Nusa
author_rank: Super Contributor
timestamp: August 01, 2019, 10:21:03 am
content: I suspect the confusion has to do with LED-based tubes for extant fluorescent fixtures. One version of the product is a plug and play replacement and is interchangeable with one in any fixture that takes that size tube. It also means the original ballast of the fixture remains in use. This is often the easiest route for many businesses to take, since their maintenance man or janitor can just replace old tubes as required.Another version of LED tubes requires that the fluorescent fixture be rewired to eliminate using the ballast. For businesses, that typically requires hiring an electrician and replacing all the tubes at once. Those fixtures will then require LED tubes for the rest of their life. So no more ballast, but more expensive and inconvenient to implement. Often replacing the entire fixture with a modern LED product is a better choice than modifying old ones, since you have to hire a professional in both cases.

author: soldar
author_rank: Super Contributor
timestamp: August 01, 2019, 12:25:52 pm
content: In the original, narrow sense "ballast" is an inductance by analogy between a physical mass and an electric current going through an inductance. Probably not the best choice of name but I suppose when you were trying to teach people with hardly any education a century ago it was helpful.So then the name came to denote electronic circuits used for the same purpose. Now it is just a name with no connection with the original meaning.To the answer "do LEDs require ballasts?" the only possible answer is "define ballast and I'll give you an answer". Without a definition of ballast the question cannot be answered. Does it require an inductance? No. Does it require a control/driver circuit? Yes. Is a control/driver circuit a "ballast"? Why would it matter what we called it? Does it need "something"? Yes. Definitely.

author: bostonman
author_rank: Super Contributor
timestamp: August 01, 2019, 01:27:18 pm
content: QuoteI didn't think they were one or the other. Typical LEDs have a voltage drop (1.2V) and a series resistor to limit the current. Are they really exclusively current driven?

author: Brumby
author_rank: Unknown
timestamp: August 01, 2019, 02:16:28 pm
content: Some may argue the semantics - but, as a guiding principle - Yes.Basically, with LEDs, you set them up to have their current controlled and the voltage will work itself out (Just as long as there is enough voltage to achieve the required threshhold).In contrast, incandescent light bulbs are supplied with a specific voltage and the current works itself out.

author: bitwelder
author_rank: Super Contributor
timestamp: August 01, 2019, 02:22:08 pm
content: Quote from: Nusa on August 01, 2019, 10:21:03 amWell, does this version of LED-based tubesallowsorrequiresto use a fluorescent-type ballast?And if it's possible to remove the old ballast, would be there any advantage in terms of LED operation in doing that? (a disadvantage could be the cost of the electrician called to do the change).

author: bostonman
author_rank: Super Contributor
timestamp: August 01, 2019, 02:30:25 pm
content: QuoteMakes more sense as the response implied one needs to inject (as an example) exactly 100mA, else the LED will not light.

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2019, 09:20:51 pm
content: LEDs require ballasting to control their current because of their low resistance. Gas discharge tubes require ballasting because they have negative resistance. Magnetrons, like in a microwave oven, also require ballasting to limit current. Incandescent bulbs on the other hand have a large positive temperature coefficient of resistance so they are their own ballast and are sometimes even used to ballast other devices.Resistance can be used as a ballasting element but this wastes power so typically inductance or capacitance is used in power applications.

author: Nusa
author_rank: Super Contributor
timestamp: August 01, 2019, 10:59:44 pm
content: Quote from: bitwelder on August 01, 2019, 02:22:08 pmFor instance, this one would require the ballast remain:https://www.amazon.com/AmazonBasics-LED-Tube-Light-Compatible/dp/B07BNJBX5T/This one would require the ballast be bypassed:https://www.amazon.com/AmazonBasics-LED-Tube-Light-Compatible/dp/B07BNGX44F/And apparently there are now hybrids, which can be used either way:https://www.amazon.com/Parmida-LED-Single-Ended-Double-Ended-Shatterproof/dp/B07FF91JHR/

author: Brumby
author_rank: Unknown
timestamp: August 01, 2019, 11:27:15 pm
content: Quote from: bostonman on August 01, 2019, 02:30:25 pmThe issue with LEDs and their current is not so much for currents that are at or below their nominal current, but when you start exceeding this. Your typical LED nominally rated for 20mA will still produce useful light output at 15mA - just not as bright. You can also see one light up with a current of 1mA - or even less - but you will need to look more closely to see it.Where LEDs have a problem is when the current exceeds the rating. Provide power with a voltage higher than Vfand you will get a brighter LED - but at the cost of a shorter life. The real danger with not having current control is that as you nudge the voltage higher and higher, your LED will start conducting current at a dramatic rate.To find out how any given LED will respond, you just need to check out the appropriate chart on its datasheet - but here is one to illustrate the general features of such a chart. You will see that different colours have similar responses, but at different voltages - and that the nominal voltage for any given LED will be for a stated current.