Electronics > Beginners
Do any LEDs require ballasts?
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Sam Hobbs:
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 ballasts limit 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 that They 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.
Dabbot:
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.
oPossum:
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.
Brumby:

--- Quote from: oPossum on August 01, 2019, 07:57:04 am ---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.

--- End quote ---

Well 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.
Zero999:

--- Quote from: Sam Hobbs on August 01, 2019, 07:40:07 am ---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 ballasts limit 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 that They 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.

--- End quote ---
You 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.
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