Transformers

[patpappas]

Stepping down 120v volts to 12v to feed LED lighting, if the LED is dimmalbe and most transformers are not, what does make a transformer dimmable? I have purdhased some dimmable LED with transformers before, the company has scince gone out of business, now I am trying to find dimmable transformers again.

 

[mivey]

There are many dimmable transformers available. Just Google "dimmable transformers" and you will see a lot of them on lighting supply pages.

As for dimming, the notes say that dimmers normally do not handle inductive loads well and can cause regular transformers to overheat.

Non-dimmable transformers hum more when dimmed, have more trouble with harmonics, and have high start-up currents.

 

[gar]

080630-2133 EST

patpappas:

It would be useful to know more about the LED lights.
How many LEDs per package, and what is the current limiting means?

Suppose there was one LED in a package, then there needs to be a current limiter. In the simplest form this is a resistor. Suppose the LED had a forward drop of 2.0 V at 25 MA and was to be supplied from 12 VDC, then the current limit resistor would be 400 ohms. This would be dimmable by changing the voltage. To run from an AC supply there needs to be a rectifier feeding the LED.

If two LEDs were in parallel and back-to-back, then an AC supply would work, but there still needs to be a current limiter.

In general it is not feasible to parallel LEDs and use a single current limit resistor.

If your LEDs require DC or have an internal diode, then one or more LED assemblies in parallel will put a DC current component in the secondary of the transformer. This offsets the hysteresis loop and increases transformer heating. A full wave bridge rectifier on the transformer output would eliminate that problem and eliminate the need for an added diode in the LED assembly.

Two ways to dim your lights. Put a Powerstat variable transformer at the transformer input, or put a DC regulator on the rectified and filtered transformer output.

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[iwire]

There are plenty of aesthetically pleasing dimmers on the market designed to 'dim' a standard magnetic transformers.

 

[HighWirey]

Stepping down 120v volts to 12v to feed LED lighting, if the LED is dimmalbe and most transformers are not, what does make a transformer dimmable? I have purdhased some dimmable LED with transformers before, the company has scince gone out of business, now I am trying to find dimmable transformers again.

Been out of the loop for a while, however you're statement "the LED is dimmalbe" is confusing to me. Do you mean that varying the input voltage to a transformer will vary the LED light output?

Wake me up men.

Best Wishes Everyone

 

[gar]

080701-1123 EST

HighWirey:

An LED's light output is a monotonic function of its current up to some failure point.

Thus, if varying the input voltage to a transformer controls the LED current, then you can use this to control the LED intensity.

If the voltage to the input transformer of a DC regulated power supply is adjusted, then no you will not control the LED intensity until the DC regulator drops out.

If internal to the LED assembly is a current regulator, then changing the applied voltage will not change LED intensity until the current regulator quits regulating.

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[HighWirey]

080701-1123 EST

HighWirey:

An LED's light output is a monotonic function of its current up to some failure point.
Thus, if varying the input voltage to a transformer controls the LED current, then you can use this to control the LED intensity.
If the voltage to the input transformer of a DC regulated power supply is adjusted, then no you will not control the LED intensity until the DC regulator drops out.
If internal to the LED assembly is a current regulator, then changing the applied voltage will not change LED intensity until the current regulator quits regulating..

You must have had alphabet soup for breakfast.

Forget that transformer vs voltage business to vary a LEDs intensity.

Yes, a LED can be dimmed to a very poor degree by varying his voltage, however the only correct way to control a leds intensity is by using pulse width modulation. Just look at those movable letter signs popping up around the world. Google the subject.

Best Wishes Everyone

 

[patpappas]

Yes we have purchased LEDs in the past were we drop the voltage to the transformer and dim the LED.

Been out of the loop for a while, however you're statement "the LED is dimmalbe" is confusing to me. Do you mean that varying the input voltage to a transformer will vary the LED light output?

Wake me up men.

Best Wishes Everyone

 

[gar]

080702-1348 EST

HighWirey:

Yes, a LED can be dimmed to a very poor degree by varying his voltage, however the only correct way to control a leds intensity is by using pulse width modulation.

What do you mean by "very poor degree"? Does this mean ease of controlling brightness, or linearity of knob position to brightness, or energy efficiency.

If a resistor is used for current limiting, then there is little difference between an adjustable voltage, and adjusting the average voltage by pulse width modulation.

If some type of control (could be pulse width) in combination with an inductor and no explicit current limiting resistor is used, then greater efficiency can be achieved.

What does the word poor mean?

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