12 vac to 3 vdc circuit

[Divetekno]

Hello. I need a circuit that will convert a 12 vac landscape lighting source voltage to a 3 vdc voltage for an LED lighting project I am designing. Any help would be greatly appreciated.
Thanks in advance!

 

[Hero999]

How much current do you need?

You do know that LEDs require a constant voltage source, not a constant current source?

What colour is the LED?

Do you have a datasheet or can you link to the supplier's website?

 

[Boncuk]

You do know that LEDs require a constant voltage source, not a constant current source?

Oops. Shouldn't it be vice versa? I learned it the other way around.

 

[Hero999]

Yes, you're right, I got that the wrong way round, an LED needs a constant current source, not a constant voltage source.

 

[Boncuk]

Yes, you're right, I got that the wrong way round, an LED needs a constant current source, not a constant voltage source.

In that case the OP could use the 12VAC rectified (and filtered) and fed into an LM317L wired as constant current source.

If the required current is more than 100mA he could use an LM317T.

Regards

Hans

 

[Hero999]

It depends on his requirements.

For example, if it's 300mA 3.5V and he doesn't want to use a large heatsink, he could use a bridge rectifier and capacitive ballast consisting of two 150µF electrolytic capacitors connected back to back with polarity protection diodes across them.

It's just a small 20mA LED, it's not worth it, a 680R resistor and a diode in series will do.

If he needs current regulation then the LM317 or switching regulator are both options.

 

[Divetekno]

Thanks for the replies

All I am trying to do is add 4 LED "candles" to my landscape lighting. The landscape low voltage lighting supplies 12 VAC and I need 3 VDC at each of the 4 LEDS. The current draw would be very low. Each LED "candle" I am adding has only 1 orange LED with a flickering circuit attached to it. Should I wire the LED's in parallel to get the same voltage at each LED "candle"? And actually I am measuring only 10 VAC at the point where I want to attach the LED's. It is at the end of a pretty long run (maybe 100 ft) of low voltage landscape lighting wire with several lights attached before the end where I want to add on the LED lights. I guess this accounts for the voltage drop at the end of the run.
Any help is greatly appreciated.

 

[Ubergeek63]

it depends on if you are interested in efficiency or just functionality

the assumption in this forum is that all you care about is functionality, not worrying about efficiency or reliability

you can not run a "flickering circuit" on individual lamps running off a current source, nor can you run LEDs directly in parallel

Interesting Electronic Schematics and Circuits - Novotill

many of the flicker circuits are micro based

 

[Divetekno]

Question

Thanks for the help, but I don't believe I fully understand you. The 4 LED lights I wish to use have the flicker circuit attached to each LED. They were normally made to operate off of a CR2032 3VDC battery. is there an easy way to drop and rectify the 10VAC source voltage from my landscape lights to the 3VDC needed by each of the 4 LED flicker circuits?
Thanks for your help.

 

[Ubergeek63]

IC you are starting with little LED flameless candles... sure just regulate it down

from AC you would need to rectify it and filter it too... sorry i tend to be a little obtuse, most of my posts are from work while a compiler is running .. at home i tend to be playing MMORPs

i always recommend a bridge rectifier but you could get away with a single diode, a capacitor, and an LM317 set up for 3V

 

[Divetekno]

Thanks

Thank you for your help. Could you refer me to a circuit diagram that would show the correct placement of these components and the values that they would need to be in order to achieve the 3 VDC at each LED candle?
Thank you so much. This forum is great!

 

[Divetekno]

1 more question

Could you refer me to a circuit diagram that would show the correct placement of these components and the values that they would need to be in order to achieve the 3 VDC at each LED candle?
Thanks in advance.

 

[Ubergeek63]

depends on how sneaky you want to be... a 3V LED candle might run on down to 2V and up to 4V ...

it just occurred to me that at 12VAC you get about 15Vfiltered, with 5mA 2.7V zeners across each one you should be able to just put them all in series

 

[Divetekno]

I would like to supply each LED candle with 3 VDC. They operate better at that voltage on my test bench. Do I need to wire the LED's in parallel off of the rectifier circuit, right? Where is a diagram of a rectifier circuit with filtering to get the 3 VDC from the 10 VAC supply I have? Any ideas? I am an amateur and need to know component values as well if possible. Thanks for your help!

 

[Divetekno]

Wait, if I understand you correctly....I can put a 2.7V zener diode across each LED and have the 3VDC or so at each LED candle? Wouldn't they need to be in parallel? Sorry for all the questions!

 

[Ubergeek63]

Wait, if I understand you correctly....I can put a 2.7V zener diode across each LED and have the 3VDC or so at each LED candle? Wouldn't they need to be in parallel? Sorry for all the questions!

if you put a zener across (parallel) each candle and put the candles in series, if you put a resistor in series with them supplying 5mA (in the case of a 5mA speced zener) more than what the candles require, the voltage across each candle will be the zener voltage.

just remember that zeners are 5% or 10% devices. if each candle needs 5mA the whole string, if there is enough voltage available, will light for the current of one plus the current to keep the zeners conducting. If the current is spuratic you might need to throw in caps as well as the zeners.