AC rectified to DC, powering large led strings idea, help welcome...

[dmachado]

Ok, the idea:

220VAC rectified to say 310VDC

Voltage divider (resistor power rating?) to get 3 100VDC "lines" to be able to power 32 3,2Vdrop leds each, or more leds with lower voltage drop

One current limiting resistor for each series depending on led voltage drop.

Does this make any sense?

All comments are most welcome! Thank you!

 

[MrAl]

Hi there,


Well, it would be better to make one string and use a smaller resistor. You still have to leave some overhead though in case of line surges etc.
Even better probably would be to use a capacitor in series to drop some of the voltage.

 

[dmachado]

Thanks for your reply.

I would add fast blow fuses on the AC side and on the DC side, depending on the mA of each string and the mA of the whole setup (for the AC fuse).

I understand the overhead concept, in case the AC voltage spikes, probably a regulator would be in order...

If the mA on the DC side are 18mA instead of 20mA, 95mA instead of 100mA, this would leave a bit of overhead, am I on the right track?

Thanks for all the help!

 

[Gary B]

I would use a saturateable reactor transformer to stabilize the output voltage in spite of fluctuations of the input line voltage. This will allow you to set the output voltage very close to the desired voltage of (32 LEDs X 3.2 vpLED= 102.4 volts) like say 105 volts. Now your current limiting resister only has to drop 2.6 volts. Even at 0.35 A. that is less than a quarter watt per string allowing the use of small current limiting resisters. It also eliminates the need for a DC regulator since the AC side is being regulated. This assumes that this is illumination rather than a sign that might change the number of LEDs that are on at any one time and thus change the output load.

 

[MrAl]

Thanks for your reply.

I would add fast blow fuses on the AC side and on the DC side, depending on the mA of each string and the mA of the whole setup (for the AC fuse).

I understand the overhead concept, in case the AC voltage spikes, probably a regulator would be in order...

If the mA on the DC side are 18mA instead of 20mA, 95mA instead of 100mA, this would leave a bit of overhead, am I on the right track?

Thanks for all the help!

Hi again,

Oh well if you use a regulator that's a whole different ball game...you have to decide if you want to do it that way first and what method of regulation or what type of ic you want to use.

I was talking about overhead voltage when not using a regulator, where you have to make sure there is enough overhead voltage across the resistor so that when the line voltage goes up it doesnt burn out the LEDs. For example, if you have 300v and you use 100 x 3v LEDs that's zero overhead, so if the voltage goes up by 10 percent it could blow some LEDs. If you use say 80 LEDs then that leaves 60v across the resistor, which is a little better but maybe not enough yet. 50 LEDs would be 150v, and that would leave 150v across the resistor which would probably be ok except the resistor would dissipate power so that's why i was talking about using a capacitor on the AC side with a smaller series resistor so the power dissipation is low.
You probably have to run the LEDs at 15ma to keep them protected for higher line inputs so when the line goes higher it doesnt hurt the LEDs.

 

[dmachado]

Let me organize my thougts:

An AC voltage rectifier is pricier than a 100V DC rectifier (ex. TL783 costs some 3-4$).

So, it would be cheaper just to rectify from AC and then setup a voltage divider using 50W rated resistors with aluminium heatsink, from which I would get in different grid voltages:

My AC is very stable at 222V, but who knows...

AC 210V
Rectified DC 294V
50W RESISTORS DC OUT
R1 100,00 OHM 96,08 V
R2 100,00 OHM 96,08 V
R3 100,00 OHM 96,08 V
R4 6,00 OHM 5,76 V

AC 220V
Rectified DC 308V
50W RESISTORS DC OUT
R1 100,00 OHM 100,65 V
R2 100,00 OHM 100,65 V
R3 100,00 OHM 100,65 V
R4 6,00 OHM 6,04 V

AC 240V
Rectified DC 336V
50W RESISTORS DC OUT
R1 100,00 OHM 109,80 V
R2 100,00 OHM 109,80 V
R3 100,00 OHM 109,80 V
R4 6,00 OHM 6,59 V


On each DC aprox. 100V I would connect a TL738 with proper heatsink, aiming to get around 90-95V, a value that would be established after measuring the voltage drop of the led string.

The 6V (or 2V or whatever) would be used to get optocouplers to control MOSFETs (something like IRF9540s), and enable an Arduino to control all the strings via optocouplers.

I know that probably power dissipation is in order, the whole setup would be in a box with a pc fan to prevent overheating.

What can I add to prevent AC voltage spikes to get to the DC side, I you know what I mean? 400V capacitors on the DC side?

Any thougts on this? Am I missing anything? Thanks.

 

[colin55]

This is all you need:

**broken link removed**

LEDs on 240v
I do not like any circuit connected directly to 240v mains. However Christmas tress lights have been connected directly to the mains for 30 years without any major problems.
Insulation must be provided and the lights (LEDs) must be away from prying fingers.
The capacitor must be X2 and add an equal number of LEDs in each string so the reverse voltage is equal across each LED.
It does not matter how many LEDs you add to each string as the brightness will be the same. As you add each pair, the current will drop a very small amount until eventually, when you have 100 LEDs in each string, the current will be zero.
For the circuit shown, each LED will get 15mA for 50% of the time. (This is NOT the same as 7mA DC). The 1k resistor will drop 15v (15mA x 1,000 ohms = 15v). No rectifier diodes are needed. The LEDs are the "rectifiers." Very clever. You must have LEDs in both directions to charge and discharge the capacitor. The resistor is provided to take a heavy surge current through one of the strings of LEDs if the circuit is switched on when the mains is at a peak. A 100n cap will deliver 7mA in full wave or 3.5mA in half-wave.

 

[MrAl]

Let me organize my thougts:

An AC voltage rectifier is pricier than a 100V DC rectifier (ex. TL783 costs some 3-4$).

So, it would be cheaper just to rectify from AC and then setup a voltage divider using 50W rated resistors with aluminium heatsink, from which I would get in different grid voltages:

My AC is very stable at 222V, but who knows...

AC 210V
Rectified DC 294V
50W RESISTORS DC OUT
R1 100,00 OHM 96,08 V
R2 100,00 OHM 96,08 V
R3 100,00 OHM 96,08 V
R4 6,00 OHM 5,76 V

AC 220V
Rectified DC 308V
50W RESISTORS DC OUT
R1 100,00 OHM 100,65 V
R2 100,00 OHM 100,65 V
R3 100,00 OHM 100,65 V
R4 6,00 OHM 6,04 V

AC 240V
Rectified DC 336V
50W RESISTORS DC OUT
R1 100,00 OHM 109,80 V
R2 100,00 OHM 109,80 V
R3 100,00 OHM 109,80 V
R4 6,00 OHM 6,59 V


On each DC aprox. 100V I would connect a TL738 with proper heatsink, aiming to get around 90-95V, a value that would be established after measuring the voltage drop of the led string.

The 6V (or 2V or whatever) would be used to get optocouplers to control MOSFETs (something like IRF9540s), and enable an Arduino to control all the strings via optocouplers.

I know that probably power dissipation is in order, the whole setup would be in a box with a pc fan to prevent overheating.

What can I add to prevent AC voltage spikes to get to the DC side, I you know what I mean? 400V capacitors on the DC side?

Any thougts on this? Am I missing anything? Thanks.

Hello again,


I can not find the TL738 you talk about on the web. Do you have a data sheet?
You certainly dont want to have to use 50 watt resistors as that will consume far too much power for what you are doing here.

I was talking about something like colin posted, except using a full wave bridge rectifier instead of two antiparallel strings.
A good solution looks like this:
A 330 ohm resistor in series with a 0.47uf capacitor, in series with the 220vac line, feeding a full wave bridge rectifier. The DC side of the bridge feeds a string of 50 white Nichia type 20ma LEDs. The average current is 15ma at nominal line, and about 20ma at 15 percent high line. That will probably do it. A 100uf cap on the DC side will protect for some surges, but you probably dont need it unless you expect a lot of surges.

 

[tvtech]

Good engineering is straightforward. Not complicated.

It is easy to make something complicated. It is however difficult to make something that should be complicated.......simple. Yet reliable.

Ask any engineer.

 

[colin55]

A 330 ohm resistor in series with a 0.47uf capacitor is not sufficient resistance for the time when the LED string is turned on and the supply is at maximum amplitude. You would need at least 2k2. The rectifier diodes are totally unnecessary. They are not doing anything.

 

[dmachado]

@MrAL
Sorry for the misleading information, it was TL783 I was talking about: http://www.datasheetcatalog.org/datasheet/texasinstruments/tl783.pdf

The 50W rated aluminium clad resistors would allow me to draw up to 500mA at 100V from the voltage divider, this is the reason for this power rating, if I draw 20mA they will be consuming 50W anyway? Excuse my ignorance...

@Collin55:the AC line powered leds would be a great solution, but I need to keep the possibility to use MOSFETs to PWM the leds with the Arduino outputs (seperate circuits using optocouplers).

This solution however could be used for the full blast light level of the aquarium lights, though, no PWM will be used with those leds, at 300 or 350mA.

Thanks for all the help I am getting!