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DC circuit question


DerrickHoffland
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DerrickHoffland

I will be putting together a LED wash light that I hoped to control off a CMB-16D board, but I think I have run into a problem with my plan.

The LED's are 3.6-4.0 forward Volt, 750 mA. I have landscape lighting transformer with 300 Watt taps for 12, 13, 14 and 15 Volts. I also have a 3 Watt led driver board that limits the current to 700 mA. It's good for 6-25 VDC input. I had planned to have 3 LED's in series connected to the driver which, in turn was connected to the LOR DC board. I would power the LOR board from the 12 VDC tap on the transformer.

If I do the math correctly (which I may not have), my three LED's will draw 8.4 Amps from one channel of the LOR board which is too high.

If I drop to 2 LED's in series, I would need to drop the input voltage to 8 VDC which would give me 5.6 Amps which would be okay with heatsinks on the board.

I would like to have a channel for each Red/Green/Blue set in my light.

My questions:

1) Am I right with what I have stated?

2) If I put a resistor in front of the LOR board to drop the voltage, how do I size it if I plan to have multiple channels running at different times? (The current draw will vary)

Any help would be appreciated.


*edit* I also considered going with the original 3-in-series plan but adding a 12 VDC cooling fan in parallel with the LOR board. This will probably void my warranty, though.

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Adding more elements in series does not change the current through the channel, only the voltage across each element.

If you put the elements in parallel, each would see the full voltage, and the currents would add...

Also, your 750mA current limiter card will limit the current, by adjusting the voltage across itself so that the voltage on the LED's is correct. As long as you don't ask it to dissipate more voltage (and watts) than it is designed to handle, two in series would still work at 12V in, with the 750mA card taking up the excess voltage.

However, with three in series and the card, you will only be drawing 750mA per channel, so there is no reason not to go that way. You do need to see how much minimum voltage is required across your 750mA limit card. Since 3 LED's is 12V, you may need to bump up to 13 or more volts...

I'm not sure I'm interpreting everything correctly though, because if you were calculating the sum of three 700mA elements in parallel, that would be 2.1A, not the 8.4 you mention..

Also, make sure your landscape lighting transformer is really a DC power supply. Most of them are AC, which will void your LOR card warranty!!:shock:

One more consideration.. The LOR DC card dims by switching on and off the channel 400 times a second, (400Hz) and varying the ratio of on and off times. In theory, this can work well with current limiting devices, but your current limiting card might not work with it, or may even be damaged by this rapid switching..

Also, not sure that it comes into play at all in your case, but the DC cards work by switching the connection to ground, not the connection to hot...

- Kevin

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Derrick

What driver are you using? most drivers have an input voltage rating that must be a certain voltage above the total forward voltage of the combined leds. Most buck drivers are about 3v. A buck driver will usually increase the consumed voltage and lower the amperage draw.

in series the current draw stays at 700mA while the voltage increases, therefore three leds = 4v+4v+4v+3v? = 15v (the leds will draw more voltage because of the limiting current)

in parallel it would be 4v+3v? at 700ma - your leds will not light as you are not supplying them enough current

I believe the mosfets are rated at 8amps - at 12v. based on this assumption, you are not comming close to their limits at .7A( i might be wrong on this - I would need to test with my ps)

on another note - most landscape transformers are AC- not compatable with most led drivers

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DerrickHoffland

Thanks for the responses.

Here is the link to my driver:

http://www.sure-electronics.net/DC,IC%20chips/LE-LL209.pdf

Does this actually limit the voltage if I put LED's in series?

In my original post, I calculated my amp draw to be 8.4. I think this is actually my power draw (4V+4V+4V)*0.700A = 8.4 W.

As usual, I know just enough to be dangerous.


*edit* Additionally, what I need is a power supply, not a transformer (?)

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based on the info provided

you need minimum 14v to power your leds

if you are only going to power one set, then you would be able to use a wall wart rated at lease 1A (enough to power the leds and the 16d board)

otherwise you will need to fid a dc power supply that will provide 14+v at 700mA x the number of clusters

Lamda makes good power supplies - though not cheap

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Bob VandenBoom

Somebody correct me if I am wrong, but I don't think you will need the LED driver if using the LOR DC board. In fact, like klb said, the LOR effects may not work with or even damage the driver board.

Let me make sure I understand the application. You want to drive 3 sets of 3 LEDs in series. Each set on a seperate LOR channel. Each series of LED's will draw 750mA and drop 4 volts across each LED for a 12 volt drop. So, a transformer that will supply 12 volts and .75A x 3 = 2.25 Amps should do the trick since you are simply switching the voltage with the LOR board. I would go with a higher Amperage transformer to power the LOR board electronics and add capacity in case you want to run anything else off the board in the future. You can find transformers all over. Here are some that should fit the bill http://www.theledlight.com/transformers.html

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Regular LEDs (20mA variety), could be driven by the LOR board without any problems but not the high-flux type. When applying pulse width modulation directly to these types, foward voltage can vary widely with fluxuations in current and the color temperature can be inconsistant.

I believe that Bob is correct when saying that a PWM signal applied to the power input of a buck regulator may damage it. You need to find a driver that has a PWM input. Then just a low voltage, low current PWM signal from the DC card can be utilized to control the driver.

Craig

Bob VandenBoom wrote:

Somebody correct me if I am wrong, but I don't think you will need the LED driver if using the LOR DC board. In fact, like klb said, the LOR effects may not work with or even damage the driver board.

Let me make sure I understand the application. You want to drive 3 sets of 3 LEDs in series. Each set on a seperate LOR channel. Each series of LED's will draw 750mA and drop 4 volts across each LED for a 12 volt drop. So, a transformer that will supply 12 volts and .75A x 3 = 2.25 Amps should do the trick since you are simply switching the voltage with the LOR board. I would go with a higher Amperage transformer to power the LOR board electronics and add capacity in case you want to run anything else off the board in the future. You can find transformers all over. Here are some that should fit the bill http://www.theledlight.com/transformers.html
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Current limiting is always required for LED applications. Often it is just a resistor in series with the LED's, but in higher power applications, some other form of current regulation is required. Some LED's like the MR-16's that Wirekat is selling have the current regulator built in, while others use an external module.

The module listed would certainly be usable with LOR in an on/off capability, but I don't know about dimming. The title on the spec sheet indicates that it is dimmable, but they provide no documentation on how... Especially having an inductor on the card I get suspicious that it is a form of switching power supply, and I would hesitate to feed it 400Hz PWM without the manufacturer's assurances that it was designed to be compatible with this dimming method.

- Kevin

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Yes, that's exactly what I said but there is a big difference between current limiting and a constant current source. Most LEDs can be current limited but high-flux types require constant current. Current limiting is dependent on LED forward voltage while a constant current source is not.

Craig

-klb- wrote:

Current limiting is always required for LED applications. Often it is just a resistor in series with the LED's, but in higher power applications, some other form of current regulation is required. Some LED's like the MR-16's that Wirekat is selling have the current regulator built in, while others use an external module.

The module listed would certainly be usable with LOR in an on/off capability, but I don't know about dimming. The title on the spec sheet indicates that it is dimmable, but they provide no documentation on how... Especially having an inductor on the card I get suspicious that it is a form of switching power supply, and I would hesitate to feed it 400Hz PWM without the manufacturer's assurances that it was designed to be compatible with this dimming method.

- Kevin
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DerrickHoffland

Again, thanks for all the input.

I selected the current limiting board because it stated that it was PWM dimmable. It may be that the documentation is wrong. My main concern at this point is to not damage the LOR board when I put this all together. (This will be some time from now as I am waiting for the sale to get the LOR hardware) If I end up smoking the current limiting board in the process, I'm not too concerned.


*edit* It may be that I would be more correct in calling it a constant current board rather than a current limiting board (?)

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