A question about led drivers

I've no experience with them so any info could be helpful.

My question is that it seems to me that there are often a series of drivers from a manufacturer with similar prices and they all output the same current only the output voltage changes and they call them constant current devices. For example a 1-3 watt may output

12-24V/300 ma, while an 18 watt output 24-36 volts/300 ma.

Well, if they are constant current, wouldn't it make sense to get one with the highest voltage so it can drive more series leds, for about the same price as a lower V output one? I figure it would be more versatile and allow me to add leds if that seems necessary.

I'm toying with the idea of building a tabletop hydroponic "garden." They get high prices for what amounts to a few LEDs and aquarium air-pump,timer and container, some of which I already have laying around collecting dust except for the leds and PS to operate them.

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default prodded the keyboard with:

LED's are current operated devices and the devices to drive them are intended for a specific use with a chain of LED's.

Calculating the voltage and current requirements is relatively easy. But you need to know what the voltage and current values are for the LED's you intend to use.

For instance a single LED might have a 3 volt and 10 milliamp rating. So putting two in series would require 6 volts and 10 milliamps, three would be 9 volts and 10 milliamps and so on.

If you put two in parallel, you would still use 3 volts but the current would double.

The normal way you would run a chain of LED's is to put a current limiting resistor in series. So a single LED run from a 12 volt supply would require a resistor that would drop 9 volts at 10 milliamps.

The formula is supply voltage minus LED voltage divided by the current equals the resistor value.

In the last example it would be 900 ohms.

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Best Regards: 
                      Baron.
Reply to
Baron

Maybe. But, 'more series LEDs' implies you are chaining ten together, and that implies that one can fail short (and the compliance of the voltage output makes the rest of 'em still light), or one can fail open (and they all go dark). A string of ten has ten times the fault likelihood as a single LED. It also implies that (to keep the specified regulation) you MUST purchase ten or so of the 300 mA-capable devices to power up. Is that really the amount of light you want to pay for?

Reply to
whit3rd

I think I see the flaw in my reasoning. Namely: heat dissipation. Taking a driver designed to excite a 18 watt string and using it to drive only one, one-watt LED would put more of a heat burden on the driver.

Reply to
default

Yeah I understand all that... I'll be working with high power LEDs; the idea is to simulate sunlight and grow herbs and vegetables indoors. So power dissipation and cooling are going to be important design elements.

Another approach may be to power each led or short led string with a series of slaved current mirror circuits then refine that a bit with thermal feedback.

Reply to
default

idea is to simulate sunlight and grow herbs and vegetables indoors."

Herbs, yeah, gotcha...

I believe LEDs cannot replace those big 400 watt bulbs. The LEDs are gettin g good, with white ones having phosphors now, but I doubt you'll ever get t he UV out of them.

Yeah I tried to grow some herbs. I had 1,200 watts of lights and a similar amount of air conditioning to not melt the house down. However all my herbs turned out to be male. So a $400 a month electric bill got me nothing. Lat er I "used" the herbs myself because nobody else would want them.

Reply to
jurb6006

No. The driver adjusts the voltage, to get the specified current.

The voltage spec just tells you how many leds you can put in series.

Reply to
Sjouke Burry

Seriously, actual herbs for culinary purposes. I did have a particularly successful "herb" garden some years back. Only three 20 watt fluorescent tube grow lights was plenty to keep my personal stash happy and then some.

I got my wife this expensive (IMO) "table top garden" for her birthday. It has a number of red/blue LEDs in a shiny reflector above the plants (with an adjustment for height) an aquarium pump feeds a longish air-stone which bathes the plant roots in a mist of highly aerated fertilizer solution, and it's on a timer.

It would be worth it to tinker with if it means fresh basil in the dead of winter...

Reply to
default

I get that.

My questions are more to do with why a 3 watt model and 20 watt constant current supply can be priced so closely with that disparity of wattage among the sizes. But if they are inefficient enough it would explain it - using one led for a source designed for 10 may result in more power being dissipated in the CC power supply.

Or they are doing something questionable, like driving a linear CC regulator from a CV supply.... That would result in a lot of heat dumped in the CC regulator. It only takes one power transistor to make a CC source, so this could just be a cheap and dirty approach.

Reply to
default

probably the only difference is the voltage rating on the capacitors

abasolutely it would

actually no it doesn't do that. the driver regulates the current, the voltage is what ever it takes.

most of the cost is all the other parts of the device, the parts responsible for max output voltage are relativley cheap.

I doubt it, such a design would cost more.

It takes one resistor fewer than a CV supply to make a CC supply.

OUT -------->|->|->|->|->|-. | | FB ------+--|

Reply to
Jasen Betts

If you look upward three or four lines he already says that the driver adjusts the V and I am agreeing with that. That's what CC is all about whatever voltage it takes. (within limits)

I hope they aren't potted in epoxy, I ordered two different manufacturer's drivers in the same power range. I'd love to take them apart...

I agree, but one never knows when you find it on Ebay...

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default

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