How to supply a 40V LED driver from mains?

What current level? A single string of 20mA LEDs? The easiest is probably a 15-0-15V transformer used as a 30V source. This gives

42.4V peak and about 40Vdc at light load with a bridge rectifier and capacitor filter. If it's easier to get one with a single 15V winding, a voltage doubler will do the job. Some transformers are wound to supply somewhat more than the rated voltage with no load.

It will be 3-6 LEDs per string, at a current of 500-1000ma (I'm thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a few volts as overhead for the current setting circuitry. You shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise, ledTech, etc.). Looking more closely I see now that such drivers accept an input range up to 40V, and require 2-3V more input than the LED string itself requires. Is the problem of supplying a lower voltage (e.g.,

20V) any easier?

thanks,

dan.

It depends on what you can get. Technically, there's no reason why it can't be run from 40V as long as a resistor or a more sophisticated control circuit sets the LED current to the proper level. However, operating it from an unnecessarily high voltage wastes that much more power, especially if the current is set by a simple resistor. That calls for a larger, costlier and heavier transformer (if it's to be based on a mains-frequency transformer). And that wasted power turns into heat which should also be taken into consideration. A 24V supply will provide enough headroom and readymade 24V supplies, preferably regulated, should be easily available.

(It's already past 2:00 am here and I have to go to bed. Heavy day tomorrow. Anything else I can help with will have to wait for some hours).

--
The Cree XP-G is an LED with a typical forward voltage drop of 3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf


The XP Power LDU series of power supplies are wide-range DC input,
constant-current DC output switchers with PWM-able outputs which
require an input voltage of at least 2 volts higher than the DC output
to operate properly.

Once that criterion has been met, the LDU acts pretty much like a
power out = power in kind of device, so if the output load is 3 XP-G
LEDs, that's  3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will need to be
about 12V, and in order to put 10 watts into the load it'll take about
830mA from the 12V bulk supply in order to do that, neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about 930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its input
voltage is raised, then the input current required to supply a given
output power will fall, so there's no advantage to be gained by using
a transformer in the bulk supply which requires larger diameter wire
in the secondary.

Would a laptop power supply be suitable then? I see that these 19V supplies are cheap to come by. I assume that I could use that to drive several of the LDUs in parallel? If I did so, would I need to match the power of the PSU closely to the required power? That is, if I had three LDUs each using 10W, if I used a 65W laptop power supply, would that be more wasteful/inefficient than using something like a 40W power supply?

Thanks for you help everyone.

dan.

Probably a "power supply." ;-)

Good Luck! Rich

The idea of using a laptop PSU actually crossed my mind earlier, but I wasn't sure what you'd be able to get there. A 19V supply will let you use up to 4 or 5 white LEDs in series and yes, you can wire more than one string in parallel as long as you don't exceed the PSU's rating. As a matter of practical consideration, I wouldn't load it right up to the max rating. Over here in India, we get cheap Chinese multi-output laptop supplies. They come with a variety of DC jacks and the output can be switched for different voltages. Some models can go up to 24V

Regarding the power rating: There's no practical reason not to use a 65W unit. In theory, the higher-powered model *may* be slightly less efficient at light load, but that shouldn't matter for your application.

Okay, I'll give that a go.

Mostly as a matter of curiosity, my (limited) understanding is that both the LED driver and the laptop PSU would be using some sort of power management electronics to try to match the required input to the required output. Does this result in funny interaction effects between the two (e.g., if the LED driver tries to pull more power, the laptop PSU supplies more, which then changes how the LED driver is pulling the power, which changes the laptop PSU again, and so on), or are they quite happy to play nicely together?

Thanks for your helpful and informative answers.

regards,

dan.

of the laptop PSU is that it would be switched-mode, and try to both the LED driver and the laptop PSU would be trying to take their input and change it to the output in some way.

Okay, I'll give that a go.

Mostly as a matter of curiosity, my (limited) understanding is that both the LED driver and the laptop PSU would be using some sort of power management electronics to try to match the required input to the required output. Does this result in funny interaction effects between the two (e.g., if the LED driver tries to pull more power, the laptop PSU supplies more, which then changes how the LED driver is pulling the power, which changes the laptop PSU again, and so on), or are they quite happy to play nicely together?

Thanks for your helpful and informative answers.

regards,

dan.

--
Yes.

Generally speaking, yes. The main job of most power supplies (with exceptions like a constant current supply) is to make a certain voltage available and the load will draw as much current as it needs. A regulated supply like a laptop PSU has control circuitry that will try to maintain a constant level of output voltage despite changes in load current or mains voltage. Most have protective mechanisms that, if you exceed a certain load current, will pull down the output voltage. Until you reach that limit, all that the PSU does is to make a constant voltage source available. "Funny interactions" will happen only in unusual circumstances.

In a way though, interactions do happen. The output voltage of a simple unregulated power supply will drop as you increase the load current, and will also change with mains fluctuations. In that sense, the control electronics of a regulated supply does interact with the load as it continuously compensates for those changes. But under normal conditions, such interactions are transparent to the user and are not the "funny" kind you envisaged.

You're welcome.