I have posted this elsewhere on the internets, but I know a few people here will have relevant skills. I have emailed Troutie about it, but I guess he’s pretty busy running a business, or just out riding.

Dear all,

if I select a Buck driver -is it’s output voltage the same is the input voltage, and it’s just the current that is regulated? Some specify an output voltage, some do not. I am assuming if they do not, they scale themselves to match the current?

I am a bit confused.

What I need is to drive 2* Bridgelux ES LED arrays of about 800lm each, and then mount them in the molex mount/cable clamp and finally fit a 50degree reflector.

links for the parts I am considering:
http://uk.farnell.com/bridgelux/bxra…0lm/dp/1787315
http://uk.farnell.com/molex/180150-0…ens/dp/1860614
http://uk.farnell.com/ledil/c11508-b…200/dp/2095794

I am really struggling with the drivers. I shall be using mains power, so I don’t want wakky voltage requirements, I’d like to use an off the shelf power supply, 12/19/24v etc.

If anyone can easily match me up an LED and Driver from those pages, that would be lovely. Or just explain how to match them.

Thanks for your time

GfS

Is this spam??

eeer, no.

It’s just too difficult for me to understand without help. Hence, I’m looking for help 🙂

Ohhhh, ‘OT’= Off Topic. My apologies…..

Have a look on eBay for power led drivers there’s hundreds of 220v to 12v dc drivers no need for a separate power supply ,look for a 800mah current output. (Sorry can’t figure out how to do links from phone)

if I select a Buck driver -is it’s output voltage the same is the input voltage, and it’s just the current that is regulated? Some specify an output voltage, some do not. I am assuming if they do not, they scale themselves to match the current?

They all have to scale themselves to match the current – that’s the only way for them to control the output current. The specified output voltage is probably a max voltage which shouldn’t be able to go above the input voltage for a buck converter

Aaaaha – I didn’t see that I can buy them potted!

OK, this should save me some soldering.

http://uk.farnell.com/xp-power/ldu2030s700/led-driver-dc-dc-constant-i-20w/dp/2082740
and
http://uk.farnell.com/bridgelux/bxra-30e0800-b-00/es-rect-800-lm-warm-wht-array-2700k/dp/2070582

driven from a 19.5v power supply should do it, right?

I’ll need 2xdrivers, 2xleds and one dual track potentiometer to synchronise the dimming for two LEDs?

That would be much simpler 🙂

Thanks !

2 sorts of output drivers:

1) Constant Current (CC)

and

2) Constant Voltage (CV)

Although the do exactly the same thing, they control a load, using either a linear pass transistor (smooth but lossey) or a switch mode inductor (efficient but noisey), the effective control term is different.

The CC driver will apply an output voltage to the load until it reaches either the maximum voltage it is possible of supplying or until it’s output current meets the target current.

The CV driver simply outputs a target voltage and the current is controlled by the loads impedance (resistance for DC)

The reason we use CC drivers for LED’s is that they have a very non linear impedance with voltage characteristic. Below some threshold voltage they are virtually non conductive (uA only flow), then suddenly they start to conduct (and emit light) and over a few tenths of volts there current demand rockets. The threshold voltages at which the characteristic changes depends upon the makeup of the LED (colour, junction type etc) but also on temperature. So if you used a CV driver, small changes in LED impedance would cause massive changes in the current being driven through the LED, generally resulting in it failing (via overheating). Using a CC driver results in a controlled current flow at all times, pretty much irrespective of the output voltage, and for an LED current = light output broadly speaking.

Various types of driver exist, with most modern ones being switch mode drivers, that store a tiny amount of charge in the magnetic field of a small inductor, and use this to transfer a flow of current between two differential voltages. They oscilate at very high frequency (typically 200KHz) and charge / discharge the inductor to force the current to flow. Depending on the architecture of the particular driver it may be able to either just reduce voltage (buck converter, i.e. Vout < Vin always), increase voltage (boost converter Vout > Vin always) or both (Buck/Boost Vout>Vin>0).

Which type of Switch mode driver you choose depends on the potential of your supply voltage and the maximum load voltage. Typically, for LED’s the output voltage is going to be less than the maximum voltage that the datasheet says you can apply to the led without damage. Where you have a choice, you would normally pick an input voltage that is higher (by some margin) that this maximum output voltage requirment, therefore you can use a simple Buck convertor. if you are designing a system for maximum efficiency, it all gets a lot harder because the efficency of the convertor is not linear with deltaV.

Hope that helps!

Here’s a typical LED Current/Voltage characteristic:

As you can see, until 2.4V effectively no current flows, then over the next volt or so there is a massive change in current flow.

Finally, If you have multiple LED’s to drive consider the most appropriate layout for those leds. It might be a mix of series and parrallel wiring.

Series:
V+ —-> Led —–> Led —–> 0v

with the same current in all parts of the series circuit, where V+ is 2x LED voltage

Parallel:
V+|—-> LED —–>| 0v
V+|—-> LED —–>| 0v

where V+ is the led voltage, but the supply current is twice the individual LED current

Parallel wiring of LEDs like that is a very bad idea – I’m surprised given the apparent level of your knowledge that you mention that. The issue is that all LEDs are a bit different, with a different voltage/current curve, so if you wire them like that it’s likely that the current in each LED will be very different, rather than being split equally between them.

Oh wow.

As I see it, if the LED is “21V” then it’s a bunch 6 3.5v LED dies in a package, therefore I need to be over 18v [21-6*.5]to start conduction, so therefore I might be too close to the lower end of the conduction voltage scale to get the 800lm, even if the driver can deliver over the target amperage.

Hmmm, well I can test it with 19 and 24V and maybe I have to persuade my manager to let me spend the money for a 24V PSU.

I’m a design intern, and working on my own.

Thank you.

aracer
Parallel wiring of LEDs like that is can be a very bad idea

EFA.

If the dies are on the same heatsink, and from a matched batch, then parallel layout is both possible and useful under some circumstances, hence i mentioned that you want to “consider” the layout if you have freedom in that area.