forward voltage/current

14 posts / 0 new
Last post
akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54
forward voltage/current

hi
the graph of forward voltage/current shows what is the max current i push out per voltage?
so for example, if i want to push 4.8A from the XHP70, i must use aroud 6.35V or higher voltage? so at 6.0V, i can get only 3.0A max current?

Edited by: akv on 06/08/2016 - 08:04
akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54

what do you think on this driver? will it drive the led for 24W?
http://www.aliexpress.com/item/Cree-XLamp-XHP70-6V-LED-Driver-22MM-DC6V-15V-1mode-5mode-Input-6-15V-Output-For/32673131364.html?spm=2114.13010308.0.47.rbWE6u

will34
will34's picture
Offline
Last seen: 10 hours 38 min ago
Joined: 12/18/2012 - 00:12
Posts: 4113

According to the specs it should work, but for how long, that’s the real question. Most 2 cells drivers for the XHP-70 Don’t last very long on turbo, with the exception of the drivers that comes with the L6 and S70.

For the price you may want to give it a try, it’s the only way to know.

And yes without higher voltage the current won’t increase, this is why for buck drivers the input voltage should be at least 1V above the output.

akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54

thank you so much for the explanation.

as for the current, i guess i can combine multiple drivers in parallel to get more current, am i right?

akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54

how the driver “knows” what voltage to forward?
for example, if i drive xm-l2 with 20×7135, how the driver knows not to overvoltage/overcurrent?
and is any 6v driver fits to any 6v led, or it needs to be a driver for a specific led model, by the its voltage/current graph?

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

You can’t really add the outputs of boost or buck drivers to the same load. A better option for more current would be an fet linear driver with voltage limited for the mcu(either by a Zener diode or LDO voltage regulator). One of those could supply all the current the led could handle but needs multiples of two cells for input.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

7135’s are self regulating, as long as input voltage is slightly above the forward voltage for the total current then each one will pass 350mA. Regulating drivers are current regulated and adjust the voltage to whatever’s needed to maintain the current that driver is designed to output regardless of changes in the Vf of the led. The Vf of LEDs varies with current and temperature so the driver compensates.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

Different LEDs have different maximum current ratings but any 6v driver will work with any 6v led as long as the max current is not exceeded. In practice, it’s hard to kill most of the 6V types if they’re mounted on a dtp copper star in a decent host.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

As far as 3V LEDs like the XM-L2 or any other, if you push 20 × 7135’s worth of current you will blow the led as that’s way too much current. The chips can’t tell when the led is about to fry so it’s up to you to choose the correct maximum drive current for the led and then feed it with enough cells in parallel to maintain the Vf at that current. From a single liion cell you won’t get 20 × 7135’s worth of current into a single led anyway but it still needs to be on a dtp copper star to handle even 4-5A.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54

thank you for the explanations, i read them many times.
the driver forward/output voltage is variable or fixed?
why when i connecting 20×7135 drivers into few LEDs in parallel, the voltage will be lower? is this because the driver trying the minimum voltage with maximum current the driver capable, or is it because the current will not flow to the LED if it’s too high voltage for too less current?
according to this, it means that if i take 8×7135 to single LED, it will do more lumes than 2 LEDs for 8×7135, because both cases it’s the same current, but with single LED it’s higher voltage.

Andrew2007
Andrew2007's picture
Offline
Last seen: 5 years 2 weeks ago
Joined: 11/04/2015 - 16:07
Posts: 590
Location: Northumberland, England

There’re plenty of graphs to look at to see what’s happening, this is a typical driver used for a 3v XM-L2 led.

The driver is designed to give a constant current and voltage output for the 3v XM-L2, you can see from the graph when the battery voltage drops to around 3.6v it cannot maintain these levels so they both start dropping.

Driver info from:

http://lygte-info.dk/info/indexLedDrivers%20UK.html

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

For the most part led drivers are current regulated making adjustments maintain a stable output current so yes, output voltage varies a bit. Even LEDs still obey ohms law and if you put them in parallel the total resistance is lower so the output voltage needed to maintain a certain current is lower. Actually, two LEDs fed 1.5A each will put out more lumens than one fed 3A. Not because of the voltage but because LED output isn’t linear and is more efficient at lower current. Along with that though, the voltage necessary to feed two LEDs 1.5A each is lower than the voltage needed to feed one LED 3A. Voltage is the push, current is the flow. Remember that in parallel current is divided between the two loads but the voltage drop across each of the two has to be the same. Look at the Vf vs current chart for any led and compare X current at Y voltage to 2X current and you will see it takes more than 2Y voltage.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

Rufusbduck
Rufusbduck's picture
Offline
Last seen: 9 months 11 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10389
Location: Golden state

Try not to get hung up on voltage, think of 7135’s as individual bundles of 350mA. Add them up to get the current you want and feed it to one led or divide it amongst several. If you need more run time add more cells. If you want to use 6V LEDs then you need 2/4/6 etc. cells instead of 1/2/3 etc.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

akv
Offline
Last seen: 4 years 4 months ago
Joined: 01/10/2015 - 14:29
Posts: 54

Rufusbduck wrote:
Try not to get hung up on voltage, think of 7135’s as individual bundles of 350mA. Add them up to get the current you want and feed it to one led or divide it amongst several. If you need more run time add more cells. If you want to use 6V LEDs then you need 2/4/6 etc. cells instead of 1/2/3 etc.

Yeah, i think i closed the xml2 and 7135 point, just add 7135 chip for extra 350ma but good to know there is too much current.
Do you mean i can use couples of lithium batteries in serial for feed the 7135 driver for 6v led, or to use couples of drivers in serial to light up 6v led?
Thanks