driver to LED config

[docb]

When using 2-3 of the same LEDs on one driver (700ma driver),
is parallel the best way?

In series, do they get less bright like an incandescent bulb would?

(I have 5v source to a driver)

 

[shortstack]

what driver and leds are you using?

 

[docb]

I have a few different drivers and a few different LEDs, many are not name brand.

Does that make a large difference?

With a 5v source, don't I have to use them in parallel?
Is that even ok?

 

[TorchBoy]

You could be using a boost driver, or red LEDs (which have a lower forward voltage than white LEDs). We don't know. :) But generally it's better to use one driver for each LED rather than simply putting them in parallel.

 

[Dave_H]

When using 2-3 of the same LEDs on one driver (700ma driver),
is parallel the best way?

In series, do they get less bright like an incandescent bulb would?

(I have 5v source to a driver)

Just keep in mind, in parallel, if one or more of the LEDs gets
disconnected, the constant-current driver will pump the same
current through the remaining LED(s), which could overdrive and
possibly damage them. Not good for expensive devices. Since
LEDs don't seem to often fail open, this would be mostly likely
due to a bad solder connection etc.

With series connection, loss of connection would kill the whole
series string, so if this is not tolerable (such as for safety reasons)
parallel can be better.

Dave

 

[docb]

Thanks guys.
Right now I have a 5v source, to a 700ma driver (very limited specs say 5-8v input, 750ma), to a white LED.

You say that:
"if one of the LEDs gets disconnected, the constant-current driver will pump the same current through the remaining LED(s)"

I'm a bit confused by that.. Isn't it constant, regardless if I have one or two hooked up? If I added another in parallel would the brightness change because *current* is split? Wouldn't it just change in series like light bulbs do, because the *voltage* is split?

Seems to me that with a 5v source, I can't run two in series even if I wished to.

 

[docb]

How does this - not the driver i'm using - work? If you hook up one LED, does it send 3v, but somehow knows you have hooked up two or three, and sends correct voltage to them?

"Output: 3 - 13.5V DC, 700MA
Please note that the driver is suitable for use with 3X 1W LED lights"

 

[TorchBoy]

Since
LEDs don't seem to often fail open, this would be mostly likely
due to a bad solder connection etc.

That (ie, failing open) would be the main failure mode that I've experienced :mecry: due to the diode junction failing or bond wires melting/evaporating.

Seems to me that with a 5v source, I can't run two in series even if I wished to.

It's possible. As I mentioned, you would use a boost driver if you really wanted to do that.

For the driver you ask about, we don't know how it's operating. It might be buck (where the output voltage is always lower than input) or boost (where the output voltage is always higher than input) or buck/boost (which does both).

 

[docb]

Some of the cheap drivers I see have hardly any specs listed.
The $5 ones I'm using now just have input and output wires, sealed in a glob.

http://www.lightobject.com/3-Watt-LED-Driver-58V-750mA-P120.aspx

In general, when they say you can use 1-3 LEDs will the driver adjust voltage for different LEDs? IOW, if my LED wants 3.4v how does the driver know that?

 

[TorchBoy]

Your LED wants a certain maximum current. It just happens to require a certain number of volts for it to draw that current.

Thanks for that driver link. I don't have it listed, although given the lack of specs I'm not sure it will get that far.

 

[docb]

Not quite sure what you mean.

I guess my real question then is can I use a Cree XPG R5 with a driver that seems to be listed as:
"800mAh Regulated Output for Cree and SSC 3.7V emitters. Accepts 3.6V ~ 9V input power"

The 3.7v part of that statement worries me.

My source is a 5v wall wart.

 

[videoman]

Perhaps a little off topic, who makes a buck/boost driver ? I really need one with a dimmer.

 

[TorchBoy]

Not quite sure what you mean.

How about: For LEDs the current is the more important figure.

"800mAh Regulated Output for Cree and SSC 3.7V emitters. Accepts 3.6V ~ 9V input power"

The 3.7v part of that statement worries me.

The mAh part worries me, because that's a capacity, which is how batteries are measured. As I may have mentioned, LEDs draw current. Does where you're getting it from not indicate what it can be used for?

Videoman, search this page for "buck" or search this page for "boost". (Either way you'll find the same ones.)

 

[docb]

It's these on DealExtreme:

http://www.dealextreme.com/details.dx/sku.3256

I am trying to find cheap drivers for Cree XPGs and I ordered these. I keep ordering the wrong things, so could really use a pointer to what WILL work.

(I am running one driver per LED. Cree XPG. 5vdc wall wart power supply.)

 

[TorchBoy]

It's these on DealExtreme:

http://www.dealextreme.com/details.dx/sku.3256

IMO it is an excellent driver and is recommended in my list. However the specifications on the DX page are out of date. http://www.videofoundry.co.nz/ianma.../driverlist.php?price_min=6.97&price_max=6.97 shows the latest known details.

 

[docb]

Ok, great to know you think it's a good combo with an XPG.

You seem to be well up on this stuff, so much appreciated.
I wish I had known about your site before today!