Mixing Dell Bricks and Bridgelux LEDs, any thoughts?

[egghead2004]

I have a couple of Dell laptop bricks. Both are 20 VDC, one is 3.5 amps, the other is 4.5 amps. I am assuming laptop bricks are constant voltage, so it will always be 20 V, and I'd have to add LEDs and resistors to consume the current, correct?

After looking at the table below I see 2 possibilities:

1- multiple 4xx series that max at over 10 V. Since the power supply is 20V, could I run 2 parallel legs and get 10V each? Obviously I would have to string multiple LEDs and resistors together to chew up the current, but am I on the right track?

2 - This is more of what I want to do, the N1203 has a max voltage of 19.8v. Is 20v really going to hurt this? My thoughts are to run four 1203s in series using the 4.5 amp supply. Straight up with no resistors that would be about 1.125 amps per LED, not far from the test current. Also, These will be outdoor fixtures scattered about, so I am bound to lose some voltage over the 250' of wire running on the ground, up, and down trees.


Below is the Bridgelux voltage spec table.
Model number, Min Vf, tested Vf, max Vf, and tested current.

Warm White
BXRA-W0401 8.7 9.5 10.3 700
BXRA-W0402 8.3 9.0 9.7 700
BXRA-W0403 25.8 28.6 31.7 250
BXRA-W0802 11.2 12.2 13.2 1050
BXRA-W1202 13.9 15.1 16.3 1200
BXRA-W1203 17.3 18.3 19.8 1050

Neutral White
BXRA-N0400 9.0 9.7 10.5 800
BXRA-N0402 8.4 8.9 9.7 600
BXRA-N0802 11.2 12.2 13.2 1050
BXRA-N1203 17.3 18.3 19.8 1050

Cool White
BXRA-C0402 8.7 9.5 10.3 500
BXRA-C0603 25.8 28.6 31.7 250
BXRA-C0802 11.7 12.7 13.7 700
BXRA-C1202 11.8 12.8 13.8 1050
BXRA-C2002 14.9 16.2 17.5 1500

 

[egghead2004]

Wait, I think I have this wrong, there will always be 20v no matter how many parallel circuits. I got to think this out more.

 

[andersonEE]

FYI, the BXRA line is now old news...you can get the new BXRB leds from Digikey. Better efficiency, >90CRI, etc.

 

[egghead2004]

Oh thanks, cheaper too!. Anyway, I still have to figure out how to use the bricks, I may have a dozen coming my way for free.

 

[cwloo]

1- multiple 4xx series that max at over 10 V. Since the power supply is 20V, could I run 2 parallel legs and get 10V each? Obviously I would have to string multiple LEDs and resistors together to chew up the current, but am I on the right track?

2 - This is more of what I want to do, the N1203 has a max voltage of 19.8v. Is 20v really going to hurt this? My thoughts are to run four 1203s in series using the 4.5 amp supply. Straight up with no resistors that would be about 1.125 amps per LED, not far from the test current. Also, These will be outdoor fixtures scattered about, so I am bound to lose some voltage over the 250' of wire running on the ground, up, and down trees.

1) You can run in series to consume the 20v. Just make sure you pulling close to the amps of the supply. I am running 2 x C361 in series over a 12v wallmart 1000mah supply and it's running fine but hot so active cooling is prefer unless you have a huge heatsink.

2) It should be fine but make sure you have enough cooling as those LED are hot.

 

[egghead2004]

I have a couple of monster heat sinks, one has 600 sq/in surface, the other over 1500 sq/in. (going to cut this in three pieces, that'll give me 3 fixtures for up to 50 watts of heat each)

I get the constant current concept, but having difficulty grasping the fixed voltage supply idea.

 

[cwloo]

Here's the pic of my setup. The 2 x bridgelux running around ~10 watts is noticeable brighter than the 18 watts compact

 

[andersonEE]

Ok, after reading your first post, it seems like you may be confused about circuit theory.

Before you do anything, make sure you understand Ohm's law (V = IR) where V=voltage, I=Current, R=Resistance. It would also be good for you to learn Kirchhoff's Current Law and Voltage law (Search for KCL and KVL).

Your laptop bricks will output a constant voltage regardless of what load (resistance) you hook up to it. The current rating (3.5A, 4.5A) just tells you the max current the bricks can supply.

Pretend for a moment that you were just hooking up a Resistor instead of your LED. The resistor has a fixed resistance (lets say 5 ohms) and because your brick is putting out a fixed 20V you know that you will get 4 Amps through the resistor (V = IR ... I = V/R ... I = 20/5 = 4). This 5 ohm resistor would try to draw 4 Amps from a 20V power supply. Your 3.5A rated brick would struggle to do this...the voltage would sag, the brick would heat up, and the supply may shutdown completely. Your 4.5A rated brick should be fine.

However, Leds are not like resistors. They do not have a fixed resistance. As you increase the Power (Voltage * Current) through the LED, the resistance of the LED goes down. This means that a small change in Voltage can produce a large change in Current. For example, if you drive an LED at 10V and get 1Amp through the LED, you would have a resistance of 10 ohms. However, it you drive it at 12V you might get 2Amps through the LED for a resistance of 6 ohms.

For this reason, it is preferable to drive LED's with Constant Current drivers which sense the current going through the LED and make small changes in the Voltage to keep the current where it should be.

If you are set on using the laptop bricks...have no fear, it can be done safely. I will give you a examples:

Lets say you want to drive 3 BXRB-30G054 Led's which are rated for 365mA and the Typical Forward Voltage at that Current is 19.1V. Your power supply is 20V so your 3 LED's will have to be wired in parallel and you will need a resistor in series with each LED to drop 0.9V. R = V/I = .9/.365 = 2.47 ohms. The resistor will need to handle 0.33 Watts (get a 1 watt resistor just to be safe).

Another example: You want to drive 3 XML's each at 3 amps. The typical Vf at 3 Amps is 3.4 Volts for an XML. Because of the low Vf, you can wire the LEDs in Series to get 3.4 * 3 = 10.2 Volts. Now you will need a resistor to drop the remaining 9.8 Volts difference between the power brick and the LED Vf. R = 9.8/3 = 3.27 ohms. This resistor will need to handle 29.4 Watts (9.8 * 3) and will therefore be huge and impractical.

Hope this clears some things up for you.

 

[andersonEE]

Haha...I read through your first post again...just to clear something up: If your power supply is 20V, you would want to run 2 10V Led's in Series, not in parallel. Similarly, if you ran 3 N1203's (19.8V) in series, you would need a power supply that could output 59.4 Volts. What you want to do is run them in parallel.

 

[egghead2004]

Thanks a lot! That does clear things up.
It's all coming back to me now, I forgot about ohms law, it's been 25 years since I really needed to use it.
I like the CC drivers better, but the bricks give me more flexibility, and I have 12 of them.
Under my grill table I will have a control box for all the outdoor lighting. Some of the new BXRBs are 6.2 volts, so I could run 3 in series at about 730mA with a 2.2 ohm 1.5 watt resister for each leg.
With the 4.5 amp brick, I can run 5 parallel legs with this configuration. The flexibility of the brick allows me to turn on only one, two, three, or all legs, any combination and the current on each leg will always remain the same. Where as the CC, restricts me, I need to make sure the LEDs can handle the CC put out by the driver, then with every additional leg turned on, the current gets cut in half, dimming the LEDs. the only way around that is having one CC driver per circuit, too much $$$$. All my halogen landscape and CFL floodlights are being replaced with my home made LED fixtures. I'll be adding deck lights, Spot lights for the Bocce and Volleyball areas, accent lights around the house, etc.

Thanks a bunch!

 

[andersonEE]

I always enjoy your build threads...can't wait to see these new outdoor setups.

Sounds like you understand it now. One thing to note: The 6.2Volt BXRBs are the LS series, which is currently unavailable at Digikey and Newark (they are listed but have an availability of 0). The only available BXRB's are the warm-white Led's in the ES series (probably because these are such a new item). So if you need cool white you might want to stick with the BXRA series.

 

[egghead2004]

cwloo, Nice, LEDs look so crisp.

 

[idleprocess]

Your laptop bricks will output a constant voltage regardless of what load (resistance) you hook up to it. The current rating (3.5A, 4.5A) just tells you the max current the bricks can supply.

This is probably true since most laptop bricks are regulated, but an unregulated brick will only spit out its rated voltage when the load is within the region of its rated current. The less current you draw, the higher the voltage. I have to keep regulated vs unregulated in mind whenever I need a DC power supply from the local surplus electronics shop - thankfully they label then and even do some rudimentary testing of unloaded voltage.

 

[cwloo]

Another example: You want to drive 3 XML's each at 3 amps. The typical Vf at 3 Amps is 3.4 Volts for an XML. Because of the low Vf, you can wire the LEDs in Series to get 3.4 * 3 = 10.2 Volts. Now you will need a resistor to drop the remaining 9.8 Volts difference between the power brick and the LED Vf. R = 9.8/3 = 3.27 ohms. This resistor will need to handle 29.4 Watts (9.8 * 3) and will therefore be huge and impractical.

Aren't we able to run 6 XML in series, ie 3.4 * 6 = 20.4 hence not require to use the resistor ?

 

[andersonEE]

Aren't we able to run 6 XML in series, ie 3.4 * 6 = 20.4 hence not require to use the resistor ?

Yes, I was just giving an example so egghead would understand how to do the calculations and choose the correct resistor.

 

[andersonEE]

This is probably true since most laptop bricks are regulated, but an unregulated brick will only spit out its rated voltage when the load is within the region of its rated current. The less current you draw, the higher the voltage. I have to keep regulated vs unregulated in mind whenever I need a DC power supply from the local surplus electronics shop - thankfully they label then and even do some rudimentary testing of unloaded voltage.

This is true. The constant 20V is correct for a regulated laptop power brick, but for a typical wall wart power supply rated at say 12V, you might get 15V unloaded and 10V if you load it up.

 

[cwloo]

This is true. The constant 20V is correct for a regulated laptop power brick, but for a typical wall wart power supply rated at say 12V, you might get 15V unloaded and 10V if you load it up.

Yup. my 12V is showing 18V unloaded.

 

[blasterman]

I've built a lot of big LED runs on laptop bricks. The most common ones being HP with 18.5 volts where either 5 LEDs or 6LEDs can be run directly in series without a resistor and you get ~700ma or ~350ma from an oldschool 3.5-7Vf white. Several have been running now for over two years 24/7.

The bigger question now is why bother with this. Current regulated supplies are cheap, and Mean Wells run well under $50. 1000x easier not having to worry about being .1 Vf off when dealing with fixed voltage in series.

 

[idleprocess]

I've built a lot of big LED runs on laptop bricks. The most common ones being HP with 18.5 volts where either 5 LEDs or 6LEDs can be run directly in series without a resistor and you get ~700ma or ~350ma from an oldschool 3.5-7Vf white. Several have been running now for over two years 24/7.

The bigger question now is why bother with this. Current regulated supplies are cheap, and Mean Wells run well under $50. 1000x easier not having to worry about being .1 Vf off when dealing with fixed voltage in series.

Local surplus electronics store sells regulated power supplies with >12V and >2A ratings all the time for close to $10. Throw in some high-power resistors at <$1 each, and your cost is well under the going rate for a constant-current driver.

In the case of this store, "surplus" generally means "takeout", so you're getting new stuff.