kcress
New member
Greetings fellow LEDites.
I know that LED lights are expensive to make and I want to show a way that might allow someone to proceed with less expense. This could allow new tank corals or allow one to reduce their electrical consumption, sometimes dramatically, from their existing HID or fluorescent setup.
If this interests you read on.
I'm going to explain a way to set up your LED lighting for bare bottom prices. How? We're going to forgo expensive drivers.
How can we do this? With a little knowledge and some work.
The only real limitation associated with this method is individual string dimming. This may or may not even matter to you.
Why do this?
Pros:
1) Substantially less expensive than using drivers.
2) Will generate less electrical noise if this is concern for you.
3) Simple!
4) Uses less space.
5) In some cases can be more efficient than using a driver.
6) Doesn't preclude one from changing over to drivers in the future when they hit the big jackpot.
Cons:
1) Daily changes to the brightness of individual strings is not convenient enough to be viable.
2) In some cases this can be less efficient than drivers.
3) Takes a little more effort to setup.
4) No built-in current limiting protection as is typically found on drivers. (But we have a safe way around this.)
Required tools are: a few hand tools, a voltmeter, and an ammeter. (This can be your typical DMM(Digital MultiMeter).
You'll of course need anything else you require for building strings of LEDs. A calculator and some basic math ability may be useful. We can help you there.
The data sheets for whatever LEDs you are using.
If you haven't perused any of the various LED threads here you should probably do this before following this thread.
OK a little background refresher on LEDs:
LEDs are strange devices. They present a back voltage or blocking voltage against the power supply voltage you are supplying to them. This means that until you feed a LED with enough voltage to overcome this back voltage nothing much happens. Then once you reach the point where something does happen it can be way too much! Something is needed to 'ride herd' on the LEDs that will limit the current thru them to levels that are safe for them. This can be accomplished several ways. One is with a driver. Here, we are going to be using a power resistor.
Let's put some numbers to this discussion.
If we have a DC power source and a LED that we want to power we have to setup a circuit to correctly do this.
Here's how:
Let's look at a typical data sheet like the one for the Cree XR-E, (a popular choice),
http://www.cree.com/products/pdf/XLamp7090XR-E.pdf
Search around to find that 'back voltage' I refered to, it's called the FORWARD VOLTAGE, or the VOLTAGE FORWARD, or VOLTAGE F, or Vf
In this case you will see it on page 4.
FORWARD VOLTAGE @ 700mA.
This is the voltage you will need to supply to this device to get it lit up properly at a current of 700mA or 0.7 Amps.
What is the FORWARD VOLTAGE for the Cree XR-E? It's 3.5 Volts.
This tells us that we need a power supply that can supply at least 3.5V to overcome the LED's resistance to passing 700mA.
In this example lets say we have a 5V power supply. (This is a very common supply.)
Now if we just hooked up our LED to this supply it would fry in about 2 seconds - never to provide a glimmer of light again. We need something to get rid of or soak up some of this excess voltage. What excess voltage? Starting with the 5V our supply will provide we then subtract the voltage the LED will carve off, 3.5V.
5V - 3.5V = 1.5V
This leaves an excess of 1.5V that we will use a resistor to eliminate.
What value of resistance? We turn to Ohms Law. 1.5V / 0.7A = 2.14 ohms.
If we put a 2.14ohm resistor in our string of one LED and a 5V supply the LED will have 3.5V across it just as it needs.
There is one other thing we must not forget. That's the power rating of the resistor. It tosses away power to drop that needed 1.5 Volts. We need to make sure it is physically big enough to handle the needed power.
We calculate the power that will be dissipated. 1.5V X 0.7A = 1.05W or 1 watt. Now the rule for sizing resistors is to always double its power rating. This means we want at least a 2W resistor. This keeps the resistor cooler. It could still be too hot to touch but that's OK.
Here's a picture to show what we have:
That's almost all there is to it. What's missing? Ummm Let's leave that for a little later as it will make more sense then.
###############################
Alright, now we'll tackle a more typical set up for lighting a tank.
Instead of a power supply, a resistor, and a single LED, we will use a bunch of LEDs.
But how many can we use? It depends on the power supply's voltage.
If you look around you will find that 24VDC is a fairly common size that can be had for very little money. We'll work with that but most any voltage will do the trick.
If we have a 24V supply how many of those CREE XR-E LEDs should we hook up as a chain in series?
Take the supply voltage and divide it by the FORWARD VOLTAGE.
24V / 3.5V = 6.86
We could hook 6.86 LEDs up in a row and the 24VDC supply would just barely run them. Except we of course can't buy or make 0.86 of a LED.
So we round down to 6.
We can hook up 6 LEDs in series to the supply with our resistor in series with the chain. We should hook these up with typical electronics protocol of ->
Power supply (+) output to the resistor.
From there we go to the (+) on the first LED
then from this first LED's (-) to the next LED's (+)
continuing on to the last or sixth LED's (-).
This last (-) gets hooked back up to the power supply's negative or (-) terminal completing this LED "chain" or "string".
That's the basic layout. But we need to figure out what resistor to use. Back to the math.
Just like our first single LED calculation we do the same here.
24V - 3.5V - 3.5V - 3.5V - 3.5V - 3.5V - 3.5V = 3V
From this we see we need to carve off 3V of excessive voltage. If we want to run the sting at 700mA or 0.7A we proceed as follows,
3V / 0.7A = 4.29 ohms. We need a 4.29 ohm resistor.
What power rating? Again:
3V x 0.7A = 2.1W. Double this and we need at least a 4.2Watt 4.29ohm resistor.
No they don't sell these at Safeway! Fret not though.
Here's what you use.
http://www.heiresistors.com/PDF/AVT_AST Spec.pdf
Note these are 25W resistors! So they have us easily covered in power capability. They cost about $5 apiece from Digikey. You can probably find them elsewhere for less than that.
That is an adjustable resistor. You can set it to 4.3ohms.
What we do is use a multimeter to set it as close as you can to 4.3ohms. You can measure this in-circuit with no difficulty. So install everything then adjust the resistors.
If your multimeter has a 1A or larger current measuring ability you can break the circuit somewhere and directly measure the current.
Alternatively you can write down the actual resistance you set the resistor to and then with the string powered up and running directly measure the voltage across the resistor. Run back to Ohms Law and calculate the actual current.
If your resistor was set to 4.3ohms and you measure 3.2V you have this:
3.2V / 4.3ohms = 0.744A
Shut down and re-adjust the resistor to make it "longer" which will raise its resistance driving down the current a bit.
That's almost all there is to it.
Hmmm what are we forgetting? Protection! We need a fuse in each string. It should be between the power supply and the resistor. For a 700mA string you will want a 1A fast blow. This will prevent a fire should something go wrong. What could go wrong? A wire from one of your solder joints could come loose. Or an LED could decide it's done and short. If any one LED shorts it will increase the current of the entire string. How much? That resistor that's there to set the current at 700ma in the above example by having 3V across it, would then have about 3V + 3.5V equaling 6.5V. Running back to Mr. Ohm this would result in 6.5V / 4.3ohms = 1.15A which would blow our 1A fuse protecting the rest of the LEDs.
You can use one of these Digikey F2313-ND.
http://search.digikey.com/scripts/D...link=hp_go_button&KeyWords=F2313-ND&x=14&y=14
These are not the conventional glass fuses you're probably familiar with. These are much less costly and need no holder. Of course you can buy holders and use the typical glass fuses if you prefer.
So lets do a walk thru of a full hood.
1) Find your heat sink.
2) Figure out your LEDs.
3) Check the Forward Voltage of your selection. Example: 3.5V
4) Decide on the drive current. Example 700mA same as 0.7A
5) Figure out how many you want to run on your fixture. Example: 60
6) Hunt down a power supply by assuming a common output voltage. Example 24V
7) Calculate the size the strings must be: 24V / 3.5V = 6.8 -> round down to next whole number -> 6
8) Calculate how many strings. 60LEDs / 6LEDs per string = 10 strings.
9) Calculate expected resistor value. 24V - 3.5 - 3.5 - 3.5 - 3.5 - 3.5 - 3.5 = 3V
3V / 0.7A = 4.3ohms
10) Calculate the minimum power rating of this resistor. 3V x 0.7A = 2.1W Double this -> 4.2W Any wattage greater than this will work. You can use the one above:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AVT25-10-ND
11) Get the mounts for the resistors to make the mounting of them much easier. (2 per resistor)
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=B1003-ND
12) Purchase 1A fuses in this case (10).
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=F2313-ND
13) Figure out your LED positions and cut the appropriate length pre-tinned stranded 20AWG to interconnect them.
14) Solder them together with the correct polarity. Then mount them to the heatsink.
15) Mount the resistors.
16) Pickup three terminal blocks like these:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=A98521-ND
17) Mount the two terminal blocks separated the exact distance needed to have those fuses clamped under the screws spanning two of these terminal blocks. This allows you change a fuse or to conveniently disconnect an individual string or disconnect one side and insert an ammeter to measure the current. Don't install the fuses yet!
18) Use the screw adjacent to the fuse to connect to one side of your power resistor.
19) The other side of the resistor goes to the proper polarity side of the first LED in this string.
20) Jumper all the screws on the other side of the fuses together. This will be your +24V bus from your power supply.
30) Mount your third terminal block nearby. Jumper all of one side together. This will be your 24V RETURN bus.
31) Connect the far end,(last LED), of all your strings to the remaining screws on the 24V RETURN bus.
32) Using your ohm meter and a screw driver set all your resistors to a little higher than the number we arrived at above. This is to protect from variation in your LEDs as the 3.5V number is only an average and not definite. So set the resistors around 5 ohms.
33) Back to the power supply!!
This one pointed out by SpacedCowboy can't be beat for value.
http://www.mpja.com/prodinfo.asp?number=16855+PS
We plan to have 10 strings and each draws 0.7A so how many amps total will a supply have to provide? 10 x 0.7A = 7A. The aforementioned supply provides up to 8.3A! So it should supply 7A just fine. I would never try to get the full rating from a supply as that is bad engineering and will reduce operating margins and system lifetime.
34) Mount the supply somewhere that is not going to EVER get wet, is out of reach of rug rats, and allows plenty of free air flow. Make sure the terminal strip on the supply can't be touched accidentally as it will have 120VAC somewhere on it. You will probably want to include a power switch somewhere.
35) Using two stranded pre-tinned 16AWG wires of the required length connect the supplies 24V RETURN to the previously prepared Ground Bus. Run these wires onto two screws that divide the bussed together side of the barrier block into thirds. This will balance the current in the bus jumpers you installed. You would not want the current to have to work its way all the way down the jumpers to the far end of the strip. You also want the wires from the supply to be big enough to handle the current if one wire is disconnected. Hence the 16AWG.
36) With too more wires connect the +24V to the bussed side of the fuse strip we've already prepared in a similar manner.
37) Review your work!
38) Power up.
39) Measure the output voltage. Is should be close to 24V. If it is power it down.
40) Measure the first string's resistor and write down the number.
41) Hook up this string's fuse. Switch your meter to Volts. Power up again. If you did everything correctly the string should light up! It will be shockingly bright. Promptly measure the voltage across the resistor. Write this down. And immediately turn off the power.
42) Divide the voltage you just measured by the resistance you previously measured. The result is the current you have running thru your string. Is it 0.7A or less? If it's not where you want it adjust it. Re-measure the resistance - power up - measure the voltage and power down. Do the math again. Repeat this until you're satisfied.
43) Now this string will give you a visual reference to compare the rest to. Go ahead, and while cycling the power, hook up more strings by installing their fuses. Power up looking for any strings that seem brighter. Check their current and adjust it if required.
45) Once they are all up and running you're there!
Here's what it looks like:
Some things to remember:
A) As you fire up more strings the system voltage will drop a little so don't go crazy trying to adjust the string currents to some crazy accuracy it's just not necessary.
B) Most power supplies have a Trim Voltage Adjust Screw. You can change the entire fixture's overall brightness by whatever that trim adjust allows. So, once the whole thing is running you could turn the whole fixture down a little to allow for some acclimation. If it doesn't dim enough you can, of course, adjust any or all strings individually to whatever you want.
C) If you want large adjust-ability of the entire fixture you could actually use a lab power supply that allows you to turn a knob to adjust the voltage of the entire fixture over a large range. This would probably be overkill unless you have some special need.
D) But what about efficiency, isn't this setup likely to be very inefficient?
We are looking at 190W to provide 147W of LED light. 147W/190W => 77%
BuckPucks land around 79% with this same design.
Mean Well efficiency is about 82%
If you want to get crazy about efficiency you adjust your supply voltage so that you need even less voltage across our power resistors. If you do this correctly you can beat the Mean Wells by a few percent, but I don't think it's required and it will reduce your system flexibility a little.
Since this build is supposed to reduce cost over active current regulation what does the cost shake out as?
Here's a single channel's expense:
$0.62 Fuse
$5.92 Resistor
$1.00 Resistor mounts
$0.66 One string's fraction of terminal blocks($6.60)
$2.00 One string's fraction of the power supply(20.00)
------------------------------------------------------------------
$10.20 To drive one string. (In this case 6 HBLEDs)
The MeanWell 60-48 costs about $50 and can run 13 LEDs. Or $23 for 6 LEDs.
The MeanWell solution costs more than twice our resistor regulation method.
It does allow dimming, (with added equipment).
This means our entire 60LED example would cost about $100 + 60LEDs. The equivalent MeanWell solution would ring up to about 5 x $50 = $250 + LEDs.
This setup also allows a very easy change to drivers if that becomes necessary or desirable.
I know that LED lights are expensive to make and I want to show a way that might allow someone to proceed with less expense. This could allow new tank corals or allow one to reduce their electrical consumption, sometimes dramatically, from their existing HID or fluorescent setup.
If this interests you read on.
I'm going to explain a way to set up your LED lighting for bare bottom prices. How? We're going to forgo expensive drivers.
How can we do this? With a little knowledge and some work.
The only real limitation associated with this method is individual string dimming. This may or may not even matter to you.
Why do this?
Pros:
1) Substantially less expensive than using drivers.
2) Will generate less electrical noise if this is concern for you.
3) Simple!
4) Uses less space.
5) In some cases can be more efficient than using a driver.
6) Doesn't preclude one from changing over to drivers in the future when they hit the big jackpot.
Cons:
1) Daily changes to the brightness of individual strings is not convenient enough to be viable.
2) In some cases this can be less efficient than drivers.
3) Takes a little more effort to setup.
4) No built-in current limiting protection as is typically found on drivers. (But we have a safe way around this.)
Required tools are: a few hand tools, a voltmeter, and an ammeter. (This can be your typical DMM(Digital MultiMeter).
You'll of course need anything else you require for building strings of LEDs. A calculator and some basic math ability may be useful. We can help you there.
The data sheets for whatever LEDs you are using.
If you haven't perused any of the various LED threads here you should probably do this before following this thread.
OK a little background refresher on LEDs:
LEDs are strange devices. They present a back voltage or blocking voltage against the power supply voltage you are supplying to them. This means that until you feed a LED with enough voltage to overcome this back voltage nothing much happens. Then once you reach the point where something does happen it can be way too much! Something is needed to 'ride herd' on the LEDs that will limit the current thru them to levels that are safe for them. This can be accomplished several ways. One is with a driver. Here, we are going to be using a power resistor.
Let's put some numbers to this discussion.
If we have a DC power source and a LED that we want to power we have to setup a circuit to correctly do this.
Here's how:
Let's look at a typical data sheet like the one for the Cree XR-E, (a popular choice),
http://www.cree.com/products/pdf/XLamp7090XR-E.pdf
Search around to find that 'back voltage' I refered to, it's called the FORWARD VOLTAGE, or the VOLTAGE FORWARD, or VOLTAGE F, or Vf
In this case you will see it on page 4.
FORWARD VOLTAGE @ 700mA.
This is the voltage you will need to supply to this device to get it lit up properly at a current of 700mA or 0.7 Amps.
What is the FORWARD VOLTAGE for the Cree XR-E? It's 3.5 Volts.
This tells us that we need a power supply that can supply at least 3.5V to overcome the LED's resistance to passing 700mA.
In this example lets say we have a 5V power supply. (This is a very common supply.)
Now if we just hooked up our LED to this supply it would fry in about 2 seconds - never to provide a glimmer of light again. We need something to get rid of or soak up some of this excess voltage. What excess voltage? Starting with the 5V our supply will provide we then subtract the voltage the LED will carve off, 3.5V.
5V - 3.5V = 1.5V
This leaves an excess of 1.5V that we will use a resistor to eliminate.
What value of resistance? We turn to Ohms Law. 1.5V / 0.7A = 2.14 ohms.
If we put a 2.14ohm resistor in our string of one LED and a 5V supply the LED will have 3.5V across it just as it needs.
There is one other thing we must not forget. That's the power rating of the resistor. It tosses away power to drop that needed 1.5 Volts. We need to make sure it is physically big enough to handle the needed power.
We calculate the power that will be dissipated. 1.5V X 0.7A = 1.05W or 1 watt. Now the rule for sizing resistors is to always double its power rating. This means we want at least a 2W resistor. This keeps the resistor cooler. It could still be too hot to touch but that's OK.
Here's a picture to show what we have:
That's almost all there is to it. What's missing? Ummm Let's leave that for a little later as it will make more sense then.
###############################
Alright, now we'll tackle a more typical set up for lighting a tank.
Instead of a power supply, a resistor, and a single LED, we will use a bunch of LEDs.
But how many can we use? It depends on the power supply's voltage.
If you look around you will find that 24VDC is a fairly common size that can be had for very little money. We'll work with that but most any voltage will do the trick.
If we have a 24V supply how many of those CREE XR-E LEDs should we hook up as a chain in series?
Take the supply voltage and divide it by the FORWARD VOLTAGE.
24V / 3.5V = 6.86
We could hook 6.86 LEDs up in a row and the 24VDC supply would just barely run them. Except we of course can't buy or make 0.86 of a LED.
So we round down to 6.
We can hook up 6 LEDs in series to the supply with our resistor in series with the chain. We should hook these up with typical electronics protocol of ->
Power supply (+) output to the resistor.
From there we go to the (+) on the first LED
then from this first LED's (-) to the next LED's (+)
continuing on to the last or sixth LED's (-).
This last (-) gets hooked back up to the power supply's negative or (-) terminal completing this LED "chain" or "string".
That's the basic layout. But we need to figure out what resistor to use. Back to the math.
Just like our first single LED calculation we do the same here.
24V - 3.5V - 3.5V - 3.5V - 3.5V - 3.5V - 3.5V = 3V
From this we see we need to carve off 3V of excessive voltage. If we want to run the sting at 700mA or 0.7A we proceed as follows,
3V / 0.7A = 4.29 ohms. We need a 4.29 ohm resistor.
What power rating? Again:
3V x 0.7A = 2.1W. Double this and we need at least a 4.2Watt 4.29ohm resistor.
No they don't sell these at Safeway! Fret not though.
Here's what you use.
http://www.heiresistors.com/PDF/AVT_AST Spec.pdf
Note these are 25W resistors! So they have us easily covered in power capability. They cost about $5 apiece from Digikey. You can probably find them elsewhere for less than that.
That is an adjustable resistor. You can set it to 4.3ohms.
What we do is use a multimeter to set it as close as you can to 4.3ohms. You can measure this in-circuit with no difficulty. So install everything then adjust the resistors.
If your multimeter has a 1A or larger current measuring ability you can break the circuit somewhere and directly measure the current.
Alternatively you can write down the actual resistance you set the resistor to and then with the string powered up and running directly measure the voltage across the resistor. Run back to Ohms Law and calculate the actual current.
If your resistor was set to 4.3ohms and you measure 3.2V you have this:
3.2V / 4.3ohms = 0.744A
Shut down and re-adjust the resistor to make it "longer" which will raise its resistance driving down the current a bit.
That's almost all there is to it.
Hmmm what are we forgetting? Protection! We need a fuse in each string. It should be between the power supply and the resistor. For a 700mA string you will want a 1A fast blow. This will prevent a fire should something go wrong. What could go wrong? A wire from one of your solder joints could come loose. Or an LED could decide it's done and short. If any one LED shorts it will increase the current of the entire string. How much? That resistor that's there to set the current at 700ma in the above example by having 3V across it, would then have about 3V + 3.5V equaling 6.5V. Running back to Mr. Ohm this would result in 6.5V / 4.3ohms = 1.15A which would blow our 1A fuse protecting the rest of the LEDs.
You can use one of these Digikey F2313-ND.
http://search.digikey.com/scripts/D...link=hp_go_button&KeyWords=F2313-ND&x=14&y=14
These are not the conventional glass fuses you're probably familiar with. These are much less costly and need no holder. Of course you can buy holders and use the typical glass fuses if you prefer.
So lets do a walk thru of a full hood.
1) Find your heat sink.
2) Figure out your LEDs.
3) Check the Forward Voltage of your selection. Example: 3.5V
4) Decide on the drive current. Example 700mA same as 0.7A
5) Figure out how many you want to run on your fixture. Example: 60
6) Hunt down a power supply by assuming a common output voltage. Example 24V
7) Calculate the size the strings must be: 24V / 3.5V = 6.8 -> round down to next whole number -> 6
8) Calculate how many strings. 60LEDs / 6LEDs per string = 10 strings.
9) Calculate expected resistor value. 24V - 3.5 - 3.5 - 3.5 - 3.5 - 3.5 - 3.5 = 3V
3V / 0.7A = 4.3ohms
10) Calculate the minimum power rating of this resistor. 3V x 0.7A = 2.1W Double this -> 4.2W Any wattage greater than this will work. You can use the one above:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AVT25-10-ND
11) Get the mounts for the resistors to make the mounting of them much easier. (2 per resistor)
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=B1003-ND
12) Purchase 1A fuses in this case (10).
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=F2313-ND
13) Figure out your LED positions and cut the appropriate length pre-tinned stranded 20AWG to interconnect them.
14) Solder them together with the correct polarity. Then mount them to the heatsink.
15) Mount the resistors.
16) Pickup three terminal blocks like these:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=A98521-ND
17) Mount the two terminal blocks separated the exact distance needed to have those fuses clamped under the screws spanning two of these terminal blocks. This allows you change a fuse or to conveniently disconnect an individual string or disconnect one side and insert an ammeter to measure the current. Don't install the fuses yet!
18) Use the screw adjacent to the fuse to connect to one side of your power resistor.
19) The other side of the resistor goes to the proper polarity side of the first LED in this string.
20) Jumper all the screws on the other side of the fuses together. This will be your +24V bus from your power supply.
30) Mount your third terminal block nearby. Jumper all of one side together. This will be your 24V RETURN bus.
31) Connect the far end,(last LED), of all your strings to the remaining screws on the 24V RETURN bus.
32) Using your ohm meter and a screw driver set all your resistors to a little higher than the number we arrived at above. This is to protect from variation in your LEDs as the 3.5V number is only an average and not definite. So set the resistors around 5 ohms.
33) Back to the power supply!!
This one pointed out by SpacedCowboy can't be beat for value.
http://www.mpja.com/prodinfo.asp?number=16855+PS
We plan to have 10 strings and each draws 0.7A so how many amps total will a supply have to provide? 10 x 0.7A = 7A. The aforementioned supply provides up to 8.3A! So it should supply 7A just fine. I would never try to get the full rating from a supply as that is bad engineering and will reduce operating margins and system lifetime.
34) Mount the supply somewhere that is not going to EVER get wet, is out of reach of rug rats, and allows plenty of free air flow. Make sure the terminal strip on the supply can't be touched accidentally as it will have 120VAC somewhere on it. You will probably want to include a power switch somewhere.
35) Using two stranded pre-tinned 16AWG wires of the required length connect the supplies 24V RETURN to the previously prepared Ground Bus. Run these wires onto two screws that divide the bussed together side of the barrier block into thirds. This will balance the current in the bus jumpers you installed. You would not want the current to have to work its way all the way down the jumpers to the far end of the strip. You also want the wires from the supply to be big enough to handle the current if one wire is disconnected. Hence the 16AWG.
36) With too more wires connect the +24V to the bussed side of the fuse strip we've already prepared in a similar manner.
37) Review your work!
38) Power up.
39) Measure the output voltage. Is should be close to 24V. If it is power it down.
40) Measure the first string's resistor and write down the number.
41) Hook up this string's fuse. Switch your meter to Volts. Power up again. If you did everything correctly the string should light up! It will be shockingly bright. Promptly measure the voltage across the resistor. Write this down. And immediately turn off the power.
42) Divide the voltage you just measured by the resistance you previously measured. The result is the current you have running thru your string. Is it 0.7A or less? If it's not where you want it adjust it. Re-measure the resistance - power up - measure the voltage and power down. Do the math again. Repeat this until you're satisfied.
43) Now this string will give you a visual reference to compare the rest to. Go ahead, and while cycling the power, hook up more strings by installing their fuses. Power up looking for any strings that seem brighter. Check their current and adjust it if required.
45) Once they are all up and running you're there!
Here's what it looks like:
Some things to remember:
A) As you fire up more strings the system voltage will drop a little so don't go crazy trying to adjust the string currents to some crazy accuracy it's just not necessary.
B) Most power supplies have a Trim Voltage Adjust Screw. You can change the entire fixture's overall brightness by whatever that trim adjust allows. So, once the whole thing is running you could turn the whole fixture down a little to allow for some acclimation. If it doesn't dim enough you can, of course, adjust any or all strings individually to whatever you want.
C) If you want large adjust-ability of the entire fixture you could actually use a lab power supply that allows you to turn a knob to adjust the voltage of the entire fixture over a large range. This would probably be overkill unless you have some special need.
D) But what about efficiency, isn't this setup likely to be very inefficient?
We are looking at 190W to provide 147W of LED light. 147W/190W => 77%
BuckPucks land around 79% with this same design.
Mean Well efficiency is about 82%
If you want to get crazy about efficiency you adjust your supply voltage so that you need even less voltage across our power resistors. If you do this correctly you can beat the Mean Wells by a few percent, but I don't think it's required and it will reduce your system flexibility a little.
Since this build is supposed to reduce cost over active current regulation what does the cost shake out as?
Here's a single channel's expense:
$0.62 Fuse
$5.92 Resistor
$1.00 Resistor mounts
$0.66 One string's fraction of terminal blocks($6.60)
$2.00 One string's fraction of the power supply(20.00)
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$10.20 To drive one string. (In this case 6 HBLEDs)
The MeanWell 60-48 costs about $50 and can run 13 LEDs. Or $23 for 6 LEDs.
The MeanWell solution costs more than twice our resistor regulation method.
It does allow dimming, (with added equipment).
This means our entire 60LED example would cost about $100 + 60LEDs. The equivalent MeanWell solution would ring up to about 5 x $50 = $250 + LEDs.
This setup also allows a very easy change to drivers if that becomes necessary or desirable.
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