Monitor resistors are large carnivorous animals that ea... Wait no! Wrong monitors!
Your question has been asked and answered about a dozen times so I'll just repeat one of those here if you don't mind.
kcress- Can you explain why you actually need the resistor and how you wire it in?
This is why.
It's required so you can measure the inter-string currents in situ.
You can put them anywhere in the string. By definition with
series, all current in a string must pass thru all elements of the string. The resistor will be an element of the string. Stick where ever you want. However.. If you stick them all on one end of all your strings, (one driver lead), when you want to compare the currents in all the strings to each other you only need to move ONE lead of your meter to measure the various strings. If you put the resistor in any other locations you would have to move both meter leads for every measurement.
In parallel string driving YOU
MUST HAVE BALANCED STRINGS. Otherwise you can have two or three times more current running in adjacent strings. This means you could plan on having 700mA running thru all your strings before you balance them and you could easily have 2A running thru one and 0.05A running in the neighbors! The 2A string is gonna die in seconds.
You cannot do this by looking at it. The human eye can't decipher in detail ANYTHING about a light source that is as bright as these LEDs. (You should never even look straight at them anyway.)
With a (one)(1)(uno) ohm resistor installed in each and every string you can instantly measure the exact current flowing thru that string with just a voltmeter. No wires to open. No careful power down, then cool down while you cut/solder open the string, followed by installing an ammeter. Then powering up. Now reading the current that is no longer the same because the whole system has cooled down. Then taking this not quite right reading, writing it down, powering down, re-soldering the open string. Powering up. Next... Only 10 more?
No, No, NO. With the aforementioned resistor permanently installed in the chain, just whip out your voltmeter, set the selector to
volts, and measure across the resistor. The number you read IS THE ACTUAL CURRENT flowing thru the string. No soldering. No temperature issues.
If you measure your strings like I just described and see 680mA, 720mA, 702mA, 698mA. You are good to go! Do not be an anal-idiot and try to get them all identical. They will all drift with age anyway. However, with the handy dandy resistors permanently in place you can check any time you want.
More likely you will probably see 590mA, 639mA, 727mA, 880mA, etc.
You then need to take your meter and while everything is running measure all the individual LED Vfs of the 590mA string. Write them down.
Do the same for the 880mA string. Write them down too.
Take the highest Vf one out of the 590mA string and swap it for the one with the lowest Vf in the 880mA string.
Remeasure ALL the string currents with the handy resistor again. You would now see something like: 690mA, 639mA, 727mA, 780mA, etc.
Keep working this same game plan until your strings are all within about 30mA. This will result in a long trouble free life for your strings. Periodically check all the string currents with the handy permanent resistors. Check maybe a month later. Make sure they are still in that 30mA range. If they are, recheck in six months. After that check every year.
Your neighborhood street signal lights can't be checked like this so in about 5 or six years the string currents shift so far that only certain strings carry most the current and fry. Not us though - because of the little miracle resistor.
The terminal blocks are to keep things clean, stable, cooling in the air, and available. They also allow easy fuse replacement. If you run dangerous sting voltages you would need to protect from accidental touches.
You use both meter probes across the resistors.
No formula. Use 1 ohm, 2 or 3W resistors. Use 1A fuses and use whatever terminal blocks you want to.
