SupraSPL
Well-Known Member
Scotch, your curiosity reminds me of me 
. Measuring voltage is very easy of course because we measure in parallel but measuring current and resistance can be tricky and dangerous for the equipment. Cheap multimeters are not very accurate. The trick to manufacturing them to be accurate is the quality of the internal resistors the meter is using as a reference, and reference quality resistors are not cheap. That said, you can get a ballpark figure with a cheap meter but it really can only give you an approximation.
If you want to invest in an accurate meter look for a used Fluke 115 on eBay. I got one a few months back for $75 shipped and sure enough it gives different readings than the pile of cheapo meters I have. The 115 also gives you more specific current readings in the range we use most (<1000mA). I wish I had done that years ago! If you are interested in multimeter reviews check out eevblog on youtube. Hosted by an entertaining aussie electronics engineer.
If you are interested in calculating the efficiency of the drivers you need to know the dissipation wattage of the string being driven. It is simple: vF of the whole string X current = dissipation watts. Then we need the input power. I just use a Kill-A-Watt meter for that. Divide dissipation watts by input power watts and you get the efficiency. The figures will change significantly as the driver and heatsink warm up, so it takes a while to test accurately.
Not surprisingly the very small cheapo drivers from DX and eBay were very poor, 50% efficiency. That is a bummer, small drivers could come in handy for cloning, vegging and outdoor lighting. Most drivers are ~75-80% efficient and some claim up to 88%. My best is 87% and that is not too far off from quality HID ballasts ~90%. If we had generator powered, solar powered or wind powered DC grows, the drivers could reach 95%+. Maybe someday...
So measuring current is the key. It is much easier to measure current if you have an extra set of test leads with clips or slide connectors (.25") on the end rather than probes. Polarity doesnt matter on the test leads but to make polarity of my drivers and strings fool proof, I always put female connectors on the negative side and male on the positive side. I do that on every heat sink, LED string and on one set of multimeter wires. That allows you to bug in with your multimeter easily at any connection point and test the current. It also allows you to swap drivers or heatsinks easily. I also keep a set of cheapo LEDs on stars with slide connectors so i can test drivers. You can daisy chain them to test the max performance of any size driver. Some drivers decrease their current as the string grows longer and you may need to know where that limit is. You also may need to check or adjust the current on a dimmable driver. The slide connectors also fit perfectly on ATC fuses and many switches.
The bare crimp connections are very solid which is important for DIY LED. Any slightly loose connections can cause a driver failure or worse burn out an entire string. Once you have a solid crimp you can heatshrink over it and color code it if necessary. You can get cheap slide connectors, colored heatshrink tubing, switches and 16awg hookup wire VERY cheaply from elecdirect. For a large DIY LED build, I cannot come up with a cheaper or better way. Surprisingly I had a hard time finding a fair price on slide connectors and had to search around the net quite a bit.
WARNINGS when testing current:
Make sure you kill the AC power to the driver before you disconnect or reconnect the string.
Make sure you have your test leads plugged into the correct slot of your multimeter. When switching between testing vF and current, you might forget to switch the leads. This can blow the fuse in your multimeter or if it is unfused (or you jumpered it!) you can cook the meter, driver and or string (I have done all of those things)
If you add a switch to your driver make sure you switch the hot wire and not the neutral.
Good luck!
. Measuring voltage is very easy of course because we measure in parallel but measuring current and resistance can be tricky and dangerous for the equipment. Cheap multimeters are not very accurate. The trick to manufacturing them to be accurate is the quality of the internal resistors the meter is using as a reference, and reference quality resistors are not cheap. That said, you can get a ballpark figure with a cheap meter but it really can only give you an approximation.If you want to invest in an accurate meter look for a used Fluke 115 on eBay. I got one a few months back for $75 shipped and sure enough it gives different readings than the pile of cheapo meters I have. The 115 also gives you more specific current readings in the range we use most (<1000mA). I wish I had done that years ago! If you are interested in multimeter reviews check out eevblog on youtube. Hosted by an entertaining aussie electronics engineer.
If you are interested in calculating the efficiency of the drivers you need to know the dissipation wattage of the string being driven. It is simple: vF of the whole string X current = dissipation watts. Then we need the input power. I just use a Kill-A-Watt meter for that. Divide dissipation watts by input power watts and you get the efficiency. The figures will change significantly as the driver and heatsink warm up, so it takes a while to test accurately.
Not surprisingly the very small cheapo drivers from DX and eBay were very poor, 50% efficiency. That is a bummer, small drivers could come in handy for cloning, vegging and outdoor lighting. Most drivers are ~75-80% efficient and some claim up to 88%. My best is 87% and that is not too far off from quality HID ballasts ~90%. If we had generator powered, solar powered or wind powered DC grows, the drivers could reach 95%+. Maybe someday...
So measuring current is the key. It is much easier to measure current if you have an extra set of test leads with clips or slide connectors (.25") on the end rather than probes. Polarity doesnt matter on the test leads but to make polarity of my drivers and strings fool proof, I always put female connectors on the negative side and male on the positive side. I do that on every heat sink, LED string and on one set of multimeter wires. That allows you to bug in with your multimeter easily at any connection point and test the current. It also allows you to swap drivers or heatsinks easily. I also keep a set of cheapo LEDs on stars with slide connectors so i can test drivers. You can daisy chain them to test the max performance of any size driver. Some drivers decrease their current as the string grows longer and you may need to know where that limit is. You also may need to check or adjust the current on a dimmable driver. The slide connectors also fit perfectly on ATC fuses and many switches.
The bare crimp connections are very solid which is important for DIY LED. Any slightly loose connections can cause a driver failure or worse burn out an entire string. Once you have a solid crimp you can heatshrink over it and color code it if necessary. You can get cheap slide connectors, colored heatshrink tubing, switches and 16awg hookup wire VERY cheaply from elecdirect. For a large DIY LED build, I cannot come up with a cheaper or better way. Surprisingly I had a hard time finding a fair price on slide connectors and had to search around the net quite a bit.
WARNINGS when testing current:
Make sure you kill the AC power to the driver before you disconnect or reconnect the string.
Make sure you have your test leads plugged into the correct slot of your multimeter. When switching between testing vF and current, you might forget to switch the leads. This can blow the fuse in your multimeter or if it is unfused (or you jumpered it!) you can cook the meter, driver and or string (I have done all of those things)
If you add a switch to your driver make sure you switch the hot wire and not the neutral.
Good luck!
