Advice: Testing long "Bus" bars for shorts/resistance - Page 1
Forum section: electronics / testgear

author: zitt
author_rank: Regular Contributor
timestamp: June 27, 2018, 11:47:13 pm
content: I'm building a RGB walkway for lighting my sidewalk using a LIDAR and an Addressable RGB strip.I've hung the strip; and know that the entire design with the proper length could draw as much as 26Amps.Because of this; I've added a copper foil +5V and GND strips on each side of the strip and have soldered connections alternatively on each 12inch section. This is done because I did a quick 5m test and show that my 5.2V supply droops to less than 3.8V at the end of the 5meter section and the Blue and Green LEDs aren't lighting (well) at the end of the strip. By adding the bus bars... I hope to eliminate the droop and enable full RGB no mater where the addressable strip is lit.I believe I'm at about 6-7meters so it's not really feasible to hook an ohm meter on each end of the installed buses to ensure there is connectivity along the entire bus.I'm looking for an free/inexpensive/local buy/amazon prime 2day way to verify both buss bars are good and has no shorts. One thought was to short the GND and 5V at the far end with alligator clip jumper (un-powered); and put an ohm meter across the +5/GND and test for resistance.Any other ideas?

author: DaJMasta
author_rank: Super Contributor
timestamp: June 28, 2018, 01:50:25 am
content: Run it out as AC (doesn't have to be mains voltage) and then every couple meters put in a switchmode converter. One converter every 2 meters means a max 1m length for the DC path using the existing traces, which probably is tolerable losses. DC over a long distance (though this only sort of qualifies) is a lot of copper and losses. You could even just use higher DC voltage and DC to DC switchers, since higher voltage means less current for the same power delivered, you get less losses in your cable at higher voltages.Maybe instead of foil bus bars, just a nice 12AWG solid core hookup wire or something. Stranded 12AWG is around 2 ohms per 1000 feet (solid should be less), and copper foil is around 0.5 mOhms per square... but that means a narrow strip may be hundreds of times longer than it is wide, a one meter strip that's 5mm wide is 200 square long, or 0.1 ohms, whereas the stranded 12AWG is around 2 ohms per 300 meters, or about 65% of the same resistance, solid core would be about 50% of the foil.Depends on how much you want to do to make it work. It may be that your environmental constraints (where it needs to fit, what can be made waterproof, etc.) will help decide between options. Also worth remembering that using PWM drive to run below maximum brightness (which should be built into the RBG strips) means you don't necessarily need to design for 26A if you can cap the output or the number of strips running full blast.

author: zitt
author_rank: Regular Contributor
timestamp: June 28, 2018, 04:33:27 am
content: I'm really already committed to the Setup I have designed and would need to change only if a problem is found with the current design. Read: I've already committed to the current design.Basically; I designed to 12VDC to 5.2VDC 13amp switching psus. On the doorway side; the main 13amp psu drives the RazPi and that side of the strip.The other end of the strip will have a similar 13amp psu driven from 12V.At this point; I'm more interested in just how to verify the buss bar isn't shorted or open. The only idea I've come up with is the resistance measurment in post #1.From there; I'll do a full scale test to see if I need to adopt a new design for the power.

author: Berni
author_rank: Super Contributor
timestamp: June 28, 2018, 05:31:52 am
content: You need to look up 4 terminal resistance measurement:https://en.wikipedia.org/wiki/Four-terminal_sensingIt lets you measure very low resistance components without including the test leads or contact resistance in your measurement results. Use a multimeter that supports this (Typically only bench DMMs have it), if you don't have one you can use 2 multimeters and a PSU. Use the PSU to send about 1 Amp of current trough it, mesure the current accurately with 1 multimeter while measuring the voltage at each end with another multimeter, Then just divide the voltage by the amps and you got your Ohms.Using copper foil is not a very good idea for 30A tho. The stuff is too thin to carry massive currents unless your strip of foil is ridiculously wide. For this application copper braid is a better choice. Its still flat and flexible like the foil but has 10x the copper thickness and as a result much lower resistance.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 28, 2018, 06:09:01 am
content: It seemed like you were looking to buy something to fix the voltage delivery problem, not just measure a resistance.Why not short the two at one end and use a meter to measure the other? You mention this with a voltage source.... but a multimeter has a precision current source for it to use in ohms mode and then it reads back the voltage between the terminals and does the math, so there's no external voltage required. You can rel out the contact resistance of the probes to get a better representation of what's actually there on a low resistance path. Yes, the 4 wire measurement mode will do better if your meter can handle it, with the same method of shorting the far end and dividing the result by two.Because it's a constant current source and with a relative mode (or just measuring probes shorted and doing the math yourself), you can take banana plug patch cables and run them out as far as you want with the multimeter - the added resistance of the test cable should be plenty low enough to leave the current source's maximums unhindered, so using a longer test lead only effects the probes shorted measurement that you subtract from your final result if you want to do an end-to-end measurement with no short at the far end.

author: Fungus
author_rank: Super Contributor
timestamp: June 28, 2018, 09:45:49 am
content: Quote from: zitt on June 27, 2018, 11:47:13 pmYou've already done the only test that counts - measure the voltage at the end of the strip. If it droops then your copper 'foil' isn't thick enough or has breaks in it.Quote from: zitt on June 27, 2018, 11:47:13 pmI'm fairly certain you'll know if there's a short.