| Electronics > Beginners |
| Measure current without loosing some |
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| haizaar:
Good day guys, I hooked 30led, 5V strip to a arduino, with additional power, and now I try to measure how much current do they draw. Spec says they should draw 60mA on full brightness, i.e. .06*30 = 1.8A total. I hook in a EEVlog BM235 multimeter and I observe significant drop in led brightness, and the current is just 650mA with DMM: https://photos.app.goo.gl/WF23rcEP3EhWsRc59 without DMM: https://photos.app.goo.gl/t1a73J7ycKD1nqs86 I then tried to get rid of clip-on crocodiles and use just the DMM test lead. Brightness improved and current moved to 1A, but still not as bright as without DMM. Am I missing something? Is there a way to measure current without such losses? My clipon crocodiles have a resistance of ~0.5ohm each, yet they caused 350mA current drop. Does it make sense? Thank you! |
| Zbig:
Get a DC-capable clamp meter like the great little Uni-T UT210E. |
| haizaar:
Sounds like a plan... but does it mean that standard DMM ammeter is not of any good? Btw, is it worth in your opinion to get a high-sensitivity one (UNI-T UT251A)? |
| homebrew:
As 'normal' DMMs use a shunt and measure the voltage drop accross it, you'll always 'lose' that voltage difference. However, in high current mode this drop should be rather small. Thus, two questions come to mind: Are you sure that all your LEDs are in parallel? If so, how is the current regulated? Dropper resistors or proper LED driver(s)? If a dropper resistor would be present on the LED strip you could instead measure its voltage drop. Knowing the resistance, you could calculate the current accurately and without the need to interfere with the circuit. |
| haizaar:
--- Quote from: homebrew on July 19, 2019, 02:03:19 pm ---As 'normal' DMMs use a shunt and measure the voltage drop accross it, you'll always 'lose' that voltage difference. However, in high current mode this drop should be rather small. Thus, two questions come to mind: Are you sure that all your LEDs are in parallel? If so, how is the current regulated? Dropper resistors or proper LED driver(s)? If a dropper resistor would be present on the LED strip you could instead measure its voltage drop. Knowing the resistance, you could calculate the current accurately and without the need to interfere with the circuit. --- End quote --- Yeah, I looked at the specs, and on the 6A range I'm measuring, the burden voltage is 0.04V/A. Since my amps are within 1-2A range, should 40-80mV make such a significant difference? The strip is a pixel strip - APA102, so I give it power and arduino drives data and clocks. Power is connected in parallel - I tested +/- wires from both ends and they "beep". The strip PCB has only one thing soldered to it besides the leds themselves: https://photos.app.goo.gl/c1FzXFRwX6CjSM8C6 I'm not sure what they are since they are inside the silicone tubing. There are two pairs of those - in the beginning and midway (it's a 1 meter strip with 30 leds). |
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