220 V LED Bulb current - Page 1
Forum section: electronics / beginners

author: den
author_rank: Contributor
timestamp: April 12, 2017, 08:50:57 pm
content: Hi Everyone,I need you help in explaining something I can't understand.On one of my LED bulb I saw accidently that the power is 7 W, but the current is 120 mA.It obviously makes it more than 26 W! My assumption was - it must have some circuitry inside, which makes a huge phase shift and a very low power factor. So I tried to see that with an oscilloscope (RIGOL 1054z). One remark here - there is no real "earth" in my house - earth lead is shortened with neutral. This is why I connected the scope like is shown in the next diagram (ground leads not connected anywhere).The idea was to measure the mains voltage with CH1 (yellow) and current as a voltage drop on the ampermeter with CH3 (purple). This is the result:The problem is that there is A LOT of noise. I can even upload a video of that, because there is a lot more, than seen on this picture.To be sure that this is a problem of the cheap bulb I made the same measurement with Philips LED and even with a halogen (same type) - all look the same! Unfortunately, I was too stupid to measure a standard incandescent bulb.What I would like to understand is - why do I see these huge spikes? Is my measurement wrong?And the primary question - why is power rating of the LED is not matching with current?

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2017, 09:20:13 pm
content: Measuring in a mains environment -http://www.tek.com/dl/51W_10640_1.pdfRegards, Dana.

author: amyk
author_rank: Super Contributor
timestamp: April 13, 2017, 02:08:04 am
content: It's probably a capacitive dropper.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2017, 02:18:42 am
content: Many of these small/inexpensive bulbs do indeed use a simple capacitor to limit current to the LED. Even with LEDs and CFLs that use a proper electronic driver they are often low power factor so the apparent power is significantly higher than the real power. Your electric meter will read the wattage printed on the box, but if you measure the current it will be higher than you would calculate from volts and amps.

author: den
author_rank: Contributor
timestamp: April 13, 2017, 05:48:24 am
content: Thank you for your replies. The video I saw, PDF I will definitely read, thanks.I understand working with mains is risky, but do you think the way I measured everything was wrong?OK, I can understand that the bulb has a low power factor (although you cannot see a huge phase shift on the scope, can you? is it just the waveform, which is responsible for the low power factor?), but why the spikes? They also are sporadic - jumping all over the screen. My only idea currently is that those come from ampermeter circuitry (meter is off, but still).I am a beginner trying to understand the basics, so any thoughts/suggestions are very welcome.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2017, 06:00:29 am
content: Connecting a scope like that is very risky. Even though neutral is tied to ground in the panel, at least in areas I'm familiar with, it will not necessarily be at ground potential everywhere due to loads pulling the neutral voltage up. You should always either use a differential probe on the scope or power the DUT (bulb in this case) via an isolation transformer.

author: Seekonk
author_rank: Super Contributor
timestamp: April 13, 2017, 06:22:24 am
content: These lamps can vary greatly in electronics. Here is an interesting test, put a .047uF capacitor in series with the power. One will be a nice dim low power night light, another will not work at all, another will flash.