Measuring the current drawn by an LED

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
This project takes place in a kitchen. Existing under-cabinet LED strips totalling 16W are fed by a 24v driver rated at 28.8W, 1.2A. I need to add a ceiling mounted LED spotlight to this circuit. Technically I have “room” for a 12W LED. (28.8 - 16 = 12.8). But I figure this might be pushing it as I’d be running the driver at 97% of its rated capacity. So I bought a 24V, 9W LED on AliExpress. When installed it seems really dim. So I connected my VOM in series and got a reading of 157ma. This would mean it’s drawing 3.8W instead of the rated 9W — which might explain why it’s dim. I assume it’s been mislabelled. Or is it incorrect to measure LED current as if it were a resistive load? (No dimmer is involved.)
 

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Tonyr1084

Joined Sep 24, 2015
7,782
Check your power supply. If it's voltage is dropping - eh, someone just posted this moment - if it's dropping then you're overloading it.
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
Sounds like it's a 3.8W, what current does the other 16W led lights take?
Good question. I haven’t found an easy way to measure them as they’re installed with proprietary connectors and I’d have to cut and strip an existing wire to measure. I’d like to avoid that. But I did temporarily hook up three 4W LEDs (i.e. 12W) that were plenty bright and they measured 425ma which X 24V comes to about 10W. So at least sort of close to rating.
 

dl324

Joined Mar 30, 2015
16,689
But could the burden voltage account for such a large drop in measured current?
It depends on the meter and the current range your were using.

I prefer to measure the drop across a resistor to avoid adding complications due to instrument limitation and having to compensate for them.

This is how to correct current readings with my Fluke 27:
1701816514348.png
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
Thanks for the in-depth calculations! I will try that. But just off the cuff it looks like my adjustment will not bring my reading of 157 mA anywhere close to 375 mA, which is the nominal current I’d expect for an actual 9W LED. Does that sound right?
 

dl324

Joined Mar 30, 2015
16,689
Does that sound right?
Have you measured supply voltage to see if it's actually 24V? What is the nature of the "driver"? Is it constant voltage or constant current?

When you observed a dim LED condition, was it in parallel with the original LEDs? Can you power the AliExpress LED with the driver by itself?
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
Check your power supply. If it's voltage is dropping - eh, someone just posted this moment - if it's dropping then you're overloading it.
Wow, never thought to check. Original voltage is 24.2V. After connecting the additional LED it’s 22.4V, a drop of 7.5%. And that’s despite the new LED drawing 58% less current than expected. Don’t see how I can be overloading the driver at such a low additional current.
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
Wow, never thought to check. Original voltage is 24.2V. After connecting the additional LED it’s 22.4V, a drop of 7.5%. And that’s despite the new LED drawing 58% less current than expected. Don’t see how I can be overloading the driver at such a low additional current.
Whoops! Sorry, I forgot to mention that the new LED is being driven through an SSR. That’s because the original LED strips are turned on by a motion-activated switch that delivers +5V to the chain of LEDs to trigger them. I used this same 5V signal to trigger the SSR which in turn delivers +24V to the new LED. The 22.4V I measured at the new LED was after the SSR and most certainly reflects the voltage drop across the SSR. This would contribute to the low current through the new LED, but not entirely explain it. I suspect I may have been shipped a 24V 5W LED mis-labelled as as 24V 9W.
 

Audioguru again

Joined Oct 21, 2019
6,612
The power rating of a light bulb does not say its brightness.
5W of light is not half the brightness of 10W because I think our sensitivity to brightness is logarithmic.
Some expensive name-brand bulbs are much brighter than cheap ones even if they have the same power ratings.

Does the 9WDC no-name-brand light bulb blink at 50Hz??
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
Yes, of course. I will do that. Thanks!
Connecting the LED directly to power supply: 23.5V x .204A = 4.8W. Assuming some loss through the meter we’re still closer to 5W than the 9W advertised. So either the product was mislabelled or the supplier has very bad QC. I’ll be trying to get a refund or a replacement. Or does anyone know of a reliable supplier for a 24V 9W ceiling-mountable LED?
 

Thread Starter

Mike_in_TO

Joined Nov 18, 2023
22
The power rating of a light bulb does not say its brightness.
5W of light is not half the brightness of 10W because I think our sensitivity to brightness is logarithmic.
Some expensive name-brand bulbs are much brighter than cheap ones even if they have the same power ratings.

Does the 9WDC no-name-brand light bulb blink at 50Hz??
I’m sure you’re right about the algorithmic perception of brightness. But still, the 24V “9W” LED should have been drawing a current of about 375 mA. At best I could only measure a current of 204 mA. That equates to 4.8W regardless of perceived brightness. (Although I’m perceiving it to be about half as bright as I was expecting). As for blinking, well, I’m attaching the LED to a 24VDC LED driver which, I’m assuming, is filtering out the frequency fluctuations of the 60Hz source voltage. In any case there’s no perceptible blinking.
 
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