Connect LED light to LED lamp power supply - Page 1
Forum section: electronics / projects

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2023, 01:09:31 am
content: I have several of these Harbor Freight LED lights. They're great and I use them a lot. I decided I wanted to make one into a desk lamp.I have several of the old IKEA jansjo LED lamps, that have a 4V power supply and figured this would be a great mate for the HF light. These are the old ones, before they went to USB.Here's the problem. In testing, if I use my junky alligator clip leads that have about 2-3 ohms resistance each it all works just fine. Although, because of the resistance there is a voltage drop. If I use better leads (soldered clips and thicker wire) that have about 0.1 ohms resistance each it bloody doesn't work! As soon as I switch on the light it comes on and then immediately goes out. If I swap the crappy leads back in it works. Ugh!This would eventually have no clip leads, just soldered connections, so the low resistance clip leads simulate the final solution but don't work.The HF LED light operated from a 18650 Li-ion battery that was about 4V and the IKEA power supply is 4V. So what the heck is going on?

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2023, 03:55:40 am
content: That power supply is 750mA, are you sure that's enough to cover what the light draws? It's possible that the resistance of your clip leads limited the current and without those it's drawing more and causing the PSU over-current protection to cycle.

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2023, 12:45:18 pm
content: That certainly makes sense, and didn't occur to me. I assumed the power supply could put out 750mA and that's it; that if I tried to draw too much it just wouldn't provide it. That's probably what it is. Thanks.

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2023, 01:29:53 pm
content: This bring up another question then. How much over-spec should I want? That is, can I expect to draw 750mA continuously from a 750mA power supply? Or how much over my expected current draw should I spec the power supply for ?

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2023, 05:46:27 pm
content: In theory you can draw the full rated current continuously, in practice it's usually a good idea to have a bit of overhead which will result in the PSU running cooler and lasting longer. I normally aim to load it to around 70-80% of rated capacity.

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2023, 07:26:24 pm
content: I like the brightness of the LED bar when the current is at least 1A. And that's about how bright it is when I run it from the Li-ion battery. So I'm going to have to come up with a bigger power supply then.

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2023, 07:41:05 pm
content: Does it not have some sort of constant current driver in it? LEDs are current driven, most power supplies are constant voltage.

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2023, 08:28:42 pm
content: I haven't busted it open yet; the case halves are glued together.

author: james_s
author_rank: Super Contributor
timestamp: April 17, 2023, 03:25:18 am
content: Have you tried running it from a bench supply and monitoring the current as you dial up the voltage? If the current reaches a certain point and then holds there then that suggests there's a linear constant current driver. If the current draw drops as the voltage increases then there's a switching regulator. Careful not to go much over the voltage of a fully charged battery. Given it's a cheap light I wouldn't be too surprised if it just uses resistors or even just relies on the battery voltage being too low to push excessive current through the LEDs.

author: MikeK
author_rank: Super Contributor
timestamp: February 05, 2024, 08:44:10 pm
content: I got back to this, with a new question...I picked up a Raspberry Pi 5V power supply to power this light. It does work, but I'm wondering if I'll damage the LEDs since it was designed to run from a single Li-ion battery. The green "full charge" LED on the light is much brighter when I'm powering the light with 5V than when using 4.2V. Can I test feasibility from 5V without just wiring it up and waiting for it to die?