AMC7135 driver in Walmart UFO 24-LED tent light

[Gadget_Guru]

Howdy all,

I picked up an LED tent light in Walmart's camping section a week or so ago. It has 24 5mm LEDs, which are much shorter and blunter than typical 5mm emitters. This shape causes them to emit a very wide-angle flood. It's not bad, stock, but a simple, inexpensive mod makes it much better, in my opinion.

Shown here is the innards of the light, stock. These batts are the cheap and nasty carbon-zinc AA cells that are packaged with the light (Why do they even bother?):

As I recall, current to the string of 24 5mm LEDs with the four included "fresh" carbon-zinc AAs was about 500mA. This means the average to each LED would be right about 20mA, not overdriven at all. But the current will immediately start to drop as the batts weaken. This gives very long run time, probably tens of hours. But I think gradually dimming battery-powered lights are annoying and crude.

To limit current from the nominally 6V batteries, the stock light uses eight 30-Ohm SMD resistors. Although all 24 LEDs are wired in parallel, there are eight groups of three LEDs, each with it's own resistor. Quite a good design as resistor lights go, I suppose.

To make this light have a constant brightness for most of the battery life, I installed a small linear regulator in place of the resistors. The resistors all came off quite easily with a tiny screwdriver and a soldering iron. You can just see them in the next photo, in the bottom left corner. They're the little black rectangles with numbers printed on them:

After the resistors were gone, I had to bridge the gaps they occupied to carry the current. I used some fine solid copper wire, tinned then soldered across the gaps to complete the circuit:

Next, the AMC7135 regulator board went in:

It's an easy job, since these boards have easy to solder pads on them. My board has just one regulator IC on it, giving a measured 337mA through most of the life of the batteries. This means that each LED is seeing about 14mA. They are extra efficient at that low current, and will last for a LONG time. Forward voltage of the string of 24 LEDs in parallel is 2.95V, quite low. The total LED power is just a fraction under one watt.

I removed the reverse-voltage diode protection from the board, since it is a bit wasteful, dropping input voltage a half volt or more. With the diode removed, I have to be certain to install the batteries with their polarity correct, or the regulator board will get fried. That small risk is more than made up for by the extra time in regulation gained by this simple mod. The battery voltage must now drop to less than 200mV over Vf before the driver board goes into direct-drive.

I have not yet tested the run time with new cells, but I did a quick test with a set of four Duracell AA alkalines that had a 2003 exp. date on them. They came in a used camera flash I recently bought on eBay, and had been used enough to be pretty weak. Here's the graph showing the very flat regulation, followed by a fairly steep decline, then a shallower curve that would likely go for hours in "moon mode":

Another option is to run three cells, with a dummy cell in the fourth battery slot. This would give shorter time in regulation, followed by a longer, shallower "moon mode" decline. Overall efficiency would be higher with three cells, as well, since the fourth cell's extra voltage is being burned off by the AMC7135 during all of the initial part of battery discharge.

 

[Black Rose]

Next, the AMC7135 regulator board went in:

Quick question....

It's hard to see in that picture, but where are the blue and a black wires connected on the inner portion?

 

[Gadget_Guru]

Quick question....

It's hard to see in that picture, but where are the blue and a black wires connected on the inner portion?

It's deceiving, but in this case the red wire is negative, and runs from the negative battery terminal to the negative INPUT of the AMC7135 board. That's the wire color that was present when I opened the light, and I saw no reason to change it.

Of the three wires attached to the the AMC board, the black middle one runs from the negative OUTPUT of the AMC7135 board to the outer negative (cathode) ring of the LED boards. The black top wire runs from the positive INPUT/OUTPUT (they're in common) of the AMC7135 board to the inner positive (anode) ring on the LED boards. Note that since I removed all the resistors and bridged the spaces left, the innermost and middle rings on the LED boards are connected, and are now common.

The blue wire runs from the positive battery terminal, through the power switch, and then to the inner positive (anode) ring(s) on the LED boards.

I hope this helps.

 

[BrighTor]

I had to chuckle when I saw this thread title as I knew exactly the lamp/light you were talking about. I bought one and promptly had to practice my fledgeling soldering skills...mine had a loose connection where the blue wire went into the resistor. Had to re-solder it. I was interested to see what you'd done with the light. I know almost nothing about electrical components in electronics so I try to read stuff in this forum to pick up info. Thanks for posting!

 

[Gadget_Guru]

After running the above test on the four old alkaline AA cells, I let the light continue to run. It kept going for days, hanging over my bed, until I got tired of looking at it. Very low light output, of course, but still just enough to read by from about two feet away.

The 24 LEDs in my light must have VERY closely matched forward voltages. When the light was running in moon mode, all of the LEDs appeared to have equal brightness. This is unusual in cheap lights with multiple 5mm LEDs in parallel. My 128LED "showerhead" light looks quite weird once the battery voltage drops, as the brightness and color of the LEDs in the array vary a LOT at low current. Some will be almost totally dimmed out, while others are still quite bright. With fresh batts in the showerhead light, they all look the same to the eye. Of course they're still going to vary in brightness quite a lot at full battery voltage, but they're just too bright for the eye to see the difference.

 

[Gadget_Guru]

I just finished a run time test with three new Duracell alkaline AA cells:

So, just about four hours in regulation, then another hour dropping down to a low level. At the end of this test the current was still at 20mA and VERY slowly dropping. It's still putting out usable light, and will for days.

I don't know yet how much longer it would stay in regulation on four alkaline cells. Perhaps another hour or maybe two?