Converting 120AC lamp to 12 V DC

Thread Starter

Pokemon801

Joined May 14, 2021
3
Hi:
I live off the grid (have for 40 years) all my lighting is 12 VDC LED. I recently picked up a really nice LED lamp at Goodwill. The lamp is powered at 120 VAC drawing 3.5 watts. When I removed the cover over the LED's I found 6 LED's mounted on a board with 2 wires soldered to it. (see photo). The 2 wires go to a little circuit board that I gather is a converter/rectifier, rectifying the AC voltage to DC and dropping the voltage to the appropriate amount for the LED's.
When I checked the voltage at the wires, I found it was 51.74 VDC. Since there are 6 diodes I figure each is drawing 8.62 VDC. I looked at the back of the board, to see how the diodes were wired, but it is just a solid piece of aluminum. Since there is just the 2 wires soldered to the board, does this mean the LED's are wired in series or parallel. Sorry I get confused by those terms.
My question: Can I cut the 2 wires loose from the converter/rectifier, put the appropriate sized resister on the positive lead and run the lamp off my 12 VDC system. What size of resister would I need?
I did something similar years ago with a pair of solar powered porch lamps, but then I was going from 4.5 VDC to 12 VDC and after a lot of reading I figured it out. LOL Any help on weather this is possible is greatly appreciated.
 

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Audioguru again

Joined Oct 21, 2019
3,173
Just guessing and assuming that the lamp does not have a transformer:

120VAC rectified makes 170VDC but the lamp probably uses a series capacitor to reduce the voltage without massive heating.
A white LED needs about 3.2V so six in series need 19.2VDC. The lamp uses something to reduce the 170VDC to 19.2VDC. Maybe the 51.74VDC you measured has the 51.74V fluctuating up and down at 120Hz producing an average of about 19.2VDC.

Since you want to use a 12VDC power supply then cut the LED circuit into two series strings with 3 LEDs that need about 9.6VDC and add a resistor in series with each string to limit the current.

3.5W divided by six LEDs equals 0.58W in each LED. The current in each LED is about 0.58W/3.2V= 181mA.
The resistors will be (12V - 9.6V)/181mA= 13.3 ohms. Using a standard 15 ohms for both resistors their power rating must be
(12V - 9.6V) squared/15 ohms= 0.4W but use 1W or 2W resistors so they do not burn things.
 

KeithWalker

Joined Jul 10, 2017
1,778
The LEDs are connected in series in the circuit board and it would be difficult to re-work it to run on 12V. The simplest solution would be to use a low power 12VDC to 120VAC inverter.
 

Lo_volt

Joined Apr 3, 2014
189
The LEDs are connected in series in the circuit board and it would be difficult to re-work it to run on 12V. The simplest solution would be to use a low power 12VDC to 120VAC inverter.
Simple, but not efficient. Since the TS is off grid, efficiency is paramount.

@Pokemon801 can you post pictures of the other side of the circuit board that the LED's are mounted to? As well let us know what other parts are in the circuit. Post pictures if you can't identify anything and we'll try to help. That will help to confirm Audioguru again's suggestion as to what the actual circuit is.
 

LesJones

Joined Jan 8, 2017
3,172
Some SMD LEDs contain more than 1 actual LED. For example the 5050 LED contains 3 single LEDs The package has 6 pins so they each have their own connections. If each chip was a single LED then the 6 in series would require about 20 volts. IF this was the case then a cheap step up converter fro Ebay such as this could be used. They can be modified to give a constant current output which is best for driving LEDs. This is how I modified one of these modules. Yet another solution would be to use the innards of a 12 volt (3- 5 watt) MR16 spotlight.

Les.
 

Thread Starter

Pokemon801

Joined May 14, 2021
3
Thank you all for your replies. As Low-Volt requested, I am attaching photos that show the back of the board that the LED's are attached to. In the same photo you can see the control board that powers the LED's. The second photo shows the back of the control board. Sorry, I can't get clearer pictures without popping the grommet that holds the AC cord.
Audioguru is correct, there is no transformer, and I believe Keith walker is correct that the LED's are connected in series.
I have thought about making my own board with LED's, I can get what I need for under $5 from Ebay. Then I would know exactly what I'm dealing with.
 

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Lo_volt

Joined Apr 3, 2014
189
The board inside looks like what Audioguru again described but with a few extra parts. My guess is that the extra parts are used to filter the voltage to the LED to reduce flicker.

The good part is that it should be easy to measure the LED voltage at the two leads coming into the LED board. While making the measurements, I'd suggest assembling the lamp back to the state shown in the first picture. That way you're reducing the possibility of you touching something with live AC on it. Also, as you are not connected to the grid, I'd suggest that you use a pure sine wave inverter while making the measurements as that's how the lamp was intended to be used.

Let us know what voltage you measure.
 

Thread Starter

Pokemon801

Joined May 14, 2021
3
In my original post, I stated that I had measured the voltage at the 2 wires. It is 51.74 VDC . My inverters are all pure sine wave so that isn't an issue. Because of the high voltage, I believe KeithWalker is correct, the lights are wired in series, because of the board construction, it would be very hard to rewire the lights. I am looking at Les Jones suggestion of using an MR16 spotlight or making my own LED board for 12 VDC.
 

anniel747

Joined Oct 18, 2020
1,034
In my original post, I stated that I had measured the voltage at the 2 wires. It is 51.74 VDC . My inverters are all pure sine wave so that isn't an issue. Because of the high voltage, I believe KeithWalker is correct, the lights are wired in series, because of the board construction, it would be very hard to rewire the lights. I am looking at Les Jones suggestion of using an MR16 spotlight or making my own LED board for 12 VDC.
You do know you can easily scratch the white coating to expose the copper to insert the resistors and new wiring, right?
 

KeithWalker

Joined Jul 10, 2017
1,778
The 120VAC lamps that I have tested contained 12 LEDs in series and each has a forward voltage of around 11V at 100mA. Those in your lamp may be similar but with a forward voltage of around 9V. You can check them if you have a DC bench supply with adjustable current limit. Scratch the paint off the copper at each side of a LED. Set the current limit to 100Ma and the voltage around 12V. Use a pair of wires with test probes connected to the supply and touch the probes across the LED. Read the voltage on the supply. If the LED is not bright enough, gradually increase the current.
 
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ErnieM

Joined Apr 24, 2011
8,150
I doubt the voltage converter board is a simple capacitor type, as there are way too many components for that scheme, and at least the one I can see (T1) is a transformer of some type indicating there is actually a switching supply at work here.

If I was doing this I would just toss everything inside the lamp and replace it with a 12V pin type LED board. Cheap from China off Ebay if you buy multiples and wait out the + month long free shipping.

Ive used similar in my kitchen for over a decade with just a few individual LEDs going dark. Also put em in my 12V pond lights to get the color of white I like,
 

Lo_volt

Joined Apr 3, 2014
189
If I was doing this I would just toss everything inside the lamp and replace it with a 12V pin type LED board.
Indeed, I think ErnieM is right. There are tons of cheap LED boards out there that will run on 12 volts with no modifications. I recently purchased a PR2 size LED for a 28 volt flashlight that I own. The LED could handle any voltage from 10 to 36 volts. There's got to be something similar that will work in your case.
 
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