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6V Lantern

My mother got me this lamp as a Christmas present to use when my wife and I go camping - something we really enjoying doing. It is a nice design, with a bright output. Instead of using the the larger 6V brick batteries, it has a compartment for 4 D cells. I created an LED insert to fit into the bulb socket, added rechargeable batteries with a charging port, and replaced the on/off switch. With 20 LEDs drawing a total of 0.4A and a set of 10Ah batteries, this lamp can last an estimated 25 hours on a single charge.

Bulb without glass.

LED bulb take one.

LED bulb take two.

The charging port.

Casing back on base.

New slide switch.

A Better Bulb

One of the first things I did to this lamp was create an LED bulb to fit in the socket. I chose to do this with this type of modification so I can still use a standard 6V bulb if I ever need to do so. To start, I took a standard 6V bulb and placed it in a zipper bag before smashing the bulb with the butt of a screwdriver. The bag catches all of the glass shards. I really can't stress enough how dangerous broken glass can be. Be sure to take all of the necessary precautions if you decide to repeat my steps. To figure out the polarity of the bulb, I put it back in the socket and carefully probed the two leads with the power on. 

For the LEDs, I wire-wrap soldered a row of 20 LEDs - 10 parallel groups of 2 series LEDs, each with a 33Ω resistor - before bending the bank into a circle. The LED bank was then carefully soldered to the leads in the bulb. I can't say that this was an easy process, but it works quite well. This first LED bulb was bright, but not nearly as bright as the original bulb. The problem was the use of two series LEDs with a resistor. These super bright white LEDs can have a forward voltage nearing 4V. Splitting the total supply between two LEDs and a resistor severely limited the maximum brightness. It wasn't incredibly noticeable with standard 1.5V alkaline batteries, but was very evident after switching to 1.2V rechargeables. The air gap was filled with hot glue to give the bulb form and support. 

In my second attempt to make a better bulb, I stuck with 20 total LEDs, but all were in parallel. I also went with two 10Ω, 1W resistors in parallel for the total series resistance. Yes, I know this isn't the best design practice. With only one resistor limiting the current for the entire bank of parallel LEDs, a lot of bad things can happen. To start, not all LEDs are made equal, so the one with the lowest forward voltage will dominate and appear much brighter than the rest. Also if one LED fails then the current is split among fewer LEDs, increasing each individual LED current. This increase in current could cause a chain reaction (however slow) that ends with every single LED failing from too much current - each time an LED fails, the current through the remaining LEDs will increase, also increasing the chance that more LEDs will fail. After considering these risks, I decided that I didn't want to use 20 series resistor per bulb, so... whatever happens happens!

Power System Overhaul

Aside from making an LED light, I wanted to use rechargeable batteries in this lamp so I didn't have to buy replacement D cells. The problem with that is the inability to recharge the batteries without removing them. In addition, the only charger I have for D batteries has a "quick" charge current of 200mA. That would take 50 hours to charge my 10Ah rechargeable batteries... I would much rather be able to use my 2.4-7.2V smart charger which has a selectable 0.9A or 1.8A charge current, but to do that, I needed to add a charging port to the lamp to connect to the entire battery bank.

After figuring out how to take the lamp apart, I was able to solder a couple of wires directly to the load connectors to put a charging port in parallel to the batteries. I had to cut a small notch in the back of the casing for it to fit, but it works, and I'm not going for any beauty awards with this one.

An unexpected side effect of taking the casing off of the base was the destruction of the power knob. It worked by sliding a piece of metal over two contacts as the knob was turned. I could never get the pieces to align correctly, so I had to replace the knob which a simple slide switch - not as elegant, but just as functional.

The finished product is not as pretty as the original, but it works really well. Although not as bright as the original bulb (which is to be expected) the LED bulb is plenty bright enough to illuminate a table or tent while camping, and the charging port makes recharging the batteries a snap with my smart charger. The only issue with the switch is that it is easier to turn on then the knob, so putting this lamp in a bag with other objects in proximity may not be the best idea, unless pointlessly illuminating a bag is no big deal. Although I don't know why I ever want to do such, the light cannot be used while the batteries are charging because the charging pulse causes the LEDs to get brighter and dim at a very noticeable frequency.

Technical Calculations

In case you are wondering, I did do the math to see what is going on with this lamp. To start, typical D cell alkaline batteries are rated as 1.5V whereas the rechargeable batteries are only 1.2V. This drops the supply voltage from an expected 6V to 4.8V. This difference is very noticeable with standard bulbs and can even cause some of them to not turn on at all. The LED bulb will work with either set of batteries, albeit brighter with alkalines. The voltage drop across the series resistor was measured at 2.02V, so the LEDs should have a forward voltage drop of around 2.8V. With two parallel 10Ω resistors, the following current is calculated.

ILED =  (VR / R ) / (Number of LEDs) = (2.02V / 5Ω) / (20 LEDs) = 20.2mA

This isn't a lot of LED current, but like I said, the lamp is plenty bright for my needs. Also, the batteries should last a lot longer. How much longer? This can be estimated! Bulbs are non-ohmic, meaning their resistance changes as the applied voltage changes. This change is non-linear, and not always predictable. The standard '6V lantern'  such as a Rayovav K13-2 needs at least 4.8V to turn on. This explains why not all 6V batteries will work with rechargeables, or why they only work when the batteries are fully charged. The bulbs are rated at 0.5A and 3.6W. Since they are non-ohmic, the typical P = VI doesn't apply, so we'll just use the rated 0.5A for calculations. Given my bank of 10Ah batteries, (and ignoring the necessary 4.8V to turn on) the bulb could last up to 20 hours. The LED bulb should light for 24.75 hours.

Bulb time = 10Ah / 0.5A = 20h ; LED time = 10Ah / .404A = 24.75h
Additional run time = 100% * (24.75h - 20h) / 20h = 23.75%


Although 4.75 extra hours of run time might not seem like a lot, there are many additional benefits to the new lamp. Using rechargeable batteries means less battery waste in the landfills, not to mention relieving the need to buy more batteries. Using LEDs not only increases the on time by 23.75%, it also means I'll probably never have to replace the bulb. Besides, a standard bulb wouldn't work appropriately with the lower voltage batteries anyway. Finally, the LED bulb doesn't put off any heat (except for a negligible amount radiating from the series resistor) so bugs are not nearly as attracted to it. Although the first version LED bulb was much more efficient (10 banks with individual LED currents of around 12mA for total on time of 83 hours) the bulb was not very bright. Having a lamp that lasted a really long time but didn't put out a lot of light wasn't very helpful. 

The problem with the second LED bulb is that so much forward voltage is wasted on the series resistance to create enough current for the LEDs. Since only 10 stings of LEDs were used in the first bulb, and each consumed almost half as much current as the second bulb, the on time was more than tripled that of the second bulb which had 20 LED strings with almost double the current through each.

The best scenario would be to find white LEDs with a lower forward voltage. Then, two could be placed in series without the reduction of brightness seen in the first LED bulb. On the other hand, using only a few high power LEDs in parallel may be better, as each will have a forward voltage nearing the supply. A small resistance in series with each would provide enough current for a very bright output. Someday I may design an even better bulb, but for now, I am satisfied.