My LED camping lantern stopped worked this summer right before a family trip to Yosemite.    When I took it apart to find out how to repair it, I discovered how dangerous the device can be while charging the lantern with the AC power connection!    While charging the device, the USB connector (used to charge your iPhone) can have 120vac present, representing a shock hazard that could be deadly!    In this post, I’ll describe the shock hazard, how to avoid it, show the teardown and hack the lantern to make it safe and give some advice so others won’t make the same purchase mistake I did.

 

LED_Lantern_Shock

SHOCK HAZARD!!    The USB Connector has 120VAC present!   NEVER TOUCH OR PLUG ANYTHING INTO THE USB CONNECTOR WHILE AC CHARGING THIS LANTERN.I purchased this hazardous camping lantern from Amazon in 2016.  It was made by Infityle and thankfully is no longer available… I have no doubts why:   You can still see the product info here:  Infityle LED Camping Lantern

The shock hazard and how to make it safe:

The shock hazard is present when charging the LED camp lantern battery with an AC power cord, possibly causing the USB connector to have 120vac present.  If you decide you want to also charge your USB power devices under these conditions, that device will also then have 120v present.   The best way to make this safe is to throw the AC cord into the trash and only use solar charging.   Otherwise, at the very least, place a piece of electrical tape over the USB connector with a NOTE saying DO NOT REMOVE so that there is no chance you can touch or use the USB cable while charging the device.  

Teardown and Repair

On our family Yosemite trip we camped at our friends family compound in Oakhurst CA with showers, and outside kitchen, and clean toilets.   It was great, but even better was that Papa Bear had a storage shed and garage full of tools to fix stuff!!  We had everything to teardown and repair the LED camp lantern.   Of course, at this point, I had no idea how dangerous the lantern was.

Taking it apart wasn’t too hard, just turning one end of the device opened up most the important parts.

Circuit Description
Taking Apart the Lantern

After Papa Bear gave me an index card and pencil, I was ready to get started on the schematic teardown.

Schematic of Lantern

The multimeter confirmed the batteries were extremely low, indicating most likely the Ni-CAD batteries were faulty.     We found three AAA Alkaline batteries at the Compound to replace the Ni-CAD batteries and this fixed the LED lantern for the rest of the trip.

Late night soldering at the Compound

Once at home, I confirmed the circuit operation and found that 2 of the 3 Ni-Cad batteries were in faulty.  After replacing the batteries with a fresh set of AAA 600maH Ni-Cads,  the device was up and running.

Operation

The schematic is composed of an AC charging circuit, a solar charging circuit, the USB discharge circuit, and a three-way switch for the flashlight, lantern, and off modes of operation.

The AC charging circuit, and the cause of the shock hazard is actually a simple charging technique that can work for many IOT devices.    However, the use of this circuit in this device is inappropriate.   In general, for any device connected to the AC output, one should follow Underwriter Laboratories (UL) reinforced insulation rules so a shock hazard is not present.

In the current circuit, the 220uF electrolytic capacitors is the bulk capacitor for powering the lantern and is charged by the 400v 1uF capacitor and the four 1N4007 diodes.    The four diodes insure the electrolytic capacitor is charged to a positive voltage under any AC voltage polarity.    The 1uF capacitor sets the charging current by the simple capacitor rule.    The 120vac voltage across the capacitor creates a current proportional to the change in voltage.  You see how this simple circuit works with this LTSPICE simulation file.

LED Camp Lantern LTSpice Schematic File

Screenshot 2017-07-15 15.31.43
LTSPICE simulation showing Average charge current of 37mA

 

The 1uF capacitor creates a peak full wave rectified a current of 1602pi601u = 60ma or an average of 60ma*2/3.14 = 37mA.   The simulation verifies this behavior.

In addition to the AC charge,   The Solar cell will also charge the battery pack.    From experiments,  it can charge the battery pack to 3.8v.   I didn’t measure the current, but I  expect is around 50mA

The USB port is powered by a boost converter that converts the 3.6v battery voltage to 5v.    The resistors attached to the data lines of the USB will allow for a 1 amp charge as described in Build your own Super Fast iPhone and iPad Charger rate .     I don’t think this is possible from the boost converter.    I expect the rate will be closer to 400ma.    Enough to charge your phone in an emergency.

The three-way switch proves power to the flashlight LED or the Lantern LED or turns it off.

A simple hack to make it super safe

This hack is simply to convert the AC cord so it is powered by a cell phone or toy power supply that plugs an AC outlet (i.e. a wall wart).     These power supplies are all UL rated and will eliminate the shock hazard.   They do this by providing galvanic isolation.       The cost is either zero if you have one already or just a few dollars.    I recycled one 9v power supply from my junk bin.  The power supply will drop to 6v at 300mA (see the current rating in the image),  but in our circuit, I will only be using approximately 50mA, so the output voltage measured around 9v.

2017-07-15 16.19.30
Recycled 10v Power Supply for Toy. UL Rated

You can see the modification to the circuit to allow this to happen.  I just added a 82 ohm resistor in parallel with the 1uF capacitor and another 454ohm resistor to the RED LED.   The 82ohm resistor provides approximately a 50mA charging current into the 600mAH AAA Ni-CAD batteries.  A safe charging current of C/10 (i..e. 600mAH/10 = 60mA) will ensure full charge in 14 hours, but won’t damage the batteries.  The 454 ohm resistor ensures the RED LED will lite independent of what polarity the DC voltage is applied to the circuit.

 

2017-07-15 21.52.01
Added a 82 ohm and a 454 ohm resistor to convert to allow a DC supply

 

I cut the AC cord and spliced it directly to the output of the power supply.

 

IMG_4340.JPG
Attaching a UL rated power supply to the the Camp Lantern AC Cord.

 

Now the system is totally safe and no longer has a shock hazard.     When using the same multimeter setup as before shows only 26vac.

 

2017-07-15 17.11.07
The New Power Supply shows only 26vac.  No more shock hazard

 

While the voltage shows only 26vac,  because of the galvanic isolation of the power supply and the UL rating, there is no possibility of a shock.

Warning Signs for the consumer

For any device the plugged into the AC outlet,    A underwriters laboratory certificate is a great guide for safety.   Any device with this UL logo has passed critical safety compliance tests for shock or fire.   The chances of a shock hazard is then super low.    Look for this rating when you get a cheap device from Amazon that’s plugged into the AC outlet.

References

A reader just found this YouTube Video of the same device.    I’m relieved that others have found and published the safety error almost 8 months ago.  This thing is deadly.

BigCliveDotCom Youtube Channel

 

Let me know what you think or if you have any questions in the comment section below .