Add on/off switch to a USB cigarette lighter adapter - Page 1
Forum section: electronics / beginners

author: gointern
author_rank: Newbie
timestamp: July 04, 2015, 11:35:24 pm
content: Hello,My first post and first request for help. I have very little experience with electronics.I bought a cigarette lighter adapter for my car and would like to mount it permanently to the dashboard.The first one I bought and connected seemed to work fine until I turned it off. After turning it on it just blew up. So before connecting the second one I thought I'll ask if I am doing something wrong.I hope the pictures make sense.Should the switch go on the positive line instead of the negative?Should I just connect everything and try again or do I need additional components to make it work?Thanks.

author: miguelvp
author_rank: Super Contributor
timestamp: July 04, 2015, 11:51:52 pm
content: I wouldn't leave it plugged all the time. Who knows what would happen if you forget to turn it off, or if you do a poor job and something shorts (like that cable what I hope is temporary)Still if it's a cheap adapter it might overheat so I wouldn't leave it unattended.It wouldn't be fun to get up to go to work just to find your car in flames.Specially because as you stated, you have little experience with electronics and you already blew one up.

author: gointern
author_rank: Newbie
timestamp: July 05, 2015, 12:18:13 am
content: So thats why I thought to add a switch. There is a light so I would see when its on or off. Wouldn't that protect?

author: Rick Law
author_rank: Super Contributor
timestamp: July 05, 2015, 01:49:36 am
content: From the looks of it, it does seem you have a cut off switch installed ok - but a few questions.(1) Looks like you have +12V (board-spring to red) always connected and the switch cut off the ground (board-red to switch-black). Since USB-negative (ground) is often constantly connected to the shielding of the USB, that could be an issue. You could be shorting via another USB plug in or other connection with ground. See if that is the case. Still, that should not "just blew up". Are you testing it in your car and how are you testing it? Your USB out is connected to what?(2) Looks like your +12V (board-spring to red) is not soldered? Is the +12V (red wire to spring) secured somehow? How about the switch wires? Are you sure when you move it they did not just "happen to touch"?(3) Define "just blew up". Tell us more. How did you determine failure? Bang!? Spark? Mushroom cloud?

author: gointern
author_rank: Newbie
timestamp: July 05, 2015, 02:19:40 am
content: The wires are not soldered yet. I just put them side by side to illustrate the way I connected them earlier in case I made a mistake and need to put the switch on the positive power cable. I will probably remove the spring or at least shorten it and solder all the cables then protect all the connections.I read somewhere that a switch should never cut a ground cable. Maybe that is just in house installations. Should I do that here?The 8 pin chip blew up. There was a sound like something shorted, chip had a black spot, and the light turned off. There was nothing connected to the USB port. I just turned it off and it smoked when I turned it back on.I bought a few of those adapters (they are cheap) so would like to give another try.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: July 05, 2015, 02:34:21 am
content: Sounds like you connected it with the polarity reversed or shorted the output somehowInstall an inline fuse holder as close to whatever the positive voltage source is, whether that is the car battery or a factory loom you're tapping into. Install the switch wherever you want between the fuse and the USB adapter.

author: bitwelder
author_rank: Super Contributor
timestamp: July 05, 2015, 06:34:26 am
content: Quote from: gointern on July 05, 2015, 02:19:40 amWhat type of light, and how powerful?

author: Hideki
author_rank: Frequent Contributor
timestamp: July 05, 2015, 09:48:22 am
content: Quote from: bitwelder on July 05, 2015, 06:34:26 amHe probably means the little LED that indicates that the device is powered up.

author: gointern
author_rank: Newbie
timestamp: July 05, 2015, 03:08:50 pm
content: Quote from: bitwelder on July 05, 2015, 06:34:26 amYes, I meant the LED light on the adapter itself.

author: Rick Law
author_rank: Super Contributor
timestamp: July 05, 2015, 10:08:05 pm
content: Quote from: gointern on July 05, 2015, 03:08:50 pmDid itcame on and then "turned off"or did it stay off and was never on?Reversing polarity should not light the LED at all and it would have stayed off from the start to finish. If it came on even for a brief moment then turned off, it was powered until the chip blew - which means it is unlikely you reversed the polarity. In this case, you have a short some where - possibly with your output actually drawing way too much current.To ensure your board is properly powered, you need a DMM to measure exactly what you put into the spring (+) and negative of the board. In your picture, you have the red wire to the board labeled negative (-). Was that soldered by you or original? Visually trace the power wire and look for shorting. Use the DMM to check would help too. In the picture, it looks like the negative power wire is router by/around the USB plug, there could be a short there.After you visually and DMM check for any shorts, if you are going to test it again without soldering, at least you electrical tape to insulate the exposed twisted "joint". It just could be the movement like pressing switch causing the exposed wire joint to touch something.Silly as it sounds, look inside the plug to see if you have metal debris or wire fragments in there. After stripping the jacket from a wire, some times, small fragments of wire might have evaded proper disposal and it get caught by and then lodged inside the plug. I had such a wire fragment that got lodged on my ~2x3 inches small PCB but took me 1/2 hour to find.Also, I hope you are not testing with the CLA in the car. The car can put out quite a lot of current. Do you have a power source to test it with? You need a 12V source with some kind of current limit. A current limiter is better than a fuse since a fuse will require replacement.First, forget the switch and power to test the adapter directly without a switch by quick-touching the +wire to spring and the - wire to the negative. Hand holding the wire so you can disconnect quickly. Start with very low current like 5mA to 10mA, and gradually increase it until you see the LED turn on.Once you get the setup to light the LED, you can increase current to the proper limit and test the output. When the output is right, only then it is the time to cut in and add the switch.

author: kolonelkadat
author_rank: Regular Contributor
timestamp: July 06, 2015, 04:22:22 am
content: personally, I wouldnt use a switch. if you leave the switch on when you leave your car, you may come back and find the battery is flat, or the car has burned down.I'd put the ground to ground. Then put the positive side to the fuse box using a fuse tap on one of the fuses that comes on with the ignition. probably use a 2 or 3 amp fuse.fuse tap video

author: kolbep
author_rank: Frequent Contributor
timestamp: July 06, 2015, 05:34:09 am
content: Normally those things have a glass fuse between the spring, and the +ve contact pin built in.Strange that yours didn't