USB cigarette light replacements

DaveInDenver

DaveInDenver

Rising Sun Ham Guru
Joined
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Messages
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About that melt down. There's several things to consider.

First is that the point of overcurrent protection (e.g. fuses and breakers) is that it shouldn't matter if you use a $1 Amazon clearance or $50 artisanal USB device. If you use the right fuse and wiring it will be safe. In fact if using a Blue Sea means you have to skimp elsewhere then you're better off spending money on legitimate fuses and quality wire and going with a cheap outlet or adapter.

For example, there's always the risk of counterfeit stuff...

Counterfeit Chinese ..... FUSES - Page 1

Counterfeit Chinese ..... FUSES - Page 1
www.eevblog.com

Also he was using a 20 amp fuse (ATC yellow is 20A) to protect wiring that I suspect couldn't handle it. He mentions it was the factory wire I think it should have been either 10A or 15A.


Now whether the fuse was sized right or not, even a 20A probably shouldn't have allowed that much cooking without warning. It's possible there's more than meets the eye with aging of insulation, connections developing corrosion over time and that sort of thing.

But sizing wire and fuses is more than just looking up a number on a chart. You have to consider that bundled wire will have a significantly lower current holding threshold than an individual wire in free space.

I'd only make the point that if anyone ever wants advice please, please, please do not hesitate to ask me or any other EE or knowledgeable wrench or electrical tech. One thing I definitely do not want is anyone turning their Cruiser or truck into a bonfire.

I've developed a fairly complex spreadsheet over the years working to do sizing of PCB traces, conductors and busbars. It's capable in dealing with copper/brass/bronze, several aluminum and steel alloys, lead, zinc using at least a dozen different insulation types, in still and moving air over various time and lengths (for voltage drop), adjusting based on Onderdonk and Preece.

Point is it helps find a good starting point and can be as conservative as you want (or don't) and has been validated on lots of real world (and even stuff not on this world anymore) projects.
 
3rdGen4R

3rdGen4R

Hard Core 4+
Joined
Jan 3, 2019
Messages
1,531
Location
Littleton, CO
DaveInDenver said:
About that melt down. There's several things to consider.

First is that the point of overcurrent protection (e.g. fuses and breakers) is that it shouldn't matter if you use a $1 Amazon clearance or $50 artisanal USB device. If you use the right fuse and wiring it will be safe. In fact if using a Blue Sea means you have to skimp elsewhere then you're better off spending money on legitimate fuses and quality wire and going with a cheap outlet or adapter.

For example, there's always the risk of counterfeit stuff...

Counterfeit Chinese ..... FUSES - Page 1

Counterfeit Chinese ..... FUSES - Page 1
www.eevblog.com

Also he was using a 20 amp fuse (ATC yellow is 20A) to protect wiring that I suspect couldn't handle it. He mentions it was the factory wire I think it should have been either 10A or 15A.


Now whether the fuse was sized right or not, even a 20A probably shouldn't have allowed that much cooking without warning. It's possible there's more than meets the eye with aging of insulation, connections developing corrosion over time and that sort of thing.

But sizing wire and fuses is more than just looking up a number on a chart. You have to consider that bundled wire will have a significantly lower current holding threshold than an individual wire in free space.

I'd only make the point that if anyone ever wants advice please, please, please do not hesitate to ask me or any other EE or knowledgeable wrench or electrical tech. One thing I definitely do not want is anyone turning their Cruiser or truck into a bonfire.

I've developed a fairly complex spreadsheet over the years working to do sizing of PCB traces, conductors and busbars. It's capable in dealing with copper/brass/bronze, several aluminum and steel alloys, lead, zinc using at least a dozen different insulation types, in still and moving air over various time and lengths (for voltage drop), adjusting based on Onderdonk and Preece.

Point is it helps find a good starting point and can be as conservative as you want (or don't) and has been validated on lots of real world (and even stuff not on this world anymore) projects.
Click to expand...
Well Dave,

Give me all the instructions. If I get one of the cheaper ones like Tim mentioned about and replace these with my cigarette lighter, could I protect myself from this with the right install, or is the only safe way is to spend $100 dollar marine or $400 aviation grade plug (https://www.sportys.com/stratus-power-pro.html). To me it's just a matter of understanding how to protect yourself with the inline draw of power. If the device starts to suck more than it's rated for, the fuse needs to blow. If it doesn't, it means my 4runner will be a nice fire to watch. This is kind of why I posted up asking for advice because I see the dangers in this. My 4runner has 3 cigarette lighters just in the front of the center console.
 
DaveInDenver

DaveInDenver

Rising Sun Ham Guru
Joined
Jun 8, 2006
Messages
13,921
Location
Grand Junction
3rdGen4R said:
If I get one of the cheaper ones like Tim mentioned about and replace these with my cigarette lighter, could I protect myself from this with the right install, or is the only safe way is to spend $100 dollar marine or $400 aviation grade plug (https://www.sportys.com/stratus-power-pro.html).
Click to expand...
3rdGen4R said:
To me it's just a matter of understanding how to protect yourself with the inline draw of power. If the device starts to suck more than it's rated for, the fuse needs to blow. If it doesn't, it means my 4runner will be a nice fire to watch.
Click to expand...
There's two questions. Your first one is the quality and reliability of the device itself and the second is safety.

Yes, no matter what your criteria are for the adapters themselves the installation can be just as safe. The power source, distribution and protection doesn't and shouldn't know what's down stream. Only that it's either within or outside the safe zone for the wiring.

The key concept is that fuses and breakers on the battery first are there to protect the wiring, not the devices. There may also be fuses on the device and if so, that's great. They'll be sized to prevent damage to the device.

Now there's nothing that says you can't size the fuses smaller than what is safe for the wiring, in which case they may protect the device as well.

But the basic things to think about are that
  1. There's always a fuse within a few inches of the source end (e.g. what's driving the current - battery, power supply, solar panel)
  2. This fuse is sized so that the smallest gauge wire in the circuit is protected (which is dictated by the copper heating and insulation temperature rating)
  3. The smallest wire in the circuit is sufficient to carry your max current for the length of time the fuse is rated to hold or not knowing that then forever (which is IMO at least 10 minutes at max current)

What I mean is a fuse is sized based on what current it will carry and for how long. I think a lot of people don't understand that. They pick a value based on what they think is the opening point.

For example, a 20A ATC will carry 20 amps indefinitely (e.g. forever) and will only open at some current more than 20 amps. Where exactly depends. It'll open at 200% (40 amps) in about 1 second but between 100% and 200% the length of time is exponential. It could hold 150% (30 amps) for several tens of seconds or a minute even and might carry 25 amp (125%) for tens of minutes or with enough air circulating to cool it may actually never blow.

So you have to balance how fast you need it to blow against how long you need it to supply to the load's demand. That's what known as a fuse (or breaker) working area under a time-current curve. It's literal, it'll always work in the blue and will be designed to trip somewhere within the red region. So you have to know the nature of your load demand to prevent nuisance trips while still protecting the wiring sufficiently.

In the case of that melted wiring the device obviously wasn't working properly but even three USB-C 27W ports shouldn't have been a problem. That's 81 watts total, so while 16A on the 5V side it's only ~7 amps on the 12V side. So clearly there was another fault or short.

That's why I doubt it's really got to do with the adapter design specifically but poor protection choices or the age of the harness. The same thing could have happened with a native 12V device running directly on the circuit or maybe even without anything even in the plug if it was a bad crimp/solder or abraded or cracked insulation.


ATO_ATC_Fuse.jpg
 
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