BMW M5 Forum and M6 Forums banner

21 - 36 of 36 Posts

·
Registered
Joined
·
6,516 Posts
Wise words. I seem to have difficulty measuring current with my multimeter. I know it is done in series to the circuit, but when I tried on the car wiring for the cig lighter, I got sparks. :eek:h: Any idea on the setup I installed, how I can test the current available at the AC plug? This way I can settle it once and for all and know what I can plug in or not.
There is no way to test "current available". You can measure the current used by any given device while it is in use, but the only way to determine "available", or maximum, current is to plug in higher and higher demand devices until you blow the circuit.

I'm not sure how you got sparks check the car wiring unless you grounded the + lead, but in simple terms, you would need to connect the neutral (big) slot of the socket to the neutral pin of the plug with a short jumper wire and connect your dmm in series between the hot (small) slot of the socket and the hot pin of the plug. Not all DMM's can measure a/c amperage.

Measuring dc amperage is the same thing - you connect the DMM in-line with the + wire and the connection on whatever device.

What did the packaging for the converter you used say about either efficiency or input/output?

d-
 

·
Registered
Joined
·
2,936 Posts
Discussion Starter #22 (Edited)
From Amazon:


  • Convert DC 12V electricity provided by the automobile into AC power
  • Can be used with a variety of electric devices with power consumption less than 100W
  • Input voltage range: DC 11V-15V; Output voltage range: AC 220V
  • Max. Output: 100W; Continuous Output: 75W
100% Brand new 12V DC to 110V-220V AC Power Inverter + 5V USB Output - It can convert DC 12V electricity provided by the automobile into AC power. Can be used with electric devices with power consumption less than 100W. With the multiple protection circuit, it will not harm the equipment or automobile. - Special versatile socket, suitable for various kinds of plugs - Input voltage range: DC 11V-15V - Output voltage range: AC 220V - Max. Output Power: 100W - Continuous Output Power: 75W - Over voltage shutdown: DC 15V-16V - Low voltage shutdown: DC 10V-10.5V - USB output: 5V.

So a continuous power of 75W @ 120V is .625A of current, .833A MAX. But if some device draws 9V, doesn't that mean it can use up to 8.33A continuous??

And you're right about the laptop then. If a laptop input in 2.5A @ 120V it will destroy the fuse or circuit or whatever.
 

·
Registered
Joined
·
2,936 Posts
Discussion Starter #23
Key lesson: Look at electronic device before plugging in, multiply voltage and amperage. If > 100, do not plug in! :)
 

·
Registered
Joined
·
6,516 Posts
...Continuous Output Power: 75...
And there you have it: 75W continuous power output.

75W / 120V = 0.625A.

Honestly, other than a small cell phone charger, I have no idea what you would plug into this device.

So a continuous power of 75W @ 120V is .625A of current, .833A MAX. But if some device draws 9V, doesn't that mean it can use up to 8.33A continuous??
You can plug a 9V device into this connection once. After that you'll have a paper weight. You cannot plug a 9V device into a 120V socket.

And you're right about the laptop then. If a laptop input in 2.5A @ 120V it will destroy the fuse or circuit or whatever.
You'll blow at least the internal fuses and likely could damage the internal circuitry, vehicle wiring, or fuse.

d-
 

·
Registered
Joined
·
4,407 Posts
Awesome work! Looks nice and OEM

Not to take away from your ingenuity, but BMW offers a AC adapter for the flashlight charger in our car's glovebox.
 

·
Registered
Joined
·
2,936 Posts
Discussion Starter #28
You can plug a 9V device into this connection once. After that you'll have a paper weight. You cannot plug a 9V device into a 120V socket.
Even if the 9V device has its own conversion or protection or whatever to convert 120V to 9V?? What types of thing can be used in an AC power outlet up to 75W??
 

·
Registered
Joined
·
3,175 Posts
For those of you who do this, please keep in mind that the AC plug in question probably can't provide more than about 1A @ 120V, and even that might be pushing it depending on the size of the wire / fuse on the cigarette lighter. I'd bet it is no larger than 10A @ 12V, which after conversion losses will be <1A @ 120V.

In other words: Don't get any funny ideas about plugging a portable fridge or anything like that into the outlet!

d-
This must surely depend on the source of the power for the inverter, not the wire to the lighter socket. If the inverter was powered by the fuse/lighter socket wire then this would be true, the inverter would be drawing a high current at 12V to provide a lower current at 120V AC.

However, if the inverter was powered by, say 0 AWG wire rated at 150A, then, assuming a large enough inverter, you could supply 10A @ 120V through the lighter socket wires. The number of Amps a wire can handle is not dependent on voltage (very rough approximation), the higher voltage allows a given wire to handle more power (Watts).
 

·
Registered
Joined
·
6,516 Posts
Even if the 9V device has its own conversion or protection or whatever to convert 120V to 9V?? What types of thing can be used in an AC power outlet up to 75W??
The key point is that you have 75W to play with. IF, and that is a big IF, you could find a 100% efficient 120V->9V transformer, THEN you could have your 8.333333A. However, especially in smaller/cheaper devices, the AC->DC rectification is performed by using only 1/2 of the AC wave (more expensive ones use additional circuitry to take advantage of the other half) in combination with a smoothing capacitor to provide a constant DC output. This gives an efficiency that is much, much lower - more like 25-30%.

75W? A cell phone charger?

d-
 

·
Registered
Joined
·
6,516 Posts
This must surely depend on the source of the power for the inverter, not the wire to the lighter socket. If the inverter was powered by the fuse/lighter socket wire then this would be true, the inverter would be drawing a high current at 12V to provide a lower current at 120V AC.
I'm not sure what distinction you are drawing here? He used the cigarette lighter wire to power the inverter, so those two are one and the same.

However, if the inverter was powered by, say 0 AWG wire rated at 150A, then, assuming a large enough inverter, you could supply 10A @ 120V through the lighter socket wires. The number of Amps a wire can handle is not dependent on voltage (very rough approximation), the higher voltage allows a given wire to handle more power (Watts).
If you are saying that he could have, for example, mounted the inverter somewhere closer to the fuse block and then provided 10A @ 120V from that remote location to the cigarette lighter location using the existing wiring, then yes, you are correct. I'd still limit it to 7.5A (the existing fuse on the circuit) in the absence of gauging the wire myself, but it could be done.

As usual, there are many hypothetical variations on the OP's mod. As performed, however, the mod is not capable of providing more than 75W continuous as limited by both the gauge of the cigarette lighter outlet and the inverter that he used.

d-
 

·
Registered
Joined
·
2,936 Posts
Discussion Starter #34

·
Registered
Joined
·
2,325 Posts
All I have ever seen is the DC plug adapter for the flashlight slot in the glove box. As shown below:

Correct, I was contemplating on using this for my V1 but unfortunately with the wire coming out of the globebox, it won't close correctly.

I really want to do the pushbutton mod though...this might be a solution.

Good work Drew :cheers:
 
21 - 36 of 36 Posts
Top