Using 120V AC plug/receptacle in 12V car circuit

Reply

  #1  
Old 12-05-10, 01:41 PM
Member
Thread Starter
Join Date: Dec 2010
Location: Canada
Posts: 5
Question Using 120V AC plug/receptacle in 12V car circuit

I am looking to make my car amplifier and sub-woofer removable, and have already fashioned a way to fasten it down and detach it from the trunk, but am curious about how to do this with the power.

I have a 120V (15A) male plug and female receptacle (the type used on common extension cords) and was wondering if it would be suitable for use in the car, to easily detach the +/-/REMOTE wiring.

Am I correct in assuming that 120V * 15A =~ 5 * (12V * 30A), or in other words in a 12V circuit the plug/receptacle should be able to handle 5x the load I will be pushing through it with the amplifier? The amplifier is fused at 30A/12V.

If this isn't a good idea, can anybody recommend a better idea for a method to easily disconnect the power as needed? There are some times I will want to remove the speakers+amplifier to use the space for storage, but most of the time it will be in the trunk.

Thanks!

kanuck
 
Sponsored Links
  #2  
Old 12-05-10, 02:08 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,631
Works fine until someone plugs that male plug into a 120v receptacle. There are plugs that will work and can't accidentally be pluged into a 120v receptacle. Cigarette lighter jack and plug comes to mind or maybe a molex connector. I'm sure they make special connectors to do what you want. One of the pros should be along with specific recommendations.
 
  #3  
Old 12-05-10, 02:56 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,968
Connection to connection is fine, but as Ray said, it is out of league. You don't want someone plugging it in to a 120v source. Someone will get hurt. Stay with 12v connections readily available at Radio Shack. AC receptacles and switches shouldn't be mixed with 12v DC stuff.
 
  #4  
Old 12-05-10, 03:27 PM
Member
Join Date: Apr 2007
Location: Near Buffalo, NY
Posts: 4,239
If the audio amplifier is rated at 30 amps, the wiring also needs to be rated at 30 amps. Voltage doesn't come into play in that part of the equation.
 
  #5  
Old 12-05-10, 04:24 PM
Member
Thread Starter
Join Date: Dec 2010
Location: Canada
Posts: 5
My only concern here is that my understanding of the math is incorrect, and that I can't compare raw wattage (ie: 12V * 30A = 360W vs. 120V * 15A = 1800W). If this is indeed correct, and the plug won't melt when carrying the 360W, it will suit my purpose.

The wiring itself is proper gauge. I'm just putting a plug on the end that has 3 prongs (for +/-/REMOTE). A cigarette lighter plug and jack would not be suitable for all 3 connections. I'd have to do something like use 2 cig lighters or 2 types of plug, and they would be a lot weaker than the 120V jack in terms of durability and not melting.

There's not really any chance that it would be plugged in elsewhere, and I could put a warning label on the end to be safe(r) about that. I was thinking of perhaps using a euro or different type of jack, but the chances of anybody else tinkering with it are next to none.

Rick: These ends are rated for 15A at 120V. Are you saying that regardless of voltage, 15A is the limit the metal in the plug will withstand - wattage is not the deciding factor? I am not talking about the wire at all here, as I'll be using 12 guage wire from a kit (barely fits into the plug but all strands do go in and will fasten).

The amplifier itself won't be drawing the full 380W, the actual load is between 270W and 300W. It's very probable that the only time this setup will be unplugged is in a case of emergency when I need to get at the spare tire beneath the speaker box.

Thanks for all of your input!

kanuck
 
  #6  
Old 12-05-10, 05:23 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,968
Originally Posted by kanuck View Post
There's not really any chance that it would be plugged in elsewhere, and I could put a warning label on the end to be safe(r) about that. I was thinking of perhaps using a euro or different type of jack, but the chances of anybody else tinkering with it are next to none.

kanuck
You want to bet someone's life on it? We have a responsibility on the forum to warn against unsafe practices. Not that it wouldn't work, as it will. There are just better ways to do it.
 
  #7  
Old 12-05-10, 05:38 PM
Member
Thread Starter
Join Date: Dec 2010
Location: Canada
Posts: 5
To be on the safe side I'll track down a twist-lock plug (the type used in marquee lights etc).

I'm pretty sure you can use the argument that *somebody might* plug it into the wrong type of source regardless of plug, ie: if somehow the equipment ended up halfway across the world or in the hands of somebody stupid enough to attempt such a thing - but I'm generally one to be on the safe side which is why I posted this question here to begin with.

Thanks again for the input, it's much appreciated.

kanuck
 
  #8  
Old 12-05-10, 05:48 PM
Member
Join Date: Jul 2009
Location: SE Iowa
Posts: 71
15A is 15A, regardless of voltage.

Perhaps something like this would work: 4 AWG Breakaway Power Connector 175A | Parts-Express.com or 8 AWG Breakaway Power Connector 50A | Parts-Express.com ?

For the remote, just use insulated male and female spade terminals.
 
  #9  
Old 12-05-10, 06:13 PM
Member
Thread Starter
Join Date: Dec 2010
Location: Canada
Posts: 5
Thank you very much for that link! This is a solution I feel a lot more comfortable with as there's no guesswork as to whether or not it will melt and I won't have to drive to the city (about an hour trip one-way) to pick it up. I wasn't having much luck finding an online store that carried these things, though it might have helped if I knew what it was called to begin with.

I think that this is what I'll be going with. The only thing that really stands out to as being peculiar, is that when 2 of these are used the +/- indicators in the connector will be reversed (on either end of the plug) so it will appear as though the + is connected to the - and vice-versa. Wire color will differentiate them and it is not possible to plug it in backwards, but it does seem odd that they don't come in a set of 2 to match those indicators together.

I still will have the issue that somebody, someday, *might* plug it in where it doesn't belong (ie: 24V source on a dump truck or semi-rig) but that's better than 120V I'd suppose.

Thanks again, you're suggestion is the best I've heard so far.

kanuck
 
  #10  
Old 12-06-10, 12:37 AM
Member
Join Date: Apr 2007
Location: Near Buffalo, NY
Posts: 4,239
Wattage isn't the deciding factor as your formula proves: 360 watts at 12 volts = 30 amps, while 1800 watts at 120 volts = 15. In your case, the fuse rating is what you have to build for. To that end, you may also want to change out the 12 awg wire. It's not rated for 30 amps.
 
  #11  
Old 12-06-10, 03:42 AM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,968
At least if some numbnut plugged into a dump truck the most voltage they would expose themselves to is 12 or 24 volts. They are just battery charging ends and you can't reverse them unless you really try hard. They do come in a matched pair and once wired to the source and load properly, they can't be plugged in incorrectly.
 
  #12  
Old 12-06-10, 10:46 AM
Member
Join Date: Mar 2006
Location: NE Wis / Paris France{ In France for now }
Posts: 4,808
The Anderson plug as one of the member posted they not only come in few diffrent size but they will have diffrent voltage rating on them as well so like example the grey one is for 12 volt while 24 volts will use the red one while 36 volts will use bleu one.

I will paste the link of real deal company they have few listing on that

Multipole Connectors

That have pretty good slection there.

Merci,
Marc
 
  #13  
Old 12-06-10, 11:53 AM
Member
Thread Starter
Join Date: Dec 2010
Location: Canada
Posts: 5
Thumbs up

I did some searching on my own and found a company (Helmar Parts / helmarparts.ca) that has a Canadian presence.. that way I won't need to spend $20 for the part + $30 for shipping + $Xpercent duty to get it shipped to me. They have a warehouse in the same province as I am in and have a large assortment of the 2-pole connectors. I did go with the gray one.

I'll also be picking up the 1000W kit just to be safe; that should also allow me to upgrade or add a capacitor if need be - this was required in my other vehicle with a 500W sub due to headlight dimming, though it was placed/fastened in a spot that doesn't block access to anything so the quick-disconnect wasn't implemented there; the wire runs straight from the battery to the capacitor and then a shorter wire connects to the amplifier from the capacitor. All terminals on the wires were also soldered after being crimped, as will be done on this next setup.

The average RMS wattage of this amplifier is 200Wx2 @ 4ohm (it is 2ohm - 8ohm capable, bridged RMS is 500W @ 4ohm), but the speakers actually clock in at 6.x ohms when tested with a multimeter so the actual total RMS load will likely be less than 400W. I won't be bridging them as the speakers can't handle that. My guesstimate was that it would be ~275W RMS at 100% capacity, and from past experience I've learned to run the amplifier at <= 75% capacity (by lowering the gain and not cranking it up at the head unit) as not to overheat it, so the actual draw when in use should be even lower than my estimate.

I was going to go with 20A fuses on both ends of the main lead as well as in the amplifier. It came with 30A fuses, but as mentioned I'd rather run it at a lesser capacity because I tend to use it for long periods of time (3-4 hour trips) and it will almost definitely get a little warm to the touch, as well as would have been using 12guage wire (the kit was $40 cheaper this way). My reasoning here is that fuses are cheaper than a new amplifier, and although the documentation states it can handle 30A continuously I'm skeptical as I've had amplifiers that were run for a long period of time die on me before. I'd much rather blow a fuse than blow an amplifier again. This method has saved my other system a couple times already, as the fuse blew much earlier than it would have if it were 40A and I have not had any issues with overheating ever since I tuned it to the lower consumption. It did take a few fuses initially to get the levels set correctly, but it has been performing as I'd expect and never given me any issues aside from a blown fuse.

Maximum performance isn't the ideal outcome, only a little extra bass. It's not intended that it be a block-shaking system. This system is going into my secondary vehicle which has other work to be done before it can handle being shaken too much.

Your comments have helped me to refine and re-think the setup, as well as do things better than I would have had I not come here and asked. Thanks again!

kanuck
 
  #14  
Old 12-06-10, 01:31 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,968
Gosh, makes living in the country taste so good!!! I was in Atlanta the other day, and man, how do they stand the noise? One was from a motorcycle. I could hear him for 2 blocks.....booom, ditty, boom.
 
  #15  
Old 12-06-10, 06:51 PM
Member
Join Date: Mar 2006
Location: NE Wis / Paris France{ In France for now }
Posts: 4,808
Originally Posted by chandler View Post
Gosh, makes living in the country taste so good!!! I was in Atlanta the other day, and man, how do they stand the noise? One was from a motorcycle. I could hear him for 2 blocks.....booom, ditty, boom.
Just try my Parisan traffic that will drive you nuts { anything goes there }

Merci,
Marc
 
  #16  
Old 12-07-10, 05:03 AM
Member
Join Date: Apr 2007
Location: Near Buffalo, NY
Posts: 4,239
Coupla things, Kanuck:

Measuring a speaker with a DMM only tells you the resistance of the coil, not the impedance load that will be presented to the amplifier. For that you need an impedance meter, but it's cheaper to trust the manufacturer's specs. Typically, a voice coil that measures 6 ohms of resistance will be an 8-ohm speaker. The amp is rated at 200 watts per channel at 4 ohms, so you can figure about half that per channel into 8 ohms.

Next, output power of the amp is not the same as the wattage the amp will draw from the power wiring. That's where (again) the fuse rating comes into play. Down-rating the fuse is a good idea, but it's not necessary to fuse the negative side of the power wiring.

Lastly, think of RMS power as an average. Music also has "peak" requirements such as the heavy pulse of a kick drum or the snap of a snare drum. The peak power necessary to reproduce those transients is where the system will draw large amounts of power for very short ticks of time. If you don't provide adequate wiring, the amp can "clip" or distort. Distortion is much more dangerous to a speaker than too much clean power. I've seen 30-watt amps blow up 100-watt speakers because the amp was clipping.
 
  #17  
Old 12-08-10, 02:43 AM
Member
Join Date: Mar 2007
Location: Norway
Posts: 251
As staded over. 30A is 30A! Wiring at low voltage needs an extra attention to voltage drop. An amplifier is extremely sensitive to voltage drop, and the cables and grounding should be well sized.
You may get nice outlets made for fork trucks etc.
Undersized wiring may compensated by use of large capacitors near the amplifier.
Voltage drop may be the most common cause to bad sound, and ruined loudspeaker-elements.

dsk
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'