AmorgetRS

iTrader: (6)

I have a slight problem with my power invertor. It is hard wired into the car at this point w/ 8 gauge wire (its just what I happened to have enough of). I hooked it up so to turn it off I was killing the ground w/ a relay because I didn't want to hook a relay into that 8 gauge wire. It worked fine for my cell phone charger and stuff, but then I hook my MP3 player into it... now it won't shut off!! When I turn off the relay the invertor "whines" telling me there is a fault, but then it keeps power the computer! How should I hook this darn thing up? Is the computer just enough mass to ground the power invertor or something?
Thanks a ton,
Douglas

Stuart Moss

Without knowing exactly how everything is wired, I can only speculate on what the problem could be.

I know you stated that you didn't want to splice into the 8 gauge wire positive wire, but wouldn't you have to also splice into an equally large ground wire to put in the relay? Both (positive and ground) should be the same gauge.

Without knowing how your MP3 player is connected, and what else it is connected to, I will take a guess that when you remove the ground to the inverter (via the relay), ground is somehow getting back into the inverter circuitry, perhaps through the MP3 itself or something connected to it (e.g. an amplifier, the ground of an RCA cable, a speaker wire that shares chassis ground, the chassis of the MP3 and/or amplifier which is also touching ground (metal) of the vehicle, etc.).

The whining could be because the inverter output is probably isolated from the input (most [all?] inverters use a DC-to-DC inverter which isolates the input from the output by a transformer), so if you remove the power input ground, but supply ground to the output may cause the whining.

This is an important reason why it is better to switch the positive, not ground, power wire. There often many points where a device will have ground available to it (not just the power ground wire), but there is generally just one point where positive power is available (the positive power input wire).

AmorgetRS

iTrader: (6)

Thanks.
The ground is not 8 gauge... its like 14. That is the wire the thing comes stock with and basically all I did was cut it and run it to the relay.

So, should I like buy 10 gauge wire or something to make it a little easier to splice... I don't even know how to put a connector on the relay that would work w/ 8 gauge..
Thanks,
Douglas

Stuart Moss

First, let me correct myself. In my first post, I said that it used a DC-to-DC inverter. That is not correct (I was thinking of a power supply like those used in mobile audio amplifiers). An inverter uses an ocsillator that switches the DC input to a step-up transformer to convert it to 120VAC. In any event, both types (a dc-to-dc power supply and a DC-to-AC inverter) have isolated input and outputs.

I am not sure I understand your problem. Do you have a problem only with the MP3 connected? If so, try plugging something else in. Does the problem remain?

It wouldn't help much to splice in a heaver ground wire (from 14 gauge) unless you connected it directly to the circuit receiving the input ground. (Splicing a heaver gauge wire into the 14 gauge wire will still have the smaller wire from the point you spliced into it and the input circuitry. To do it correctly, you'd have to replace the entire length of wire). If it came that way from the factory, then I would imagine that the wire size is okay, but wonder (doubt) why the input positive is a heaver gauge.

I'm either not understanding your connection/problem or something is wrong. Both power input wires should be the same size (gauge) unless someone fiddled with it after leaving the factory. At a minimum, I would suggest that you switch the positive and not the ground to turn it on/off.

[This message has been edited by Stuart Moss (edited May 15, 2001).]

AmorgetRS

iTrader: (6)

Ooopsy, looks like I wasn't clear enough either.
I was saying splice 10 guage wire in for the positive power instead of the 8 gauge wire just to make it easier to hook it up to the relay.

Yes, it is only when the MP3 player is on, for my cell it doesn't happen or when nothing is plugged it turns off nicely...
Thanks,
Douglas

Stuart Moss

I am curious why you are using an inverter. Do I understand you correctly that you are taking 12VDC, converting it to 120VAC (through your inverter), and then converting it back down and rectified to ~10VDC? If so, I can't understand why you'd want to go that route. It is more expensive, less efficient (you'll use a lot more energy [amperes], much of it wasted in the form of heat for the conversion), more complicated (more things to go wrong) and bulkier than if you simply converted vehicle power (12-15VDC) down to what your cell phone and MP3 player use.

Of course, if your cell phone and MP3 player use only 120VAC, then I could understand the need. But if you're simply using these devices through their respective wall transformers, I would use a simple 3-terminal voltage regulator which would cost <$5 to convert the 12-15VDC to whatever the devices use (I presume less than 12VDC).

There are many car adapters to use a cell phone off the DC power in a car, and if your MP3 uses batteries, the same should apply with it too. I think Radio Shack and/or J.C. Whitney carry voltage converters for just such applications if you don't want to make the simple circuit yourself.

AmorgetRS

iTrader: (6)

See the computer you are typing these message board messages on? My MP3 player have everything it does, minus the monitor. I would use a 12v power supply, but the total cost of the mp3 player has been like 40 bucks (mostly the LCD screen) so I don't feel like buying a 12volt power supply for 100 bucks (seems the price I see on them normally)

The cell phone is just the car adapter died so I used the normal plug in my car w/ the power invertor... just me being lazy.

That help?
Thanks,
Douglas