6V pos ground to 12V neg ground power inverter question

[Jeff Balazs]

I have a question on how the unit I bought actually works.

It only has 3 leads......one red lead is 6V neg from ignition switch.......another red lead is 12V positive to stereo.......and one black lead goes to ground (positive in this case) which the manufacturer claims is a common ground ??? The diagram also shows the ground lead from the stereo (negative in this case) going to a common ground. It sure looks like a short circuit to me.

 

I don't see how this could be. I called the local vendor I bought it from and asked for an explanation of how this works. He said he couldn't begin to explain it.......but it works fine and it is not even necessary to isolate the radio or antenna. He said he has put dozens of these in and never had an issue. But could not explain how it does this bit of wizardry.

 

Anybody have an explanation of how this actually works?

 

Thanks in advance. Jeff

[Dennis_MN]

Yep, the unit actually puts out 18volts, so the diff between the truck ground and the out put is 12volts.

Dennis

[Jeff Balazs]

Ok......perhaps.... but what about the difference in polarity? How can a 6V positive ground and a 12V negative ground be common in the chassis?

 

Jeff

[TodFitch]

Putting out 18v would be one way to do it.

 

I'm not sure but I think they work by turning the 6v DC to AC (kind of like the old vibrators) then run it through a transformer to step up the voltage and then rectifying and regulating it down to the desired voltage. With a high frequency on the AC you could have a very small transformer. The way a transformer works you could easily have a positive ground on the input connected to a negative ground on the output as they are isolated by the AC section of the circuit. With modern electronics there are probably better (lower cost, less power loss) ways of doing it but I think that concept handles it.

 

I've got one on order to replace the 6v to 5v regulator I was using to power my cell phone via a USB plug. I was relying on the fact that the cell phone has a plastic case so I could cheat on the grounds. However the shield on the USB connector can (and did) come in contact with the metal dash when the cellphone is unplugged.

 

I doubt that I'll tear it apart to see what is in it when I get it. Just going to hide it under the dash and mount a 12v accessory plug on the bottom of the dash. With that I can pop in one of those cheap USB power plugs. Seems a bit of a waste to go from 6v to 12v to get to 5v but at least I shouldn't have to worry about the grounds touching anymore.

[Jeff Balazs]

Can't say I understand how this gadget works........but work it does! Still a bit baffled about how the common ground functions. On one hand I have a 6 volt positive lead from the battery cable to chassis......and on the other a 12 negative lead from the stereo hooked up to the chassis as well. I must be missing something??? But what the heck it works fine. And it sure is nice having tunes. Yeah!!!!.

 

All the extra effort I put into insulating the cab paid off very nicely........it sounds truly amazing. May just be an old truck on the outside.....but it is almost a bleeding concert hall on the inside . I couldn't be happier and I spent less than 200$ on a stereo and 4-3 way speakers. With my trusty Ipod hooked up I am good to go. No commercials.....no crap....just music I really enjoy. Gotta love that!

 

Jeff

[TodFitch]

The Custom Autosound PGPI-HC unit I ordered arrived today a day or two earlier than I'd expected.

 

They write "check the output voltage-it should be about 12.6v with no load". But the output seems to be 11.88v with no load regardless if my engine is on or off. That might be a bit low for some devices that have a low voltage shutoff but does not affect what I want it for.

 

The blurb says that there is no transformer in it which allows the quiescent current draw to be about 10 milliamps (I did not measure so I don't know if that is accurate or not). I hooked mine up to work off the ignition switch as the car sometimes goes for long periods of time without being started and I would like to reduce the odds of draining the battery during those periods.

 

Anyway, I think I'm good to go now and if I accidentally touch the ground shield of the USB cable to something in the car nothing bad will happen.

[Jeff Balazs]

Tod;

That is the same unit as I put in. I think the output voltage is directly affected by the voltage going in. Anything less than 7.2V is going to produce less than 12.6 V according to what I have read. I have all new upgraded wiring, battery and a 50A alternator so I should be OK. Not sure how it would do if used on an original electrical system? Might be ok if the system is completely up to spec.......but I would guess it might suffer if the electrical system has any weakness.

 

Jeff

[TodFitch]

Tod;

That is the same unit as I put in. I think the output voltage is directly affected by the voltage going in. Anything less than 7.2V is going to produce less than 12.6 V according to what I have read. I have all new upgraded wiring, battery and a 50A alternator so I should be OK. Not sure how it would do if used on an original electrical system? Might be ok if the system is completely up to spec.......but I would guess it might suffer if the electrical system has any weakness.

 

Jeff

 

My reading of their instructions and looking at the chart on the the back in indicates that the no load output should be 12.6v as long as the input is between just over 4v and 16v. The amount of power/current you can get out before the voltage sags depends on input voltage with about 1.4 amps at 4v and nearly 5 amps at 15v.

 

My battery and generator are good. 6.3v reading with engine off and when the engine is started I go up to 6.7v, both measured where the wire to the inverter is connected to my ignition switch.

 

All my readings done on the same volt meter that is showing 13.66v on a 12v battery I have connected to a trickle charger.

 

Anyway, the output voltage is enough that the USB power adaptor is happy. So my smartphone with its navigation software is happy to run all day and keep the cellphone battery charged. So it is working for me.

 

Just wanted to let some others know that at least one of their units was putting out a bit below 12v so if they had a voltage critical application they'd know to check.

[Jeff Balazs]

Tod;

I could be wrong. Maybe it is the amperage output that is affected by the voltage from the electrical system? I know I saw something about this.

Hey though as long as it works I am happy with it.

 

Jeff

[TodFitch]

Tod;

I could be wrong. Maybe it is the amperage output that is affected by the voltage from the electrical system? I know I saw something about this.

Hey though as long as it works I am happy with it.

 

Jeff

Yes. And probably cheaper than putting a 12v battery with a limited service life in the trunk with a Harbor Freight solar panel on the tray below the rear window.

 

Especially if, like me, you have no trunk and no tray below the rear window.

[Jeff Balazs]

Tod;

Yes exactly. I am glad we have this option. I thought about going that other route.....but I honestly I would rather have it this way.

I suppose if I had my project to do all over again the switch over to 12 volt negative ground would be something I probably would do. It would be less expensive than sticking with 6V and and make several of the upgrades I did much easier. Oh well.......

 

Jeff

[48Dodger]

Hey Jeff,

 

   Didn't have time to read everything in the thread, but if my understanding is correct.....forget the term "ground" for a moment, and just think positve to negative. The inverter is going to steal the 6v thats coming in and make it 12v and and send it to the stereo. Think of a Y adapter (3 leads).....both those legs are going to go back to neg. One of those legs is going to the stereo as 12v.....the neg wires all physically go back to the neg side of the battery. The invertor is a square that holds the guts that uses diodes, transitors and capacitors to change one of those legs to 12v.....the neg wire from the stereo will need to catch a ride with another neg wire back to the battery.

 

48D

 

schematic for homemade invertor    

Edited June 29, 2014 by 48dodger

[Jeff Balazs]

Tim;

I get the part of stepping up the voltage from 6V to 12V. What I am having trouble understanding is how my positive ground chassis "accepts" a negative ground lead from the stereo and how they peacefully coexist. The unit I bought is made specifically for this purpose but I expected it to have a separate ground lead for the stereo instead of just a single common ground that goes back to the chassis. It does work just fine but I will be damned if I understand how.

 

It could be my own "wiring" that is preventing me from grasping this. You may appreciate this. When I was 11 I was thrown from a horse and suffered a fractured skull and severe concussion. Unfamiliar horse....no saddle and of course no helmet.....meets angry dog. Aspiring young cowboy takes a bad fall. I never lost consciousness so instead of being kept in the hospital I was Xrayed and sent home. Turns out they missed the fracture the first time. My folks rushed me back in and this time the Doctors tell them it isn't possible to have this severe damage and not pass out....and if I was that badly injured I would not still be alive. This time they xray again and I imagine they have an "Oh S%$#!" moment. And I was sent home again where I spent about a month in bed while the swelling went down and my brain rewired itself. No therapy in those days......just a few EEG's. No wonder I have a few crossed wires huh?

 

Jeff

They say what ever doesn't kill you makes you tougher.......but sometimes I have trouble getting things through my thick skull.

[Don Coatney]

When I was 8 I was tossed from a pony up against a barn, The pony did not like me. After tossing me he backed up and kicked me square in the head as I was pinned against the barn. I still carry that scar today.

 

Think of it this way. Any DC system has a positive and negative connection. The circuit is not complete until the positive side returns to the negative side. This circuit can take many paths and there may be other DC or AC circuits taking the same path. But all other circuits are ignored as they have no meaning. The only thing that has meaning to the circuit is the original source.

 

If you are welding on a vehicle and connect the ground lead to the frame. The circuit is only complete when the arc is struck thus completing the circuit either AC or DC. The battery potential is still in the frame at the same time the welding potential is still in the frame but the two potential signals only react to the source. Hope this makes sense.  

[Jeff Balazs]

Don;

Thanks it sorta does. Circuit specific.....check!

 

Glad to know I am not the only one here who has had a serious run in with a horse.  My accident which was actually caused by a stray dog ....squashed all hope of me becoming a root'n - toot'n cowboy. Never could come to grips with the "lost of trust issues" I had with large animals after that.Too bad too as I would have enjoyed riding the range......or at least what was left of it.

All that is left of the old dream is that I finally have the right truck for the job. If you watch the movie "the Rounders" you will see that the two aging bronc busters drive a '52 Dodge 3/4 ton. I have the truck and I am aging....but I will leave the horse part to someone else.

 

Jeff

[Don Coatney]

I continued to horse around for a few more years. When I left California in 2000 I sold this mare to the folks who give wagon rides in Yosemite. This horse could jump a 5 foot tall fence and did so many times but she was a gentle giant.

 

[Jeff Balazs]

Don;

Good thing she was a gentle giant.....'cause if that thing had kicked you in the head you would have known it!

Or perhaps...... not.

 

Jeff

[Merle Coggins]

Like Don said it... I like to think of the chassis as a big pool for the electrons to return to their original source. They can flow however they wish and they will find their original source to return to.

 

Merle