12 volt positive ground need help with 7905 circuit

[1Dangerous1t00]

Hi all - I am having problems setting up a circuit for a 7905 negative voltage regulator. Needing to boost the current to a max of 10 amps with a NTE180 or 181 transistor. I want to use this on a 12 volt positive ground vehicle. I need to get negative 5 volts and up to a max of 10 amps for the ignition coil and fuel gauge. Not quite sure how to set up the transistor with this regulator to make it work. Any help would be greatly appreciated.
I have large heat sink and fan for the transistor and regulator to keep it cool.

 

[Gryd3]

What are you trying to build?
You may be able to simply use the chassis of the vehicle as the positive supply rail and build the circuit like normal... The catch here would be if you plan to interface your circuit to another in the vehicle, you need to make sure the signalling and connections will be the same polarity.
Of course, the 'ground' of your circuit must remain floating in this case.

 

[1Dangerous1t00]

Thanks for the reply. Heres what I am working with - a 1936 Chrysler Airstream that was originally a 6 volt vehicle positive ground. A 12 volt alternator has been recently installed and converted to a positive ground. The fuel gauge and sender and ignition coil, which ties to the body ground, needs to be 6 volts. The amperage draw from the ignition coil and fuel gauge can at times be up to 10 amps draw. I know that the LM7905 can only handle 1-1/2 amps max. Wiring in that part is no problem. Problem I am having is interfacing the transistor into the regulator circuit for high current. The transistors I have are NTE180 (PNP) & NTE181 (NPN). Not sure which of the two to use with it or interface it in to the regulator circuit. I was thinking of possibly using it as a bypass transistor, but not sure how to wire it in. The collector current can handle 30 amps, base current 7.5 amps and the emitter-base voltage is 4 volts. Vcer and Vcb are rated at 100 volts. Vceo is rated at 90 volts. Any ideas?

 

[Gryd3]

Thanks for the reply. Heres what I am working with - a 1936 Chrysler Airstream that was originally a 6 volt vehicle positive ground. A 12 volt alternator has been recently installed and converted to a positive ground. The fuel gauge and sender and ignition coil, which ties to the body ground, needs to be 6 volts. The amperage draw from the ignition coil and fuel gauge can at times be up to 10 amps draw. I know that the LM7905 can only handle 1-1/2 amps max. Wiring in that part is no problem. Problem I am having is interfacing the transistor into the regulator circuit for high current. The transistors I have are NTE180 (PNP) & NTE181 (NPN). Not sure which of the two to use with it or interface it in to the regulator circuit. I was thinking of possibly using it as a bypass transistor, but not sure how to wire it in. The collector current can handle 30 amps, base current 7.5 amps and the emitter-base voltage is 4 volts. Vcer and Vcb are rated at 100 volts. Vceo is rated at 90 volts. Any ideas?

Eep. That sounds a little involved. To be honest, I think you should probably get some help from some of the more experienced members on here.
@hevans1944 , and @KrisBlueNZ , Do either of you have any suggestions?

I don't want to assume, guess, or base any suggestions off of what I 'think' is a good solution on this one.

 

[hevans1944]

This is a tough problem to solve with a linear voltage regulator because of the large (10 A) current required combined with the need to drop about 14 - 6 = 8 V across an external pass transistor to boost the current capability of the LM7905. The transistor will dissipate about 80 W under these conditions, and its maximum power dissipation with a perfect heat sink is only 100 W. There are plenty of one-transistor "bypass boost" circuit examples, but they all share the common problem of what do you do with all that heat?

It gets pretty hot under the hood of your 1936 Chrysler Airstream... you wouldn't want the ignition to fail while out for a cruise! Why not upgrade the ignition system to a nice 12 V capacitive discharge system? You could probably run the fuel gauge with a suitable current limiting resistor from -12 V too. But while we're at it... since you ditched the original 6 V system (which presumably had a generator, not an alternator) why not ditch the positive ground too? About the only thing you would then need to replace from the factory-original configuration would be the radio, unless you want to provide a +12 V DC to -6 V DC converter to power just the radio...

If you want to continue down the path you are on, I would seriously consider using a buck switching power supply to drop from a nominal -12 V (maximum -14 V) positive-ground power supply input to a -6 V positive-ground power supply output.

I would imagine @KrisBlueNZ has more experience with this than I currently do. Finding an off-the-shelf isolated DC-to-DC buck converter would probably also do the trick. Something with 12 to 15 V DC input and 6 V DC output at 10 to 15 A sounds about right, but nothing comes to mind yet. Try Google with those search parameters.

If you can find an isolated positive input/output converter, you can power it up by connecting its positive "input" to your automobile ground and its common to your -12 VDC battery supply. The -6V DC output would then be taken from the converter "common" terminal with its "output" terminal connected to the automobile ground. Again, this only works if the converter common can be operated in isolation from the automobile ground, and the converter input is isolated from the converter output. Might have to wrap the whole thing in plastic tape to prevent "accidents" from happening... and it will confuse the hell out of anyone trying to troubleshoot it later unless you document what you have done and why.

Edit: Re-reading what I wrote, I hereby withdraw any recommendation for plastic-wrapped DC-to-DC converters, isolated or not. Too many things to go wrong. You need a buck converter that accepts -12 V DC input and provides -6 V DC output. Period. Good luck with that... maybe Kris can design one for you. I should probably go back to messin' 'round with amateur radio and vacuum toobs.