12V DC Voltage regulator question

[Big Cory]

I am attempting to build an AC inverter that can be plugged in to a cigarette lighter in my car for a school project. I am using a simple circuit I found online. I want to add a 12V regulator so the circuit has a constant 12V instead of the 12.6V that I have measured from my battery. Can anyone suggest a good regulator and show a schematic of how i need to hook it up?

 

[skulenov]

the problem is that the input voltage of 78XX regulators needs to be few volts above regulated output voltage, so you would need some ~14v at the input of regulator...
hmm, maybe you could try searching on the google for some discrete version and suit it for your needs...

 

[Big Cory]

Thank you skulenov, now I know why I couldn't get the regulator to work in the simulator. I guess I will have to try and build my own simple regulator circuit out of Zener diodes or something. Time to dig out the old books...unless, of course, one of you knows how to do this.

 

[audioguru]

A "low dropout" regulator works perfectly if the input is 12.6V and the output is 12.0V. That is what they are made for.

A voltage regulator has a max allowed current spec. But you forgot to say what is your max current requirement from the regulator.

 

[Big Cory]

Thanks audioguru. I would like the inverter to produce 300 watts, so I need 25 amps. Do you know the part number for a "low dropout" regulator rated for 25 amps and how to hook it up? I am thinking it should be simple voltage in, voltage out, ground, but i have seen some circuits that use capacitors, etc. Also, I don't think the circuit I am building will handle 300 watts, but this is a school project and all i need to show is 120 volts at 60Hz. I have seen a better, more complicated design that will handle 500 watts that i would like to build in the future, but I am a newbie, and this project is just something to get my feet wet, so to speak.

 

[skulenov]

well, 25 amps is really lots of current from my point of view! 300W also!
I wouldn't recommend building such high wattage/current circuit for someone who calls himself newbie
maybe it's safer to build a low power version just to prove concept and see potential problems that may arise and then go for a higher power version...?
go to Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view and search for low dropout regulators, but i think you won't find such high current one... biggest i saw was rated 3A...

 

[bokbaard]

Also, 25 Amps over the cancer-stick igniter(lighter?) is a bit heavy, it's not really meant for that

 

[audioguru]

Most simple cheap inverters do not regulate the voltage.
More complicated inverters regulate the output voltage, not the input voltage.

 

[Big Cory]

Thanks everyone for the help. I have seen commercially available inverters that are capable of producing over 500 watts of power through the cigarette lighter plug. I am attempting to build a lower wattage version as a proof of concept, but i would like to regulate the voltage of the circuit at 12 volts to regulate the output frequency. perhaps i should use the 555 timer version instead (i am trying the dueling capacitor/transistor timing circuit) and regulate the output like audioguru is suggesting. Thanks again.

 

[audioguru]

A half-decent oscillator doesn't change its frequency when its supply voltage changes. Look at the datasheet for a CD4047 oscillator that has two opposed outputs. The frequency change is less than 1% when the supply is changed from 6V to 15V. It doesn't need a regulated supply but it could use a supply that clamps supply spikes to 15V or less.

I hope you are not thinking about making this piece of garbage that blows up its backwards capacitors and has hardly any output power:

 

[Big Cory]

Actually, that is the one i was going to build for my project. I was able to simulate it in multisim so i figured i would be able to build it and at least show that it puts out 120V@60Hz. I was also going to build a circuit that I saw (posted by audioguru if i remember right) at the same time in case the simple one didnt work right. The other circuit had many more components and a 555 timer to regulate the frequency. The problem is it uses a transistor (2sc1061) that i am unable to simulate. Also, i saw a couple different versions of the circuit online and am not sure which one works properly and I cannot simulate the circuit without the transistor.

 

[Big Cory]

I'm going to try to attach my circuit here, it simulates 120V@6oHz with 12V DC input. If I change the base resistors to 196 ohms it gives me 60 Hz at 12.6 Volts, which is why i want to try some sort of 12v regulator. Probably a dumb idea, but i dont have the spice file for the sc1061, so i cant simulate the other circuit to see if it works properly or if i can modify it somehow. Thanks again for your help.

 

[Norris]

Something you may not have noticed is that a lot of invertors advertise 300 watts from the lighter socket, but when you read the instructions it then tells you that to get 300 watts you need to wire it in with a seperate high current circuit. Drawing that much current from the lighter circuit could be risky, if the fuse has been bypassed you could have a nasty dashboard fire.

 

[audioguru]

Multisim is stupid:
1) It doesn't know that the original schematic has the polarity of the capacitors backwards so they explode.
2) It doesn't know that the transistors have a max allowed reverse voltage of the emitter-base diode of only 7V but the circuit tries to put 23V on them which causes avalanche breakdown and very high currents in the capacitors which causes them to explode even if the polarity is correct. The wasted power is almost the only power.
3) The 200 ohm base resistors supply a base current to the transistors of only 55mA. The collector current is only 550mA for the transistors to saturate well so the output power is only 6.5W. If you load it more then the output voltage drops.