I have a little unit that plugs into the wall (110v). The transformer output reads: AC 6V~300mA. I want to connect it to my 12 volt car battery. I used a 22ohm 2 watt resistor to take the voltage from 12v to 6v at 300mA. I cannot get the unit to turn on. Are my calculations correct? Am I missing something else? Any help would be greatly appreciated.

 

Transformers don't work with DC.

Simply. Not. Work.

No amount of DC current will make a transformer run backwards or forward.

 

use a rectifier circuit to convert the AC into DC first. Use capacitor that can act as a filtering device. U may use LM7812 IC to get a constant 12V Dc O/p. You may get the circuit online.

 

use a rectifier circuit to convert the AC into DC first. Use capacitor that can act as a filtering device. U may use LM7812 IC to get a constant 12V Dc O/p. You may get the circuit online.

I think you have the requirements confused, harishramprasad.

12VDC to 6 VAC

 

easiest to just buy a small 12V to 120V "inverter" and plug your "little unit" into that.

 

Why do you want to connect a 6V AC supply to a 12V DC battery? Are you hoping to charge the battery (which would need a DC charging voltage > 12V and a current > 300mA to be practical)?

 

It looks to me like he wants to plug the 110V/6V transformer output into a battery and get 110 VAC out of the other end.

 

I think he means he wants to replace the 6V AC supply with the 12V battery, as power for his unnamed device.

He MIGHT get away with a DC supply to replace the AC, but it's impossible to know.

A resistor divider as he tried will NOT work unless the load is constant and known. So far it is neither. He has misunderstood the transformer rating of 300mA as the load current, which is certainly much lower. So he may have ruined his device by supplying it 12V.

More detail about the device to be powered is the only way out.

 

My interpretation is that he has some device that is powered by a wall transformer that outputs 6VAC @300mA and wants to run it off the car battery instead. He thought he could do this by simply using a resistor to drop 6V at 300mA.

This might work, if the 6VAC simply goes to a bridge rectifier and the unit always draws 300mA, but not likely.


Bob

 

I think the ball is in the OP's court to clarify what he wants and we haven't heard back from him. I am with Bob and Wayne's interpretation that he wants to replace a wall wart with a battery for his device, which requires 6VAC as input.

 

Input : AC 6V~300mA.
Output : 12Vdc Car battery.

If you using 6Vac through diodes rectifier and cap filter, it could be 8.484 Vdc, it is not enough voltage to charge the 12V battery.

Io_dc=Vac/1.414=300mA/1.414=212.2mA
How about the current of car battery, 1A,5A,10A?
If you using 212.2mA to charge the battery could be a long journey ...

 

You mean like one of these?