split 12v to 2 6v supplies

[justwantin]

Is it possible to take, say, a 12v wall wart and split the voltage to two seperate supplies in the same circuit? I was figuring on using two lm317's but being very new to this as a hobby I am not sure if there would be anything to look out for. TIA

 

[k7elp60]

If you can be a little more specific I'm sure we can help you.
All power supplys have a common lead. So do you want two +6V supplies with on common lead, or do you want a +6V supply and a -6V supply both with a common lead.
It will help if we know the current capabilities of the wall wart also.

 

[justwantin]

I'm thinking about an application which will require 4.5 volts to a microchip blinking some LEDs and 6 volts to 2 X 1w LEDs.

I'm doing the microchip circuit with an lm317 now but I'm now thinking of adding a couple 1w LEDs which will not be blinking. They will require at least 6v and I was thinking of doing this with another lm317.

So basically it would be a 12vdc supply to 2 lm317's one which regulates to 4.5v and the other to 6v.

I haven't figured out a way to google this so I thought I'd ask here.

 

[k7elp60]

A couple of LM317's should work fine. They may require heatsinks depending on the load current. Do you have a data sheet for the LM317's?

 

[justwantin]

Yes, I've used the lm317 before and have already breadboarded both voltages. I just returned from finishing off a job and will now try to build the complete circuit with both regulators supplied from the same source. Heatsinking so far has not been required on the 4.5 volt circuit. I've redone my numbers and the 1w LED circuit will get 7v. I don't think a heatsink will be required there but I may be wrong.

Current is something I haven't quite wrapped my head around yet. Maybe I should read more and make less smoke.

Thanks for the help.

 

[colin55]

A 12v adapter will produce about 16v on no load. This voltage drops to 12-13v when the load is 500mA for a 500mA adapter or 1amp for a 1 amp adapter.

If your load is 200mA, the voltage from the adapter will be higher than 12v. It could be about 14v.
If your output from a 3-terminal voltage regulator is 7v, the difference between input voltage and output voltage will be 7v and the current will be 150mA. This produces a wattage of 1watt and this wattage must be dissipated by the regulator. A 3-terminal regulator requires a heatsink when the wattage is above 400mW.

 

[justwantin]

Thanks for the info. I haven't yet hooked this up to a wall wart. All I have in the junk box is a 12v at 600ma. I'm not sure it will be adequate.

I had the lights running last night off a homemade benchtop power supply ( **broken link removed** ) set to 12v. I have no idea what the current would be.

There is an lm317 regulating to 7v for 2 parallel sets of 2 1w LEds each in a series. Each LED specs at 3.5Vf at 350mA. This lm317 is on the hot side of warm and without any math I'd say it needs a heatsink

The second lm317 supplies 4.5v to 15 5mm LEDs in 5 parallel strings of 3 LEDs each which are turned on/off randomly via a microchip. As a separate circuit this lm317 never gets hot with a 6 or 7.5v wall wart nor does it now when sharing the 12v supply with the lm317 set up at 7v.