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+18V, 5V power supply help

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smuel

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lm2976

Hi,

I need to build a power supply with 18V and 5V DC. I seem to have 2 options...

Transformer (18V)-> rectifier -> 7818 regulator -> 7805 regulator

or

multitap transformer -> 2 seperate rectifiers -> regulators

i figure in the first case, the 7805 could get pretty hot. but in the second case i need double the filter caps.

any thoughts?


Thanks

Sam
 

7818 regulator

Hi !

You did not inform what are the currents required. The 7805 will become hot only if the current is high, for low currents a heatsink attached to it could be the solution (and cheapest).

I suggest you to use a LM2976-5.0 switching regulator, with the 18Vdc input and regulating to 5Vdc on the output. You need only as additional components: an inductor, a small capacitor and a schottky diode. This alternative has a better power efficiency than a linear regulator.
 

18v regulator

For currents > 100mA why not to consider switching regulators?
And this can apply to both outputs: 18V and 5V ..
One unregulated voltage of ≈20V can be easily converted to enything you need ..
Take a look at this switching regulator:
**broken link removed**
Other versions, fixed or adjustable, are also available ..

Regards,
IanP
 

18v 5v psu

Thanks for the fast replies!

I dont need much current, just to drive a couple of chips and an lcd. I'll chuck a small heatsink on the regulators anyway just to be safe. I do need to run the USART on an ATMEGA16 which I've heard needs a bit of extra oomph, anyone know how much? I'm sure it's in the data sheet, ill have a look later on.

I'm not keen on a switching supply because i need a super clean 5V for the ADC in an avr micro. I've been driven totally nuts by noise already, so i dont want to introduce even more.. So i'd prefer to stick with the good old regulators.

Do you think i should go with feeding the regulated 18V into the 5V regulator? and a 13V drop wont cause heat problems at low currents? Would it be best to feed the regulated 18V into the second reg, or simply feed it straight from the unregulated voltage.

Thanks again

Sam
 

18v linear regulator

If you use a regulated voltage to the input of the 7805 it will be better, since the voltage drop across it will be lower (13V) and the output even cleaner.
However, the 7818 will get hotter, since now it will source the entier 5V current, too.

The best solution for linear regulators is to use separate transformer windings. That way you can minimize the power dissipation in both regulators.
I don't know what you mean by doubling the caps. Why do you need to double them?
Or are you referring to the fact that you need two of them?
 

lm2976-5.0

Check another solution:
you can use 7818 for 18V output
make switching regulator about 8V with cheap switching regulator MC34063 (up to 500mA)
and then use 7805 to regulate 8V input to 5volt output.
:D

Regards
Davood.
 

7818regulator

I think ill just run the regulated 18V into the 5V reg. The 18V rail will be drawing almost zero current, so the fact that the 7805 will be stealing it's current from the 7818 hopefully wont be an issue.

with doubling the caps, i was referring to the fact I'd need a large filter cap for each rectifier, so to save on space and parts it might be a better choice to go with the other option and just deal with a little heat.

I'm still open to suggestions if anyone has a better way to do it.

Thanks

Sam
 

make 18v power supply

5V voltage regulators can work with input (uregulated) voltages of up to 35V, so leave the 18V regulator alone and feed 5V regulator from the same unregulated voltage as the 18V IC .. just give it a little bit bigger heatsink and your local "Tricky-Dicky" store should have plenty to choose from ..

Regards,
IanP
 

super linear 5v power

If 18V needs hardly any current, then I think it's still best to power the 5V regulator off the 18V rail.
That way you need a single cap and the power dissipation will be shared by both regulators (though not equally).
 

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