# 5v to 12v Circuit

Discussion in 'Electronic Basics' started by LED Man, Feb 23, 2005.

1. ### LED ManGuest

Looking for a very simple (Newby) 5v to 12v circuit, or device, low
cost, to run 120 ma 12v led off 5-6v

TIA

2. ### Robert MonsenGuest

You need a boost circuit.

You can buy DC-DC converters for this kind of application, but they may
violate your 'low cost' requirement. You can probably get them surplus.
I know I've seen them at halted specialties in san jose for a few bucks.

Here is a web page that describes the basics, if you want to try to
build your own:

http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/converter.shtm

You can also use switched capacitor circuits to do this, but it's hard
to get 120mA from one of these.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

3. ### CWattersGuest

Does it have to be a 12V LED lamp?

I would probably be cheaper and easier to use ordinary LEDs and run them
from 5V.

4. ### William P.N. SmithGuest

As Colin said, you need a different LED. Most of them are
intrinsically around 3 volts or so, so a 12V LED is just an LED in a
package with an appropriate resistor. Building a power supply to
overcome the resistance is a lot more complex than changing the
resistor (or getting the same LED with a resistor sized for 5V
supply).

5. ### philoGuest

just use a different dropping resistor...

a "12v" led and a "6v" led are really the same...

it's just a matter of the dropping resistor (which is in series)

6. ### petrus bitbyterGuest

It helps if you tell us what kind and type of 12V LED you're talking about.
I never saw 12V LEDs so your 12V type is an appliance of some kind. May be
electronic, may be some LEDs in series or only a LED with a simple series
resistor. An optimum solution depends on this information.

petrus bitbyter

7. ### petrus bitbyterGuest

It helps if you tell us what kind and type of 12V LED you're talking about.
I never saw 12V LEDs so your 12V type is an appliance of some kind. May be
electronic, may be some LEDs in series or only a LED with a simple series
resistor. An optimum solution depends on this information.

petrus bitbyter

8. ### LED ManGuest

It's not a single led, it's a 30 led package MR16, which is a good area
light, in a nice neat package.

Works well, 10 hours off 10xnimh AA, just looking to run off 4 AA

10. ### TerryGuest

Struck me too that AFIK most LEDs operate from 1.5 to 3.0 volts?
So the 12 volt package must either have a) A dropping resistor which wastes
some 8 or 9 volts, or b) LEDs in series (unlikely?).
Simplest would be, if possible, to modify the package so as to waste only
two or three volts of the 6 volts that four AAs will provide?

11. ### Antipodean Bucket FarmerGuest

I seem to recall hearing of 12 volt LEDs designed as
drop-in replacements for automotive/truck lamps in
tail-lights, etc.

12. ### Duane C. JohnsonGuest

12V visible light LEDs can't exist.
They would be in the hard ultra violet.

What you are describing are circuits of multiple
LEDs and ballast resistors and such.

Duane

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13. ### John BeardmoreGuest

Or a switch mode constant current power supply ?

Why unlikely ?? Works for us !

J/.

14. ### EcnerwalGuest

or b) LEDs in series (unlikely?).

I believe it was already mentioned that the item this thread started
about was a 10 or 12 LED in one package lighting unit, which is probably
a few series strings in parallel, depending on the LED operating voltage.
LEDs in series makes all kinds of sense. If you change from wasting
power on a dropping resistor to running as many as you can (depends on
voltage available) from a constant current supply (easily made from an
LM317 voltage regulator [for less than a dollar], among other methods)
you get more light out for the same power in. And you can more precisely
control the current (which is what LEDs care about and die from too much
of) so that you can run them at maximum rated current (for maximum light
out) without paying too much attention to the supply voltage. With a
dropping resistor, the resistor value must not permit more than rated
current at maximum voltage in, and in most applications is taking more
power than the LED is.

Nothing is free, in that the regulator is eating some power (it
basically acts like a smart resistor) but for a 12V supply, with a
typical LED operating voltage of 2 or 3 volts, the same power that would
be running one LED and a dropping resistor can run 3-5 LEDs. One can
approximate this with a smaller dropping resistor, but the constant
current supply does a better job.

15. ### CWattersGuest

I suspect that your LED lamp might be designed to replace a normal halogen
lamp. Remember that normal 12V halogen downlights operate from 12V _AC_ not
12V DC.

16. ### Bill Kaszeta / Photovoltaic ResourcesGuest

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