It depends on the forward characteristic of the LED. Using a typical
value of Vf=2V at If=20mA, you need to drop 15.5V at 20mA, which
requires 775 Ohms. The closest standard 5% values are 750 and 820 Ohms.
750 Ohms at (17.5-2=15.5V) will produce 20.67mA, which is probably a
little too much current. 820 Ohms would limit the current to 18.9mA,
which is definitely safe for an LED rated at 20mA. Or, you can get
precision resistors at 768 or 777 Ohms. In all cases, you're dissipating
something under 1/3 Watt, so you'll need a 1/2 Watt resistor.

Note that while many LEDs are rated at If=20mA, the Vf values for
typical LEDs vary quite a bit--between 1.2 and 2.2V at a minimum. You'll
need to know the Vf value for your LEDs to correctly calculate the size
of the resistor needed. If you can't get the specific value, go with an
820 Ohm 1/2 Watt resistor as a minimum--it is good for <20mA down to a
Vf of 1.1V. If in doubt, more resistance => less current => less chance
of burning out your LED--especially if you wire it in backwards.

FWIW, I seriously doubt that you have a transformer that produces
17.5VDC, at least not without some additional help ;-)


> -----Original Message-----
> I have a transformer that will pump out a maximum of 17.5 
> volts DC and
> wish to use it to power 12 volt DC LEDs. What do I need to 
> put in between to
> keep from burning out the LEDs? I do know I need to make sure 
> my amp draw is
> not over my output amps.

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