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# LED voltage problem

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#### djcypher

##### New Member
Hi,

Please excuse the noob question, which is probably a no-brainer but I think I'm making it out to be more complicated than it should be...

I'm trying to build a simple LED circuit using the following:

1 10W LED (Forward Voltage : 9V-12V, Forward Current : 1000mA)
1 12V battery pack

My gut tells me that I should be able to run this with a 1 Ohm resistor. Is this correct? I'd like to use a 1 Ohm, 2W resistor but will this reduce the LED output?

Any guidance would be much appreciated

Thanks,

S

Is it a self-contained LED assembly with a built-in current regulator, or is it a "naked" LED. If it is just a naked LED, then a suitable current regulator is MUCH more complicated than just a resistor. The variation in battery voltage as the battery discharges requires the more complicated circuit.

Since the "forward voltage" is a range of voltages then it must be a naked LED and if its forward voltage is actually 9V and your battery is actually fully charged at 13.8V then with a 1 ohm current-limiting resistor the LED will instantly burn out with a current of (13.8V - 9V)/1 ohm= 4.8A.

Yes, it is a "naked" LED.

Actually MikeML you've just answered a question that I didn't know to ask - I'd seen something online (not in English) that involved an LED driver but I didn't really understand what it was for. You've now given me an insight into what its purpose must have been - to keep the voltage constant as the battery discharged.

The LED that I have came with what I believe is a current regulator and a voltage transformer:

The problem is that obviously the input calls for 85-265V input, whereas I only want to use 12V. Is it then a question of building/buying a step up transformer?

Thanks again,

S

... You've now given me an insight into what its purpose must have been - to keep the voltage constant as the battery discharged.

Actually, the regulator is used to keep the CURRENT through the LED constant regardless of what happens with the input voltage.

The LED that I have came with what I believe is a current regulator and a voltage transformer:

The problem is that obviously the input calls for 85-265V input, whereas I only want to use 12V. Is it then a question of building/buying a step up transformer?...[/QUOTE]

No, the AC line powered supply that came with your LED is of no use in your application. Using a 65% efficient step-up switching power supply (DC-AC) to feed a 70% efficient step-down power-supply (AC-DC current) is not the way to go.

For 12V battery (RV?) operation, I would have bought a "naked" LED (one without any electronics), and paired it with a Buck-Puck or similar current regulator.

Last edited:
Hi Mike,

Ok - I'll pick one of those up and have a look.

Thanks again for all of your great help. I really appreciate it.

And in case you were wondering: yes I'm going to go buy a basic electronics textbook and read through it before I do anything else!

Sash

Have you decided how to cool the powerful LED?

Yup, I've got it mounted on a large heatsink and have checked that the dissipation is within the LEDs params.

Thanks,

S

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