LED ballest resistors

[Ron Renko]

I would like to use my off grid solar system to power some LED's. If I have a 12volt, 900 ma LED chip. I calculate a one ohm 2 watt ballast resistor. Will this be a problem as the voltage drops on the battery? Os a costant voltage driver a necessity?

Your thoughts/experience please. Thanks.

[Sunking]

If your LED is rated for 12 volts you do not use any type of resistor.

[Ron Renko]

It is 12 volt rated. I thought as the battery voltage changes the current will change at the LED and that was a no no as it is an exponential with voltage. The resistor is supposed to limit this?

I am only babbaling what I have read. Just confused and need some insight.

[Sunking]

It is 12 volt rated. I thought as the battery voltage changes the current will change at the LED and that was a no no as it is an exponential with voltage. The resistor is supposed to limit this?

I am only babbaling what I have read. Just confused and need some insight.

Got a link to the LED. I assume they come with a fixture and ballast. If you put a 2 ohm resistor in series as you suggest 1 of 2 things will happen.

1. Will not work.
2. Limit the current below spec, and turn your 13 watt lamp into 10 watts and burning 3 watts in the resistor.

If the LED has its own ballast/driver it is self regulating. All you do is supply battery.

[inetdog]

It is 12 volt rated. I thought as the battery voltage changes the current will change at the LED and that was a no no as it is an exponential with voltage. The resistor is supposed to limit this?

I am only babbaling what I have read. Just confused and need some insight.

Since you refer to it as an LED chip, are you referring to the bare LED without external driver circuitry? If this is the case, you do need to look at the specification sheet from the chip manufacturer to see what is required, and without the part number or a link we can't tell any better than you can.

Some LED chips include driver circuitry but need external components added. You may have one of those.

The ideal solution is to use a driver circuit which delivers a regulated current, often by pulse-width modulated switching to minimize the power wasted in the driver. And yes, if you supply a voltage greater than the LED itself starts conducting at there must be current limiting somewhere.

If all you add is a series resistor, then it is quite possible that the range from 12 volts to the 15 volts or more than you can get from a 12 volt battery system which is being charged will require a different value resistor than just a regulated 12 volts would.

[Sunking]

You cannot do a Driver with a passive resistor. It needs to be active control.

[inetdog]

You cannot do a Driver with a passive resistor. It needs to be active control.

You can, it is just very inefficient and have really poor light intensity regulation as the voltage varies.
Same way you can use a resistor as a "driver" for a neon bulb in a 120 volt circuit.
Or you could just use the resistor and not call it a Driver if that is what is bothering you.

In addition, if the circuit/chip as delivered contained some circuitry in addition to the LED, it could just make use of an external resistor for input voltage programming. (I am not saying that any currently available units necessarily do this, but I seem to recall such a package.)

[Sunking]

You can, it is just very inefficient and have really poor light intensity regulation as the voltage varies.

OK IO will give you that but that is the problem with cheap LED lights, they use resistors, and in many cases the resistor uses more power then the LED making the 150 L/W now 50 L/W. A cheap CFL is better than that. A good LED Ballast converts a voltage supply into a current supply with very little loss.

[inetdog]

OK IO will give you that but that is the problem with cheap LED lights, they use resistors, and in many cases the resistor uses more power then the LED making the 150 L/W now 50 L/W. A cheap CFL is better than that. A good LED Ballast converts a voltage supply into a current supply with very little loss.

No argument from me there. If the LED voltage is close to the supply voltage, there will be less power wasted in a passive resistor ballast, but also much poorer regulation.
It pays to know just what you are buying. When your are making your own fixture, the priorities and choices may be different, but you still need to know what the trade-offs are. (Sort of like building your own panel in that respect. You can learn from it, but it should not be seen as a way to save money. )