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Comment by londons_explore | Hacker News Reader

The transformers are usually less reliable than the LED's themselves.

Also, LED's unlike nearly every other electrical device are constant current devices (as opposed to constant voltage, which is what a USB port, battery, or nearly any other power supply offers for example).

Constant current devices must be wired in series rather than parallel, meaning that if you wanted more than one LED per transformer, you would need to break the existing circuit to add an extra LED. That isn't really compatible with standards around electrical plugs - you would need to short out all unused electrical outlets in a constant current system.

Constant Voltage vs Constant Current is another one of those 'DC vs AC' or 'electron has positive or negative charge' issues - We have chosen one standard, and everything is built on it. We could go back and redecide, but all infrastructure would need updating.

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LEDs are not constant current devices. They are diodes with non-linear I-V characteristic curve.

The question of supplying LEDs with CC or CV power supply is only the question of efficiency and cost.

Most LED strips are made as a serial/parallel combination of LEDs with an added resistor for each segment, to get the desired current through the LEDs in a segment from a ~12V CV power supply.

You don't really have that much choice when it comes to LED strips. CC power supply would need to generate thousands of volts or you'd need a bunch of them. And that would be costly.

LEDs are better modeled as constant current because it's the I part of the curve which is typically more unreliable. The junction voltage is largely consistent, but due to the exponential increase in current, to balance the amount of power it's easier to regulate the I component of P=I*V for consistency than it is the V portion.

You can run any current through the LED. (well, up to a point :D) It's one way to regulate the brightness. I'm not sure what "modeled as constant current" really means when talking about a passive device.

If anything I simplify leds as maintaining a "constant voltage", no matter the current I send through them (in the right direction). Just like other diodes.

> it's easier to regulate the I component of P=I*V for consistency than it is the V portion.

I understand, and we're on the same page. But it doesn't change the fact that almost no led strips do this. You can iron out over individual LED differences by putting 3-4 in the series, and regulate the current using a resistor.

In fact, most of the LED indicators you'll see are driven like this. LED + resistor [+ optionally PWM or multiplexing].

Only for power LEDs you'll usually see anything more complicated invlving DC/DC CC switching power supplies.

No, I was talking about the LED light bulbs that replace incandescent light bulbs. Every one of those comes with a transformer, which we throw away when the LEDs die (or vice-versa). Could we not separate the two and have a transformer supplying constant current, and then we could swap that or the LEDs out when one died?

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