What is AC? when suppling LED lamps?

[ericmark]

I like I am sure most others consider AC as alternating polarity, but AC actually stands for alternating current so with pulse width modulated power supplies the polarity may remain static but the current still alternates between zero and maximum. Add some capacitors to smooth it and then yes DC but if no smoothing is it AC? And if a "driver" for a LED lamp is pulsed can it be called DC?

So if I get 10 - 30 volt DC lamps and try running them off something called a driver although voltage not current regulated could it damage the lamps or cause them to malfunction and if it did could I claim from the supplier as labelled DC when really AC?

 

[winston1]

It is not AC. It is pulsating DC.

 

[JohnW2]

It is not AC. It is pulsating DC.

Indeed. Whether or not the voltage alternates in polarity obviously depends upon what reference voltage one uses but, with a 'two-wire' system, with pulsed DC the current (when it is flowing) will always flow in the same direction.

To partially answer eric's question, a pulsed DC source intended for incandescent bulbs/lamps will presumably have the potential to possibly damage LEDs, since, although high voltage pulses which average out to a bulb's nominal voltage may well be OK for an incandescent (e.g. halogen), LED ones may, or may not, tolerate those high voltage peaks. A lot presumably depends upon what is inside the LED 'lamp'.

Kind Regards, John

 

[ban-all-sheds]

It is not AC. It is pulsating DC.

 

[ban-all-sheds]

To partially answer eric's question, a pulsed DC source intended for incandescent bulbs/lamps will presumably have the potential to possibly damage LEDs, since, although high voltage pulses which average out to a bulb's nominal voltage may well be OK for an incandescent (e.g. halogen), LED ones may, or may not, tolerate those high voltage peaks. A lot presumably depends upon what is inside the LED 'lamp'.

AIUI using pulses is an established technique to make LEDs run brighter. The peak voltage is not enough to damage them in its own right, but would be enough to damage them by overheating if applied continuously.

Persistence of the phosphor and the human eye is enough to produce a brighter light.

 

[JohnW2]

AIUI using pulses is an established technique to make LEDs run brighter. The peak voltage is not enough to damage them in its own right, but would be enough to damage them by overheating if applied continuously. Persistence of the phosphor and the human eye is enough to produce a brighter light.

Indeed, bernard often mentions that, and I think that someone (eric?) did it as a 'classroom experiment'.

However, there is presumably a limit to how high a peak they can tolerate, and quite possible a lower limit that would be OK with incandescents (when, incidentally, one has persistence of the heated filament, as well as visual pigments in the eye). As I said, a lot presumably depends upon 'what is inside' the LED lamp, since the LED itself may well not 'see' the same degree of 'peaks' as exist in the supply.

Kind Regards, John

 

[bernardgreen]

The peak voltage is not enough to damage them in its own right

Being pendantic the peak current is not enough to damage the die (LED element ) provided the average current is less than the continuous maximum.

Pulsing at 4 to 5 times maximum continuous current with a 1 to 10 mark / space ( ON / OFF ) ratio make the LED appear to be 4 times brighter ( than if driven with steady maximum current ) but using only half the power.

Pulsing LEDs creates some strange effects, for example in a CCTV recording of a pulsed LED display it can appear that the display is being turned ON and OFF when the pulsing of the LEDs does not coincide with the camera's reading of the image on it's target ( retina )

 

[JohnW2]

Being pendantic the peak current is not enough to damage the die (LED element ) provided the average current is less than the continuous maximum.

Indeed, that's why that I said that a lot depends upon "what is inside" the LED lamp - if there were something approaching 'constant-current' circuitry, the LED elements might see much the same peak current regardless of how high the voltage peaks were.

Kind Regards, John