voltage determines amperage drain on p7?

[jasonsmaglites]

ok, i've seen even before the p7 that when you drive more voltage to an led, it wants to draw more amps. it almost like you can control how many total watts are going to it by ramping up or down the voltage, making the led more hungry. can someone explain this to me. incandesents dont have this property do they?

this may be a seperate question but i also want to know, if you have a driver that wants to send 2800 mah to a p7, how does it know the vf of the led? the led at a certain voltage wants certain amps right (see question above)? if so, how does the driver limit the current. by just stopping excess current from getting to it, or adjusting the drive voltage so that the led "wants" the correct amount of amps?

 

[Lynx_Arc]

LEDs and bulbs operate like a resistance load, with bulbs pretty much predictable LEDs are a little more dynamic as they require you to reach a certain voltage before they start using power and it isn't quite linear when in the power range but predictable based upon
using ohms law. An example: if you have a 6v source a 1 ohm resistor(load,bulb,etc) then you have E(volts) = I(amps) x R(ohms or resistance).
6=1xI or 6amps. You can see that if you keep the resistance the same the current will go up with the voltage. Bulbs tend to hold their resistance value steady but LEDS are a little more dynamic and vary some on the resistance in their operating range.
Two ways you can limit the power (volts x amps) to an LED. Either regulate/limit the voltage or regulate/limit the current.

 

[Alan B]

ok, i've seen even before the p7 that when you drive more voltage to an led, it wants to draw more amps. it almost like you can control how many total watts are going to it by ramping up or down the voltage, making the led more hungry. can someone explain this to me. incandesents dont have this property do they?

this may be a seperate question but i also want to know, if you have a driver that wants to send 2800 mah to a p7, how does it know the vf of the led? the led at a certain voltage wants certain amps right (see question above)? if so, how does the driver limit the current. by just stopping excess current from getting to it, or adjusting the drive voltage so that the led "wants" the correct amount of amps?

I was looking at a chart of LED voltage vs current earlier today. Below 2.5 volts they essentially turned off and drew no current. Above 2.5 volts the current rapidly increased, approximately doubling for each 0.2 volt increase.

A current limited driver will sense the current and adjust the voltage to get that current. A low valued resistor in series with the LED generates a small voltage that is proportional to the current. The regulator adjusts the voltage to the LED until the proper current is reached. It does not care about the voltage, so long as it is within the range the regulator has available.

 

[ledstein]

I was looking at a chart of LED voltage vs current earlier today. Below 2.5 volts they essentially turned off and drew no current. Above 2.5 volts the current rapidly increased, approximately doubling for each 0.2 volt increase.

Try here: http://www.zled.com/en/product/prd/zpowerLEDp7.asp

Open: Electrical drive management pdf.

 

[yellow]

If You feed the incan with a higher voltage, it also draws more current,
only the change with led is considerably higher

 

[rantanplan]

[...]
this may be a seperate question but i also want to know, if you have a driver that wants to send 2800 mah to a p7, how does it know the vf of the led? the led at a certain voltage wants certain amps right (see question above)? if so, how does the driver limit the current. by just stopping excess current from getting to it, or adjusting the drive voltage so that the led "wants" the correct amount of amps?

A good LED driver is a constant current source and for that purpose it doesn´t need to know the Vf of the LED ... it is in the responsibility of the "human designer" to make sure that the chosen driver can operate at the expected voltage range.

The driver circuit has a small sense resistor in series with the LED, which tells the driver circuit what the actual current flow is (Ohms law: Voltage across a known resistance gives you the current). The driver tries to adjust the actual flowing current to the values it is "programmed" for ... which is done by raising or lowering it´s output voltage.

Every driver is limited in its output voltage, mostly by tolerances of its components, but as long the Vf of the LED (for a certain current) lies in the output voltage range of the driver, the current is regulated.

 

[Egsise]

Current determines brightness, voltage drain on LED can be anything.
Voltage drain is individual to every LED. Voltage drain increases if LED heats up, causing more heat.
Regulate the current.