For power LEDs, is there a downside of using a resistor, compared to a constant current driver?
(DC wall wart type supply.)
That depends, if you're just driving a couple and looking for a simple circuit - as long as you've got a constant voltage source that isn't going to spike and allow excess current through the LEDs, and size the resistor appropriately, then there's no problem.
Don't forget the thermal management aspect of power LEDs.
For a power LED if you use resistors you'll be wasting a lot of power as heat and the resistors required will need a high wattage rating. With a switching (not linear) current regulator there is much less power wasted and you have more freedom in selecting a power supply -- the voltage doesn't need to be as closely matched to the LED voltage as when using resistors/linear drivers.
db2db:
For power LEDs, is there a downside of using a resistor, compared to a constant current driver?
(DC wall wart type supply.)
a) It uses a lot more electricity.
b) It means you can't run them at full rated power (or shouldn't try to - you can get runaway conditions near the limits)
Regarding (b), I'm not sure if you should try to run most big LEDs at full power anyway, they get awfully hot if you do (which is more wasted electricity). It's generally better to underpower them a bit..
See
This series has a great explanation of the different ways of driving a power led. There is also a really simple design for an efficient driver. I'm a software guy and even I could handle the hardware.
jimbarstow:
This series has a great explanation of the different ways of driving a power led. There is also a really simple design for an efficient driver.
That driver may be simple, but it's not efficient. It will waste as much power as a resistor.
fungus:
jimbarstow:
This series has a great explanation of the different ways of driving a power led. There is also a really simple design for an efficient driver.That driver may be simple, but it's not efficient. It will waste as much power as a resistor.
Replace the fet with an npn ...
What's your complaint with this type of circuit?... inefficient how (for what it does)
Replace the fet with an npn ...
What's changing to an NPN do for us?
db2db:
Replace the fet with an npn ...
What's changing to an NPN do for us?
For us?
....I was talking to fungus, using the n channel fet you need to apply 20 volts or so according to my simulation, which concurs with the indestructible's thread...
Switching to a bjt means you can significantly drop the voltage and still power the LED.
I'm not saying you should, I'm intrigued by fungus' s comment, It's not as wasteful as he might think.
cjdelphi:
fungus:
jimbarstow:
This series has a great explanation of the different ways of driving a power led. There is also a really simple design for an efficient driver.That driver may be simple, but it's not efficient. It will waste as much power as a resistor.
Replace the fet with an npn ...
What's your complaint with this type of circuit?... inefficient how (for what it does)
emm... off-topic but... may I ask what this simulation app is?
looks fun!
Everycircuit for android and ios....
cjdelphi:
I'm not saying you should, I'm intrigued by fungus' s comment, It's not as wasteful as he might think.
(Vin - Vf(LED)) * I = power wasted. You can make up any arrangement of mosfets, bjts, or resistors that you like but the above formula still holds true.
Chagrin:
cjdelphi:
I'm not saying you should, I'm intrigued by fungus' s comment, It's not as wasteful as he might think.(Vin - Vf(LED)) * I = power wasted. You can make up any arrangement of mosfets, bjts, or resistors that you like but the above formula still holds true.
I'm refering to the waste of the transistor (heat dissipation) ...
cjdelphi:
I'm refering to the waste of the transistor (heat dissipation) ...
Did I just get baited into your LED argument where the voltage available is always perfect and heat stays constant? What are you trying to say?
Yes, some wasted energy comes with these solutions, but I don't think that's the primary issue that most people care about.
Try to somewhat match V source with the vf times the amount of LEDs to use, and the waste is reduced.
Still better than incandescent.
Having a cheap circuit with some control over current seems a good solution as constant current drivers are a bit pricey.