Dimming High Power LEDs?

Mussawar

Joined Oct 17, 2011
95
so I could use any general purpose NPN? like a 2N3904?

I'd need something much larger for dimming this LED though, I think. It's pulling more than 1A when it runs.
Yes you can use any NPN transistor that can handle more than 2 Amps. This kind of LED draw less than 2 Amps. Power transistors can handle it happily. It handles a 100W TV set and LED is about 20W.
It is quite simple dude ;)
Transistor is operating in emitter follower mode. Output voltages would be equal to base voltage. Output current also depends upon the base current. If you feel deficiency of current just replace the pot with a lower value (e.g. 1k or 2k etc.).
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
As soon as the proper driver arrives I'll probably use that method for dimming. I think I have a couple of very large transistors I pulled out of a power supply or something,
 

Mussawar

Joined Oct 17, 2011
95
As soon as the proper driver arrives I'll probably use that method for dimming. I think I have a couple of very large transistors I pulled out of a power supply or something,
What do me actually means by proper driver? Should it be made by a company or must it be expensive? Proper is the one which works good for you. This improper is working properly for me and I'm happy with it. If you want to go more advance, play around PWM. PWM is much more proper than simple single transistor drivers but definitely would be a bit complex design. Try around 300 Hz to avoid flickering.
If you have decided to purchase a patent circuit from a store then debate on circuits and techniques etc. is useless and just a wastage of time. Good Luck.
 
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Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
Something I just thought of, using the circuit Mussawar provided, how would I stop the LED from dimming all the way??

I don't necessarily want to turn it off with the pot, but would rather have a lower limit where the LED won't dim any further.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
So I assembled this circuit and used a 10K pot, but as you can see in the video in doesn't work properly.

At the start of the video it is at full brightness, then I start adjusting it dimmer, and you can see it doesn't really get dim, then suddenly starts pulsing, and keeps doing that more slowly until it's off.

Then I started turning it back up again.

What would the problem be here??

https://www.youtube.com/watch?v=dWRJH9HK3FY
 

spinnaker

Joined Oct 29, 2009
7,830
You don't want to try and dim an LED with a pot. It is really not going to be very linear in relation to brightness and is awfully inefficient. The best way to dim an LED is through PWM. You turn the LED on and off very quickly at different duty cycles and your eye perceives it as a change in brighness.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
This was the circuit posted previously, using a D1761 power transistor, a 10K pot, and a 47ohm 10W resistor.

all powered by the 30W LED driver
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
so, can someone explain why this dimming circuit doesn't work right? or how I could build a simple pwm controller?
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
Well looking at the parts in the image I'd need to purchase, and considering the time involved in assembling it, I may be better off just buying one of THESE



I do have some questions about Bill's PWM though.

Where would "square wave out" go? what's it's purpose?
What is U2B ? It's just an LM393 that's all grounded out.
 

wayneh

Joined Sep 9, 2010
17,496
...I may be better off just buying one of THESE
Couldn't agree more. But you asked how to build one. ;)


I do have some questions about Bill's PWM though.

Where would "square wave out" go? what's it's purpose?
What is U2B ? It's just an LM393 that's all grounded out.
The square wave is just a matter of fact, it isn't used for any purpose in the figure. Bill showed with U2b how you blind off any un-used comparators, to prevent spurious oscillation.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
545
I asked to build one just to see what was involved. There are a LOT of variations on the web.

It's too bad the original dimmer posted here didn't work. It seemed like a sound idea using a transistor to control the current flow to the LED.

but at $8 and free shipping, I may pickup a couple of those bad boys when I get some money.
 
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