30th July 2009 01:49 PM
No they wont. Just mount them on the rear of the housing. The dissipation of a single LED will be 4.4W at the recommended operating point. Not much heat to get rid of considering the 5000lb heatsink it is bolted to.
Originally Posted by audioguru
30th July 2009 03:30 PM
Here is a hack at making a linear constant-current regulator for a string of three of the W724C0 LEDs using my favorite LM431. I configured it to deliver ~1.6A. Note that it keeps the LED current constant while the battery voltage sweeps from 12.2 to 15V. Below 12.2V, the NFET is saturated, and the current decreases. R1 sets the current.
In the third plot pane, see the power dissipation in one of the LEDs, the FET, and the resistor R1. At 14.5V (motor running alternator voltage), the dissipation in the FET and R1 are about 4W each; quite manageable.
Let me know if this seems to be heading in the right direction. If it is, when I get some more time, I will simulate the effects of transients.
30th July 2009 05:20 PM
I made it adjustable by adding a pot.
30th July 2009 08:23 PM
See page 34.
Obviously you can use a discrete comparator rather than a MCO and add a pull-up resistor if it has an open collector output.
30th July 2009 08:34 PM
My 0.02: I used "10 W" LEDs very similar to this on my car's reversing lights.
I strung 3 in series, and utilised the car's exisiting wiring as a 'current regulator'. They lasted a suprisingly long time, some weeks, and well long enough to evaluate them.
My conclusion was the coloUr rendering was a bit poor, so I didn't persue it any further.
A plus with these LEDs is the efficiacy, 70 lumens / Watt, a lot better than incandesant at 5 to 10 lm/W
Last edited by marcbarker; 30th July 2009 at 08:38 PM.
31st July 2009 06:29 AM
This circuit is wrong. It won't oscillate as drawn. You would have to reverse the inverting and non-inverting inputs to the comparator to make it oscillate. It also has an awful start-up transient until C1 charges which causes the current through the load to be very high initially.
Originally Posted by Hero999
Last edited by MikeMl; 31st July 2009 at 06:29 AM.
31st July 2009 11:08 AM
I think SoftStart was ommitted for clarity of illustration. I'd say the LED would tolerate the high starting current anyway, going on the longievity of my "10 W LEDs setup" in my earlier posting.
The 'Comparator' doesn't have to be a comparator, it can be a transistor. It doesn't need to compare accurately, just fast.
Could the posted circuit have its flaws fixed enough to become a practical circuit?
Last edited by marcbarker; 31st July 2009 at 11:10 AM.
31st July 2009 12:24 PM
Yes you're right, another option is to use a N-channel MOSFET and move it to the low side.
Originally Posted by MikeMl
That shouldn't happen, the current ramps up until the voltage across the sense resistor exceeds the reference slightly, the MOSFET is turned off and the current falls. This makes the start up time slow which is only a problem if you want to PWM it which would be a rubbish way of varying the brightness, a more sensible way is to vary the reference voltage.
It also has an awful start-up transient until C1 charges which causes the current through the load to be very high initially.
31st July 2009 09:39 PM
Hysteretic converters like this are good but they do have their limitations, namely that they can be unstable under certain loading conditions. Not a problem if you just want to drive a couple of LEDs though.
1st August 2009 04:53 AM
This 3 transistor constant current smps oscillates/regulates around load current and could be adapted for 12v-3v or 12v-6v use;
2-transistor Black Regulator
You could use a logic level PFET for the main switch device to get currents up to a few amps with no problems.
high power led,
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