A few questions if I could

Hi all,

I have a few things I need to clarify for a project that I am doing.

1.Will using a switched PSU get around me needing a power regulator to limit power to the LED's?

2. I want to use a 12v source and approx 12 LED's. I have read that Parallel circuits are not ideal for LED's so I was going to use two series circuits containing 6 LED's each both connecting to the same 12v supply. Assuming the overall load does not go over the PSU's max load, do you see any problems with this?

3. If i did need to use a regulator, such as the lm317, where would if fit in the circuit?
__________________-> RESISTOR->LED->LED->LED->LED->LED->LED->Ground
Source->regulator
__________________-> RESISTOR->LED->LED->LED->LED->LED->LED->Ground

4. I have been told if I use a regulator I don't have to worry about voltage. Why is this?

Many thanks from a novice:thinking:

First off, if we're talking white LEDs, your Vf for each is going to be somewhere around 3.1V-3.9V at typical currents. YMMV. What this means is that each string is going to have a max of 6 x 3.9V = 23.4V, so you're better off using a 24V supply if you can, or going to 4 strings of 3, each string having a current limiting resistor.

Now, a note about resistors that I always like to put in my posts. Vf's vary, and LEDs are brightness binned on current. When you have a set resistance in this circuit, lets say 2 ohms to get 350mA out of the max total Vf in my 4 strings of 3 suggestion, then at the low end of the Vf range, you have 3 x 3.1V = 9.3, for a gap of 2.7V from the supply to the load. Now your 2 ohm resistor is causing the current through the LEDs to be 1.35A! That could have drastic consequences in terms of brightness variation and LED life.

Now instead, take that lm317 (not the best regulator for current control, but we'll use it here). The lm317 will adjust an internal resistance to ensure that the adj pin is at 1.25V lower than the output pin. So, if you make sure you have 1.25V + 0.3V for the internal voltage drop = 1.55V of overhead from your supply voltage to the maximum Vf of a string, then you can put the LED string from the adj pin to ground, and a resistor that is 1.25V/desired current between out and adj. Do that for each string, and the current will be guaranteed to be what you want, and you'll get even brightness.

take that lm317 (not the best regulator for current control, but we'll use it here)

Click to expand...

What would you suggest as better regulators?

--
Thanks,
Mohit.

Hi Steve,

Thanks for the info. Funny as I read one of your previous posts on here and you mentioned about the resister problem but you did not go into detail. I was going to contact you direclty:twothumbs

So rather than use the lm317 what would you suggest. I need it to be as small as possible. I was looking at a regulator on led1
http://www.led1.de/shop/product_inf...ma-p-526&cName=constant-current-sources-c-112

Would this be suitable? Would the 20ma one be suitable in the 4 strings of 3 setup, or would I need an 80ma (20ma per string). Or may be 4 x 20ma regulators.....

Also, how hot do the regulators get? My design would mean it would be in an enclosed space with no cooling.

Thanks

markf said:

So rather than use the lm317 what would you suggest. I need it to be as small as possible. I was looking at a regulator on led1
http://www.led1.de/shop/product_inf...ma-p-526&cName=constant-current-sources-c-112

Would this be suitable? Would the 20ma one be suitable in the 4 strings of 3 setup, or would I need an 80ma (20ma per string). Or may be 4 x 20ma regulators.....

Click to expand...

OK I guess you understand a switching PSU won't provide the regulation you need.
The part you list there, one per series string. So 4 of them. That type of regulator, like an lm317, just burns up the extra voltage as heat. It's got a 500mW dissipation which would be (Vin-Vledtotal) <=25V @ 20mA. But that's totally burning hot to the touch, and only if the board is sitting in like 70F air, and it may not live all that long at that temperature. So don't run it anywhere near that hot. Lesser Vin-Vout will reduce the heat.

What we normally use are switching regulators like the ZXLD1360. There are a bunch of these now. They're not only very efficient but provide accurate regulation under virtually all conditions and don't have a significant minimum Vin-Vout. But you do need a few basic components to build the circuit, including a small inductor and a low-ohm shunt resistor that may be difficult to get from Radio Shack.

OK Thanks for that.
Heat would be an issue for me as anything that will be needed to control the power or voltage would be embeded in the structure of the what I am making, and thats made out of wood. heat and wood don't really go together in the home


I am using a everyday 240vAC to 12 VDC PSU and, this is external. There must be something similar that would give me the 12V and regulate power in one product? This would solve my problem of heat and space as it would be external.

Thanks

How much electronics experience do you have? If you do not have a lot, switching regulators are not always the best place to start. As you will be having several strings, it also becomes a lot of circuitry.

If you have 4 strings of 3 leds each, a linear regulator per string would be fine. If you use TO-220 package, heat is unlikely to be much of an issue as you will be spreading it over 4 regulators and are unlikely to have that much voltage on any given one. If you can attach the regulators to a piece of aluminum (even better if it is the case, even better).

I would not suggest the LM317 though. Even at 20mA, it still has a large dropout, about 1.5V. You will be also losing 1.25V for the feedback, which off a 12V supply leaves you only 9.25V. Depending on your LEDS, that may not be enough voltage. Something like the Micrel MIC2941 may be better for you http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=576-1138-ND it has only about 300mV dropout at 20mA. With the 1.25v feedback, that gives you about 10.4 volts to play with for driving 3 LEDs. That may still be a bit iffy though.

There is one other trick you can play. As opposed to connecting from the feedback resistor directly to the feedback pin, you connect the two with a forward biased diode. Typically the feedback resistor will have 1.25v across it. For 20mA, the value is 1.25/0.02 or 62 ohm. If you have a diode between that resistor and the feedback/adj pin on the regulator, then the voltage on the resistor can be approximately 1.25 - 0.7 (forward voltage of the diode), or 0.55V. You also need a resistor (say 10k) between the adjust pin and the 12V power supply. Depending on your diode, you are likely going to be more in the 0.65v range for that diode. You will not get quite the same accuracy on currrent, but for the most part, it is good enough. You will now have 1.25 - 0.65 or about 0.6v across the resistor and its value will be 0.6/0.02 or 30 ohms. With the 0.3v drop out, you have now 11.1 volts for your 3 diodes. That hopefully will be enough for your LEDS to get to 20mA.

Semiman