Driving LED at two intensities

[csphere]

I would like to drive this led at two intensities. Would it be possible to use PWM with a fixed resistor and put an Inf./0 Ohm resistor in parallel (i.e. a switch) to achieve this?

 

[Gunner12]

A switch that changes between the resistor and an open circuit should work if the LEDs are wired in series, but it might not be the most efficient method. Those 7 LEDs seem to be wired in series, so you should be fine. A lower duty cycle PWM would probably be more efficient though.

 

[csphere]

Thanks for your reply. I have seen some drivers at taskled.com that could do this, but they seem to be about "clicking" to a new level and staying there. I would prefer a pushbutton approach that makes the leds burn at full intensity when the switch is active and another, to be determined intensity when it isn't. Btw, you wire those LEDs any way you wish, you can drive them all in parallel, or in series. If all else fails I suppose I could drive 2-3 of them at full intensity and another 4-5 at full intensity at the flick of a switch. But that is not really what I would like.

 

[evilc66]

I would like to drive this led at two intensities. Would it be possible to use PWM with a fixed resistor and put an Inf./0 Ohm resistor in parallel (i.e. a switch) to achieve this?

If you are looking to drive the LED with pwm (assuming it's coming from a microcontroller), why not just change the duty cycle of the pwm to change the intensity?

 

[csphere]

I'm looking to drive the led with an off-the-shelf driver or something with a schematic I can build. There is no chance I can design something like that myself in the forseeable future.

 

[AnAppleSnail]

With most "boost" driver architectures, a resistor between the driver and the batteries will act as a throttle. It is not especially efficient, but it is very simple to operate. You'd want a SPST momentary switch in parallel with a few ohms of resistance setting your 'dim' mode. I've used 350mA drivers with 1 and 100 ohm "settings" to make a low mode and a brighter mode.

 

[arek98]

You can use blue shark driver, remove trimpot (you can order it with removed) and use resistors to get levels you want.
Then you could use two switches as you want, one to turn light and other to switch level.
First switch would need to be installed on input power and other would connect/disconnect one resisitor to change level.

See driver here and there are links to CPF with lot of info about driver
http://theledguy.chainreactionweb.com/product_info.php?products_id=1136

 

[arek98]

Actually maybe you could keep original trimpot, it would give you possibility to regulate low output.

Then you would need to solder resistor with switch in series between PAD1 and PAD3 (center pad that controls driver output)
http://www.candlepowerforums.com/vb...Shark-driver&p=1950180&viewfull=1#post1950180
Extra resistor, call it R3 would need to give correct resistance for required current (in parallel with R1).

LEDs are rated to full 1A so you can actually use full output of Shark, this means that you could just solder switch to PAD1 and PAD3 and skip extra resistor all together.

When on, it will short both pads and PAD3 (control) will see 1.235V and will result in full power. Opening switch will go to low mode set by trimpot.
Tricky part is solder switch to pads where trimpod is already installed, but you need just thin wire, AWG30, 32 will be enough.

 

[csphere]

That sounds interesting. So basically you are suggesting to mod that driver with an extra wire and a resistor. Unfortunately this assembly is rated at 700mA, not 1A, but maybe that can be overcome as well. I'll try to wade through the specs of the driver later tonight.

On a side note, Avast! complained about a Trojan in tabpane.js in the link to the driver.

 

[Changchung]

I have done I electronic circuit that have this feature, it work with 12v, let me know if that help you

 

[arek98]

Edited because it was incorrect.

Little changed drawing from one of other posts:

Rfixed​ together with R1 and R2 (R1+R2 are created by original trimpot and they sum to 20kΩ) create voltage divider that controls voltage on PAD3 and this controls Shark output current. Shark outputs max 1A (with default sense resistor). Changing voltage on PAD3 moves this output current between min (which is about 10% of max, so 0.1A with default sense resistor) and max 1A.

To make it work the way you want (I understand you want one switch for power and another for low/high mode) you can do something like that with Shark.


Remove original trimpot and create resistors network that will give you levels you want.

I’m not sure about voltage levels required to get currents you want but let say you require 500mV for high (at PAD3) and 50mV for low.

With switch off current flows from reference voltage source thru Rfixed​, R1 and R2.
1.235V / (Rfixed​ + R1 + R2) = 0.5V / R2
R2 = (0.5/(1.235-0.5))*(Rfixed​+R1)
We have two variables but we know that we want R1 + R2 little more than 20kΩ. If we assume R1=5kΩ then from above equation R2=17kΩ.
This will give 0.5V at control PAD3 when switch is open.

Now to get low mode we close a switch connecting R3 in parallel with R2, lowering resistance and voltage at PAD3.
To get 50mV we need Rx (R2 and R3 in parallel) to be 1.05kΩ (you can use equation above again).
From this we can compute R3
R3 = R2*Rx/(R2-Rx)
R3 = 1.12kΩ



About Avast!, I guess is false positive, this website is well known here and been there forever but on the other hand you never know what snicked in somewhere. My AV does not complain though.

 

[csphere]

Thanks for your elaborate post. I think I understand it for the most part. Contrary what I may have led you to believe, I am looking for a driver that has a default setting (low) and a high/full setting when a switch is pressed. When you let go of the switch it should return to the default level. This weekend I have more time to dive into it, perhaps I can work up some Google magic to find something related to PWM to suit my needs.

 

[arek98]

With two switches light will turn on at level that depends on SWlevel beeing closed or open.
If you want low all the time and only momentary high you can use "normally closed" momentary switch for SWlevel.
This way turning on power you will have low and pressing switch will give you high as long you hold it.

Of course it will be easier if you can find driver that suits you but if you would need to build/add PWM circuit then it gets complex (and you need a driver that can either be controlled by PWM or turns on/off fast enough to PWM power.

 

[Changchung]

Thanks for your elaborate post. I think I understand it for the most part. Contrary what I may have led you to believe, I am looking for a driver that has a default setting (low) and a high/full setting when a switch is pressed. When you let go of the switch it should return to the default level. This weekend I have more time to dive into it, perhaps I can work up some Google magic to find something related to PWM to suit my needs.

Here is a video of that I have done;

 

[csphere]

Hi Changchung, that looks very nice. But in my case I could have a constant output with power on, and a high output with the red switch you press, reverting to contant (low) output when you let go of the switch. Is that possible with your setup?

 

[AnAppleSnail]

Momentary switches can be NO (Normally Open) or NC (Normally Closed) as needed.

 

[csphere]

With most "boost" driver architectures, a resistor between the driver and the batteries will act as a throttle. It is not especially efficient, but it is very simple to operate. You'd want a SPST momentary switch in parallel with a few ohms of resistance setting your 'dim' mode. I've used 350mA drivers with 1 and 100 ohm "settings" to make a low mode and a brighter mode.

Do you have an example of such a driver? I'm looking for something that is power efficient for both settings, not burn off half the power in a resistor at the low setting :D

 

[AnAppleSnail]

Do you have an example of such a driver? I'm looking for something that is power efficient for both settings, not burn off half the power in a resistor at the low setting :D

I used a "350 mA boost driver" from DealExtreme and throttled it with a resistor. I never measured the efficiency, but the runtime was reasonable (A few hours on 'high' with one Eneloop AA). This light was perfect for me because all the moving parts were self-cleaning and it could be fixed with duct tape. I find that cheap boost drivers do not work well with over 100 ohms between them and a AA cell. I have not tried this with expensive ones.

Even a strictly resistor-based solution will not get much worse than 66% efficient with decent Vf matching. I think (But have not shown) that putting a resistor between the driver and the battery simulates 'low battery' conditions for the driver, so however it acts on low battery, it will act here. In general if the voltage to the driver gets below 0.8v, the light cannot function. This tells me that the efficiency is something like (1.2v nominal - 0.8v general minimum to work) is 30% loss as a rough estimate, before the driver.

 

[Changchung]

Hi Changchung, that looks very nice. But in my case I could have a constant output with power on, and a high output with the red switch you press, reverting to contant (low) output when you let go of the switch. Is that possible with your setup?

Yes, my circuit can do that, you just need the led driver, I dont really like the led star that you choice, 7 leds is kind difficult to drive at 12volts, I will choice something different, maybe two or three stars with 3 leds in serie.


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