quick simple schematic check, please...

[grimxneko]

hi, im quite new to electronic design and circuitry, but (with the much appreciated help of others) i did just recently design a simple schematic for this project im working on (pictured below), and i just want to get some final opinions on it before i actually put it together. now, the specifications for the two outter columns of LEDs can be seen **broken link removed**, and the two inner columns can be seen here. so, i guess just let me know if i did anything wrong or if you have any other suggestions. thanks.

 

[audioguru]

Your circuit operates the LEDs at or slightly more than their max allowed current so they will probably overheat and fail soon.
The brightness pot will instantly burn out if the brightness is turned up.
The poor little 9V battery will last for only a couple of minutes when the brightness is at max. You will see the battery dimming the LEDs as it quickly runs down.

 

[grimxneko]

oh
that's pretty dissappointing. so... how would i fix that?

 

[audioguru]

oh
that's pretty dissappointing. so... how would i fix that?

Don't you know how to reduce the current in your LEDs?
Look at Pulse-Width-Modulation for dimming high power LEDs.
Use a much bigger battery that will supply the high current for a longer time.

 

[Sceadwian]

grimx spend some time learning how to use 555 timers. There's tons of information on them just use Google for 555 LED dimmer.

 

[grimxneko]

thanks for your responses, but to be honest it's all concepts im very unfamiliar with. all i got from that is that apparently my circuit generates too high a current. ... ugh, all i want is a big adjustadle IR flashlight yet this has turned into a long complicated endevor for me. ... now im just a little frustrated and tired at this point, so instead of suggesting these entire concepts i'd have to learn, cant someone just please design a simple schematic or a specific explaination of exactly how im supposed to construct this in the most efficient way? and i need to keep the basic design i have (4 collums of 5 different LEDs), but i dont care if you change the battery or whatever. i hope im not asking for too much, im just frustrated and confused at this point.

 

[Sceadwian]

grimx.. I want to not have knees and a lower back that are going on me, but I don't expect to go to an online forum and find a solution for those problems that I can do at home... I need a doctor for my problems, or a lifestyle adjustment.
Just as you can't expect to have a desire for a specific goal no matter how simple you think it is and then have it occur without gaining the reasoning behind the issues involved in what you want. You are, actually asking too much, if you don't want to learn EVERYTHING behind what you want to do then you don't want it enough, which means you have no other choice that to find someone that does know what you need for what you want and will give it to you, and that always costs. We can't instantly teach you, or expect you to want to learn; but what you're asking for is not a matter for casual slapping together.

 

[Mr RB]

...
exactly how im supposed to construct this in the most efficient way? and i need to keep the basic design i have (4 collums of 5 different LEDs), but i dont care if you change the battery or whatever.
...

If you MUST keep the 4 columns of 5 LEDs (I assume they are already on a PCB) that what I would do is use a LM317 and one resistor, setup as a constant current source. A resistor of 15 ohms will give you a total of 80mA. That will be shared by the leds so each of the 4 columns gets 20mA.

You can leave the 25 ohm resistors on the columns if they are already assembled on the PCB, otherwise with the LM317 you won't need those 4 resistors.

 

[grimxneko]

huh, thanks. that actually helped a bit. i kinda understood that. im just a little confused of how to set it all up, and whether or not i would still be using a potentiometer in this. so i mapped out how im assuming it'd look, soooo... is it right? any corrections would be great. thank you.

 

[Mr RB]

Close, now lose the 500 ohm pot, reverse the LM317 and its resistor, and change to 12v battery not 9v.

 

[grimxneko]

oh, ok. but now 2 things i dont quite get: how would i adjust the brightness without the pot?? and i wasn't too sure of what exactly you ment by "reverse" the LM317 and resistor. idk if i shoud've switched the in and out leads, or put the resistor on the other side like i did. but anyway, i tried and came up with this...

 

[Mr RB]

Yes you still need to switch the in and out pins.

You said this is for a torch, how much adjsutment do you need?

The 15 ohm resistor sets the LED current (and brightness). If you switch in a different resistor you can have a different brightness setting.

 

[grimxneko]

ah, ok. and sorry, i assumed you knew i wanted a manual brightness adjustment. but yea... is there a way i could add in some kind of variable resistor that'd control the brightness (and if yes, what value of resistance would it be and where would it go)? like what if i replaced the 15 ohm resistor with a potentiometer, would that work (just a thought)?

 

[vne147]

hi, im quite new to electronic design and circuitry, but (with the much appreciated help of others) i did just recently design a simple schematic for this project im working on (pictured below), and i just want to get some final opinions on it before i actually put it together. now, the specifications for the two outter columns of LEDs can be seen **broken link removed**, and the two inner columns can be seen here. so, i guess just let me know if i did anything wrong or if you have any other suggestions. thanks.

Why did you start a new thread for this? Anyway, they schematic I gave you had a typo. It should have been a 50Ω pot not a 500. Sorry.

I agree with Audioguru that the resistance values should be bumped up a little. The legs that you placed 10Ω resistors on if you ran the equations I gave you actually come out to around 10.7. By rounding down you will increase the amount of current going through the LED and since you're already operating at your maximum allowed current, that's not good. You should bump up both resistance values. I'd change the 25Ω to 27 or 30 and the 10Ω to 12 or 15.

There are more elegant and power efficient solutions than this for certain but it seemed like you needed the most basic solution. This is it.

Also, you haven't followed the advice from the last thread about the undersized 9V battery. At full brightness the 9V will last maybe an hour at most and that's if you don't fry it from the amount of current draw.

Your circuit operates the LEDs at or slightly more than their max allowed current so they will probably overheat and fail soon.
The brightness pot will instantly burn out if the brightness is turned up.
The poor little 9V battery will last for only a couple of minutes when the brightness is at max. You will see the battery dimming the LEDs as it quickly runs down.

Audioguru,

Why do you say the pot will instantly burn up when brighness is turned up? I don't think the power dissipated by the pot will ever get that high. What am I missing? Thanks.

 

[Sceadwian]

What the power handling of the POT? By the way, pots are horrible for changing brightness, sure it gets dimmer, but only cause the POT is burning the power off as heats, wasted energy. There are two common methods for switching brightness levels. The simplest is to.... simply use more than one circuit. Have a multi position switch which allows you to turn one two or three banks of LED's on at a time, this is the simplest solution and very efficient. The next best method I would think would be to use a 555 timer (with a current pass transistor if required) to pulse width modulate the LED's with a variable duty cycle. This would allow from about 2% to 98% brightness control. For a stupid simple variable brightness nothing beats a switch on each LED bank. The right multiposition switch is the only important part.

 

[audioguru]

Most pots are rated at 1/2W over the entire resistance. When it is turned to half then only half of the resistance is heating and its rating becomes 1/4W max. When it is turned so that only 10% of its resistance is used then the current is almost maximum and only 10% of the resistance is heating a lot with only a 1/20W rating.

 

[vne147]

What the power handling of the POT? By the way, pots are horrible for changing brightness, sure it gets dimmer, but only cause the POT is burning the power off as heats, wasted energy. There are two common methods for switching brightness levels. The simplest is to.... simply use more than one circuit. Have a multi position switch which allows you to turn one two or three banks of LED's on at a time, this is the simplest solution and very efficient. The next best method I would think would be to use a 555 timer (with a current pass transistor if required) to pulse width modulate the LED's with a variable duty cycle. This would allow from about 2% to 98% brightness control. For a stupid simple variable brightness nothing beats a switch on each LED bank. The right multiposition switch is the only important part.

Sceadwian,

I don't know what the power handling of the pot is but I have seen panel mounted pots up to 5W. This pot will not have to dissipate nearly that much power. I haven't run the numbers yet but I'm going to now because I don't understand why using the pot is a problem. I understand why it is inefficient but efficiency was not my goal, simplicity was. I hadn't considered using a switch like you suggested but that would be simple too. I completely agree with you about there being more efficient ways of controlling LED brightness but you've been following this thread so you'll probably agree when I say that I didn't think this guy was willing to delve into a PWM solution for his problem.

Most pots are rated at 1/2W over the entire resistance. When it is turned to half then only half of the resistance is heating and its rating becomes 1/4W max. When it is turned so that only 10% of its resistance is used then the current is almost maximum and only 10% of the resistance is heating a lot with only a 1/20W rating.

Audioguru,

I think I follow what you are saying. The power dissipation capabilty of a pot approaches zero as it's resistance approaches zero. But doesn't the voltage drop across it and therefore its need to dissipate power also go to zero? I mean it seems there has to be some breaking point or else any pot as soon as it was adjusted from 0 resistance and there was the slightest amount of current flowing through it would burn out.

 

[audioguru]

When the pot is turned down a little from max brightness then its current is very high. The dissipation is the current squared times the resistance of the small portion of the total resistance so it burns out.

 

[grimxneko]

ok, so i pretty much understand the inefficiency of using a pot in this situation. now, ultimately, i think i like "MR RB"s suggestion the best: to use a LM317 IC with a 15ohm resistor. but i just want to know if it's possibe to use that idea and still be able to control the overall brightness manually (and if yes, how?). like, what if i used the Lm317 and a 555 dimmer or PWM; would something like that work? though, if that yeilds some complicated or inefficient solution, i suppose i could just leave the brightness adjustment fearure out. but i still want to know my options before i decide. -thanks-

 

[audioguru]

Add a 500 ohm ohm pot in series with the 15 ohm resistor in the LM317 current regulator circuit. When the pot is max then the current is reduced to only (1.25V/515 ohms=) 2.4mA plus the idle current of the LM317.

You must use a 12V battery because 9V is not enough.