Moderator Edit:

Welcome to AAC!

A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

This was split from Low/High setting on LED

Sorry for the bump, but figured it better than a new thread on same (ish) issue.

I recently broke a decent work lamp and am going to repair it (as my first electronic project as will hopefully learn lots).

As if I've not got a steep enough learning curve, I also want to make a modification to, and hopefully your expertise can help.

My mod/issue is that I'm looking to have it run on half brightness and full brightness, rather than just on and off (it was very bright when dark, thus looking to have half brightness).

It has 40 LEDs, and as is battery powered, so I wonder if:

Is it better (more efficient) to light up 20 LEDs on 'mid mode' and then all 40 of them on 'full' or

is it better to use some other 'dimming' technique to acheive the less dazzling night time functionallity?

I've read a fair few posts on 'dimming' and see PWM mentioned a fair bit, and some others that just increase resistance to lower the flow etc. So if it'll be beneficial to use another dimming technique, please can you advise what I should read up on and I'll take that away to research.

Just theory at this stage so I can go and read up (hense no specs etc), but any guidance is much appreciated.

Many thanks all,

Upex


As per this actual thread - I'm not after complicated button presses or micro controllers etc, I'll be happy with a 3 way switch etc. This is litterally just after the best method to dim the thing and not waste loads of energy via heat etc.

Could you live with a 3 position switch, off-low-high?

- Hell Yeah, sounds ideal!

 

Welcome to the forum, but you have hijacked someone else's thread. Always best to start your own (a) from politeness and (b) because would-be viewers may disregard an old thread.
Depending how your LEDs are wired and powered (e.g from an inverter) it may not be easy to separate them into 2 groups of 20. How are they connected to each other and to the battery? Can you post a schematic/sketch?

 

I have a tutorial on LEDs you might find useful, it also has circuits to do much of what you are trying to do...

LEDs, 555s, Flashers, and Light Chasers

Bill's Blog

 

Is it better (more efficient) to light up 20 LEDs on 'mid mode' and then all 40 of them on 'full' ...

I think you meant the opposite, 40 on mid versus 20 on full? The answer is, Not really. The brightness of an LED is nearly proportional to the current (and power) passing through it. So your two scenarios would deliver a similar amount of light with a similar power consumption. At least in theory - some details of the circuitry could change that.

...is it better to use some other 'dimming' technique to acheive the less dazzling night time functionallity?

PWM is a great technique for controlling LED brightness. It is efficient (compared to a current-limiting resistor, for instance) and can give you smooth control from 0 to 100% brightness. An additional advantage is color. LEDs shift in color (slightly) as the current changes. PWM delivers a steady current while the LED is on, but reduces the brightness by turning it off, not by reducing the current. This maintains a consistent color regardless of brightness. Very clever. Maybe not relevant to a simple work light.

I've read a fair few posts on 'dimming' and see PWM mentioned a fair bit, and some others that just increase resistance to lower the flow etc. So if it'll be beneficial to use another dimming technique, please can you advise what I should read up on and I'll take that away to research.

Do you know how the LEDs are currently wired? There might be a simple solution like turning off half of the lights. But there might be some sort of complex driver circuit already in place that will be tricky to mod. Can't speculate without knowing the starting point.

 

Hi all, many thanks for input so far.

Bill, thanks for moving my post (other forums I'm on for cars and bikes like to keep them linked together where poss, but I like this way better ), sorry for dumping it in the other one. Definately don't want to hijack and will ensure it doesn't happen again. Thanks for pointing it out and correcting me.

Anyhow, back on topic.

I have smashed the led board, so am going to use new board and new LEDs whilst retaining the case and battery.

I guess it could fit more LEDs in, but going on the assumption of 40 until I understand what I'm doing.

Alec, ill be starting from scratch really, so no schematic at this time. Will draft some up when know the direction regards dimming.

Bill, thanks again. I'll read the post/s you've linked to. I'm Brand nee to this, but kindda excited as to learning new stuff, especially considering the help already displayed. Nothing better than a whole wealth of friendly and knowledgeable folks to instil confidence and guide.

Wayneh, sorry if I'm off spec, but (without having read bills links yet) I assumed running 20 LEDs would be half light, ie mid, and they switching the other 20 (so all 40 are then on) would be a way to get it brighter (full power). This I assume was a way to achieve the dual mode. Ie 2 seperate circuits of 20 LEDs.

In terms of pwm, I've read a few bits on it, but not clear on how its achieved. Do I need to introduce a new part? As will need to scope out the size vs space.

I assume though, that the pwm is the way to go, ie more efficient?

Many thank all and here fingers crossed eh.

Upex

 

I have smashed the led board, so am going to use new board and new LEDs whilst retaining the case and battery.

Oh boy, so we're starting from zero as far as the circuitry. This thing is battery powered, not mains? What sort of battery? I suppose you have no specs on the LEDs, other than that they are white?

Is the smashed board still around? A good photo of both sides of it might help folks here understand how the lamp used to work, and thus learn a little about the particular LEDs you have.

I think two settings, half on and full on, should be the least of the problems.

 

Oups, sorry no. In a range I binned the broken stuff lol.

Did scope it out though and by my limited knowledge, it was pretty basic. LEDs, some resisters and thy was pretty much it.

I not got any LEDs etc yet, as wanted to figure out the half on vs dimming first, then once I know how best to tackle that, then ill look to
design the build and get components etc.

Battery wise, not sure. No markings on it or the charger, but the case opens and ill tackle that once I know where we heading regards the dim (ha ha that's me right now) and bright stuff.

 

A very simple approach would be two parallel strings of 20 LEDs in series, with a switch on each series. Each string would need the appropriate current limiting by either a resistor or a control circuit. But this would require a relatively high voltage power supply, about 60V or so, to overcome the forward voltage drop across 20 LEDs. If your power supply is only 12V, for instance, you can only put 3-4 LEDs in series for each string. So you really need to know what you have to start with.

 

Complete agreement. Half the LEDs on, then all the LEDs on. They will have to be broken up into short strings of 3 or 4, depending on the specifications, but that is dead simple to do. No fancy switching, no high frequency complications.

 

That was my thinking gents. Say 1 circuit of 21 LEDs in a 7 x 3 LEDs, for half on (dim) and the turn the other circuit on of say 18 (know that's only 39 in all, but I'm not precious) in a 6 x 3 LEDs format. Obviously the number of LEDs will depend on the led forward voltage etc, which I don't know yet lol.

The battery (i think, is 12v). Will check when I get a chance and start looking at the pars etc.

Had a quick read of some of bills pwm info and its confused me and schem looks very complex. I was hoping there was a little component that did it, that I could just put inbetween the power and LEDs type approach, but this looks well beyond me lol.

So if we take 12v as the battery and use the leds in 7x3 for 21 of them and the second part being 6x3 for 18leds. Will it be very inefficient? As I assume, and having finished reading up yet, that this will need bigger resisters and thus lose more power through heat, than having all LEDs on together with lower resisters?

Thinking it through now as I type, that doesn't sound right.

Is it true that each string of 3 LEDs would get a 12v in, and the resister would be there to reduce the 12v to the combined total of the 3 LEDs forward voltage drop?
Ie 12v to each string, the LEDs have vf of 3v, thus giving total of 9v for the three, so each string resister would dissapate 3v?

If the above is a true(ish) way that it works, then would it be more efficient to lose the resister and add in another led, ie 4 LEDs per string, total vf of 12v against a 12v supply?

If this is true, then the current would increase with each additional string added to the circuit? How would one calculate the amount of current per string / overall?

Thanks all, I think I'm learning something already (although all the above maybe bull, in which case ill retract this statement!)

Upex

 

You are correct in the ideas about 3 LEDs and a resistor. Trying to do 4 LEDs and no resistor makes the current go towards infinity. You can't do that unless you want a single use smoke bomb.

Quit worrying about efficiency. The resistor waste for 13 strings (3V @ 20 ma) is less than 1 watt.

 

I agree. Sacrificing a tiny amount of efficiency in exchange for simplicity is a good bargain.

 

Ok folks, thanks for the heads up. Didn't know how efficient or not it would be. 1 watt doesn't sound too bad lol.

So am I right in thinking the pwm idea is fairly complicated and thus, for my very first time at this, should just stick with the 20/20 approach?

Thinking as its my first go, that ill buy some cheapo LEDs and board to build version 1, then all being well and me a bit practiced at assembly can build a better version later.

Will scope out some specs and do some math regards the resisters and check back here to make sure I've got the idea and concepts (before I buy anything) and will then redo once have them, just to be sure.

Thanks for all the help, very much appreciated.

Upex

 

Take my advice: Later won't happen. Buy some good LEDs now and be done with it in one try.

and, yes. Forget the PWM circuit. You can't imagine the complications that can happen while trying to save less than one watt.

 

Prudent advice. PWM would be worth looking at if you wanted smooth dimming over the whole range. For 2 or 3 fixed settings, it's not worth it.

 

Cool, thanks folks.

I get what you say #12, but if I do make a cheap version, I get to make another and then create a case and can have 2 lights lol.

This all got me thinking about making lights for my garage as well as it ain't got power! First thing first though eh LOL.

 

Just fiddling around. If that wasted .78 watts ran for 8 hours a day, it would take 4 years to amount to $1 on your electric bill. Now, guess how much the parts will cost to save most of that .78 watts...
Then think about how many hours you are willing to work to save 25 cents a year.

 

#12 how cheap is your electric lol. I spend about £75 a month on mine, and only have a small house.

Guess its like comparing you gas to our petrol lol. $2+ dollars per litre, so amounts to rip off Britain LOL. And let's not go near our insurance prices.

I wish I lived in the states :-(

 

Ok, so having looked up some LEDs I've picked some to run through the math.

I gives a Vf of 3.2 typical and 4 max at 20ma

I'm not sure if I use typical or max Vf though, so using the typical, I get:

Max of 3 LEDs per string with total Vf of 9.6.

12v - 9.6 = 2.4v to get rid of.

2.4v / 0.02a = 120 ohm resister per string of 3 LEDs.

How am I doing? Lol.

If I need to calculate based on 4v max, I get 0, would I add the lowest resister possible and restrict them a bit to be safe? Ie better than no resister.

Having looked at loads of LEDs I see many variations. Am I right in assuming that I could different LEDs for the dim (first half of LEDs) compared to the second circuit, so that the second set are brighter spec? If so I assume I just treat them seperately and work out resister value for each circuit?

Thanks all

 

Something else to consider is the '12V' supply. Depending on your battery state of charge and type the voltage could be lower or higher and will vary with time. You should design for the worst case extremes of LED Vf and supply voltage.