3 banks of 5 Leds in series and an mains power supply, does this require resistor?

[kielo]

Firstly ... hi everybody,

i have a bit of an odd question regarding LEDs and a wall powered power source.

Basically... its for a friend who has asked for 3 "LED baseball hat lights" ( **broken link removed** ) to be wired up to a mains power source.


the "led cap lights" consist of 5 leds (super white, probably 3.3v and 25ma), these were initially powered by 2 X CR2032 batteries in series with output at 3v so i guess its a 6v to power leds.
From here on i will call each bank of 5 leds a set... so there are 3 sets of lights.


the 3 sets have already been wired together by a friend who said we only need to connect to a power source. (he said 9v... i think it should be 6v)
see picture of the internal (on spare) showing that the leds are wired in parallel...



there are two wires.. red and black... these are soldered onto the battery terminals on the sets...
set 1 has two red connected to positive, one leads to set 2 positive and the other is connected to the power supply positive. the black is connected in same way but to negatives.



set 2 is connected to set 3 with the red (positive) going to positive terminal and black to negative. (and previously mentioned connections).



set 3 ONLY has the connections to set 2 with red for positive and black for negative.





my questions are:

1. is this correct wiring?
   
2. if i have a power supply of 6v (switch-able upto 12v) dc (max out 1.5 amp) will i need a resistor in place? i was told not but the leds blew! but i was also told to set at 9v...
   
3. if i do need resistor where should it go? as it seems a mix of parallel and series

...

any help on this would be greatly appreciated.
please dont say to rebuild as this is just for prototype and so i will make a proper set of banks for friend (where i know more of what im doing, and resistor for each led!), we just need this working asap!


once again any help would be great as i just cant work this one out!

thanks

kielo

 

[vne147]

Each LED does need some way to limit the amount of current flowing through it or it will burn out. A resistor is usually the easiest way to do that. If the hat lights worked fine and the LEDs didn't burn out when powered from batteries, then there are likely resistors already in the circuit somewhere. Or the current was limited by some other means. Not sure which is the case with you.

What's on the other side of that little circuit board in your first picture? Can you post some clearer pictures showing both sides of the board and the traces?

As far as how it's wired up, it looks right to me. The 3 sets are in parallel.

 

[crutschow]

Likely the resistance of the button batteries was limiting the LED current.

With a low resistance source, such as a line power supply, you must add resistors to limit the current.

 

[vne147]

Likely the resistance of the button batteries was limiting the LED current.

With a low resistance source, such as a line power supply, you must add resistors to limit the current.

If those 5 LEDs in each set are in parallel, what balances the current between the 5 LEDs?

 

[kielo]

if i switched the voltage power supply to 3v ... would i need a resistor at all?
given tat tey are rated at 3.3v (well average for super bright whites are)... would 3v power them? or would they try to use too much current?

 

[crutschow]

You always need some resistance in series with LEDs to limit the current. Otherwise they will take as much as they can and blow.

 

[vne147]

if i switched the voltage power supply to 3v ... would i need a resistor at all?
given tat tey are rated at 3.3v (well average for super bright whites are)... would 3v power them? or would they try to use too much current?

Are there any components on the other side of that circuit board aside from the LEDs?

There are two blue wires coming from what looks like a switch that are soldered into the circuit board. What are they soldered to?

What is the red wire soldered to on the circuit board?

Clearer pictures of both sides of the board would be really helpful.

 

[kielo]

hummmm, post didnt work...

ok,
the other side of board is blank, only has the leds poking out.
the blue wires are to push switch: on (flash), on (permanent) and off.
the red wire goes from battery to some white stuff on circuit. (the blue wires also go to that).


i wired it up and put it on with 3v and they all switched on.
should i just put a 1ohm resistor on? or.... do i need to put a much bigger one on?

i really am confused... if i put on a 1ohm resistor will it stop alot of current? i thought it hit voltage? and that electronics only withdraw as much current as they need? ? ?

 

[crutschow]

i really am confused... if i put on a 1ohm resistor will it stop alot of current? i thought it hit voltage? and that electronics only withdraw as much current as they need? ? ?

Most electronics do only draw the current they need but LEDs are one exception. They are current devices and will tend to take all the current you give them, so it must be limited by a resistor.

The correct resistor value is R = (Vs - Vled)/Iled where Vs is the supply voltage, Vled is the rated LED voltage times the number of LEDs in series, and Iled is the desired LED current.

 

[vne147]

hummmm, post didnt work...

ok,
the other side of board is blank, only has the leds poking out.
the blue wires are to push switch: on (flash), on (permanent) and off.
the red wire goes from battery to some white stuff on circuit. (the blue wires also go to that).


i wired it up and put it on with 3v and they all switched on.
should i just put a 1ohm resistor on? or.... do i need to put a much bigger one on?

i really am confused... if i put on a 1ohm resistor will it stop alot of current? i thought it hit voltage? and that electronics only withdraw as much current as they need? ? ?

You say this thing has a blinking mode. Because of that I'm not really clear on everything that's going on in this circuit. I don't know if this will work but without knowing anything else about the circuit, here's what I would try first:

1. Disconnect one of the sets from the other two.
   
   
2. Put the batteries back into the disconnected set and turn it on.
   
   
3. Measure the voltage across one of the LEDs and call it V1.
   
   
4. Measure the current being supplied by the batteries and call that I.
   
   
5. Calculate the size of the resistor you need by using the equation R = (VS-V1)/I. Where VS is the supply voltage (either 6V or 9V based on your previous posts).
   
   
6. Solder the resistor between the positive battery terminal and the red wire. You'll need one resistor per set so 3 total.

If you don't have a multimeter or don't know how to do any of what I said, just choose a resistor and try it out. Start high at first and gradually decrease the value until you get the brightness you want without burning out the LEDs.

For a supply voltage of 6V any value between 50Ω - 100Ω should be a good starting point. When in doubt, always start with the higher value.

 

[kielo]

ahh, will give a load of resistors a go!
ps thanks to all for help!

 

[vne147]

ahh, will give a load of resistors a go!
ps thanks to all for help!

You're welcome. Come back if you need any more help.

 

[kielo]

cheers, will do

think a redesign of a well planed circuit will be better and done for a proper job but thats easier!

 

[4pyros]

the blue wires are to push switch: on (flash), on (permanent) and off.

If this is the case it has a flash circuit under that white blob and should have current limiting built in.
Try with your "load of resistors" first and if thay are not as bright as with the batterys than hook them up to 6 volts DC.
Andy

 

[kielo]

seems to be working fine on 3v.
i am just a bit worried that if its trying to pull the 3.3v it might draw too much current and get hot after a while... i think thats how it works??? or i could just be exhausting my electronics ability and getting fluff in the brain!

 

[kielo]

ps, thats working fine on 3v supply

 

[4pyros]

It has a flash circuit under that white blob and should have current limiting built in.

 

[kielo]

sweeeeeeet. so no need for resistors? should i boost power to 4.5v? guess its ok at 3v so....

 

[4pyros]

should i boost power to 4.5v?

No because you dont know whats under the white blob!
Andy

 

[vne147]

Didn't you say that you already tried to just put 6V to it but the LEDs blew?