I need assistance with bulbs and resistors..

Thread Starter

Techster151

Joined Jan 24, 2019
4
Idea Source:
I have a project at hand and need some assistance from the geeks out there. (I hope to return the favour someday...)
If you view the attached image and the video linked, you may get a nice idea about what I am trying to do. I have replaced the coin cell battery with 9v batteries. In the image red lines represent bulbs and the black lines represent the tracks of foil (both positive and negative). All the bulbs are 3v. In the image, in the top part 5 bulbs are connected to 1 9v battery. In the middle, 5 bulbs are connected to 1 9v battery and in the bottom, 6 bulbs are connected to 1 9v battery. The problem is when turned on some lights flicker and some don't turn on (the mount board on which they are placed on is a little curved) when adjusted some light up)(I tried changing the bulbs). Do I need resistors? If yes please tell where should I place them? Thanks for your help in advance...:)
 

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Alec_t

Joined Sep 17, 2013
10,907
Welcome to AAC!
Are the 'bulbs' filament type or LEDs?
How much current does each draw?
Can you post a link to their specification?
Are the connections soldered joints or just twisted wires?
 

Thread Starter

Techster151

Joined Jan 24, 2019
4
Welcome to AAC!
Are the 'bulbs' filament type or LEDs?
How much current does each draw?
Can you post a link to their specification?
Are the connections soldered joints or just twisted wires?
Thankyou for replying!!
the bulbs are leds. Each draws 3 v. I bought it from a local store and there is no way I would get specifications. The batteries are just taped to the foil with battery connectors.
 

Alec_t

Joined Sep 17, 2013
10,907
Each draws 3 v.
That is not the current draw; it's the nominal forward voltage (Vf). 3V implies either a white or blue LED.
Your present arrangement has a number of problems:
1) The Vf of LEDs varies quite a bit from one sample to another, so connecting them in parallel across the foil tapes means that the ones with the lowest Vf will hog more current than the others, leading to uneven brightness.
2) Without series resistors to limit current, battery life will be short and some LEDs may have a short life.
3) Sticking LED wires to aluminium foil strips does not make reliable connections. Thin tinned copper wire (perhaps stripped out of a length of stranded cable) would be better than the foil and can be soldered to LED wires.
4) Parallel connection of all LEDs of a group is not the most efficient way. With a 9V battery you will be 'wasting' about 6V. It would be better to connect 2 LEDs and a current-limiting resistor in a series string and then connect seversl strings in parallel. If you choose a LED current of, say, 5mA then the resistor value per string would be (9V-2*3V)/5mA = 600Ω. The nearest common value would be 560Ω.
 
Last edited:

dl324

Joined Mar 30, 2015
10,003
For tape traces I use self stick copper foil tape from a stained glass store or internet.
Yup. Don't buy tape sold for electronics - it's much more expensive. IIRC, the 1/2" stuff they sell as a slug repellant is the most economical.
 

Thread Starter

Techster151

Joined Jan 24, 2019
4
That is not the current draw; it's the nominal forward voltage (Vf). 3V implies either a white or blue LED.
Your present arrangement has a number of problems:
1) The Vf of LEDs varies quite a bit from one sample to another, so connecting them in parallel across the foil tapes means that the ones with the lowest Vf will hog more current than the others, leading to uneven brightness.
2) Without series resistors to limit current, battery life will be short and some LEDs may have a short life.
3) Sticking LED wires to aluminium foil strips does not make reliable connections. Thin tinned copper wire (perhaps stripped out of a length of stranded cable) would be better than the foil and can be soldered to LED wires.
4) Parallel connection of all LEDs of a group is not the most efficient way. With a 9V battery, you will be 'wasting' about 6V. It would be better to connect 2 LEDs and a current-limiting resistor in a series string and then connect several strings in parallel. If you choose a LED current of, say, 5mA then the resistor value per string would be (9V-2*3V)/5mA = 600Ω. The nearest common value would be 560Ω.
How can I connect them in series (I have to use foil tape) as in all the videos they are connecting in parallel. If you use one line(of foil tape) you are connecting both the negative and positive ends of the battery to the same place and you aren't allowed to do that because then it wont work. I placed a switch on one of the tracks and it works...so I think a resistor should too. I just need to know the places I need to place it in or at least how to know (calculate and know) where to place them. Can I calculate if I get my hands on a multimeter? If yes, then how?
 

Alec_t

Joined Sep 17, 2013
10,907
How can I connect them in series (I have to use foil tape)
Here's how one LED string could be connected to copper foil tapes :-
LEDstring.PNG
Resistor-to-LED and LED-to-LED wire joints should be soldered.
For ~5mA LED current in this string the resistor value would be 560Ω. For ~9mA, use 330Ω.
If the two LEDs of a string are further apart than their lead wires combined, then a third length of tape could interconnect them.
 

Bernard

Joined Aug 7, 2008
5,395
Who is setting project requirements?
Best practice & how it is done do not always agree. I was asked to make a string of 50 LEDs, 3 AA battery box
with a small ckt. board throb. Assumption made that LEDs were series parallel with a boost ckt. but no- they were all parallel with one resistor; ckt. board was a timer. With throbbing average total current is 35 mA. Later found
20 & 50 LED strings from Garden Décor that looked the same.
With no guarantees you might match Vf of a bunch of LEDs & make strings of = Vf with a resistor to give = current for each string.
 

Thread Starter

Techster151

Joined Jan 24, 2019
4
Here's how one LED string could be connected to copper foil tapes:-
View attachment 168677
Resistor-to-LED and LED-to-LED wire joints should be soldered.
For ~5mA LED current in this string the resistor value would be 560Ω. For ~9mA, use 330Ω.
If the two LEDs of a string are further apart than their lead wires combined, then the third length of tape could interconnect them.
Thankyou so much. This really will be of assistance will tell you if it works...Btw, will they always be either 5/9 mA and what about the voltage?
 

Bernard

Joined Aug 7, 2008
5,395
How much current for an LED, depends on many things, power availability, viewing conditions, & personal preference. If AC power is available & portability is not a problem I'd go for an AC adapter like in a recent project, a 48 V output driving 10 series connected bright 3W LEDs with a 40 ohm 12W resistor. For brightness
try different Rs & a battery , be surprised on how bright a LED can be on 1 mA.
Another case portability was required so 3 AAAs, boost converter, 4 LEDs in series, 8 strings giving a marquee
effect. Think it was 15V & 300 ohm series R. LEDs spiral wound on a pointy hat. Or from Macado, " make the punishment fit the crime ".
 

Alec_t

Joined Sep 17, 2013
10,907
Btw, will they always be either 5/9 mA and what about the voltage?
If the supply voltage is Vs, the number of LEDs in series in a string is N, the forward voltage rating of each LED in that string is Vf, the chosen LED current is ImA, and the resistor value is R , then these values are related by the equation ImA=1000*(Vs-N*Vf)/R.
 

Norfindel

Joined Mar 6, 2008
326
You could use 4 AA or AAA cells. That's 6v. Then, assuming every led is around 3.2v Vf, to get 5 mA per led, you will need to connect one 560 ohm resistor in series with every led, and then connect the other leg of the resistor and the other leg of the led to the battery. (6v - 3.2v) / 5 mA = 560 ohm. You can check the brightness with one led, and then decide if it's ok, or you need more or less current.
If you're planning to use nimh rechargeables. They have 1.2v and not 1.5v like normal primary AA or AAA cells.
 
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