Simple Question about LEDS and Resistors

[Ajaylat]

First off, I want to say that I'm not an electrician and don't know much about electricity.

My question... Is it ok to have two resistors connected to a 3mm led? Please don't ask why two resistors.

I just want to know if it would harm the led or have any kind of negative effect on the circuit or whatever. The two resistors may or may not have different ohm values.

I know this sounds like a dumb question, but I just need the answer. Here's a picture to illustrate what I'm talking about:



Thank you very much!

 

[Marvo]

Hi Ajaylat. I'm not sure I fully understand your question. Putting two smaller resistors in series with an LED will create a voltage divider circuit that's the equivalent of just have a single larger resistor equal to their sum in series. If the resistors are in series then they won't both be connected to the same 3mm LED so I really don't understand your question.

Maybe you can clarify.

 

[uksparks]

The only thing I can see it doing is adding a resistance that's going to make the LED dimmer than without them, I don't know the resistance of the LED or the resistors but I would say its just like adding another LED in series in effect, it's just going to be dimmer if not totally zapped for power and no light up at all.

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As Marvo says, we could do with a bit more clarity and info really.

 

[shanky887614]

Probably asking cause of dimmers

 

[bawall]

use this to work out the resistance required to drive a led LED calculator for single LEDs

the resistance is the total of both resistors in series R1 + R2 etc.

 

[tazz]

I presume this is a college project to find the resistance required to limit the current and voltage to an led....
Ok the formula is R= (Vs - Vl) / Il This is to say R= resistor required, Vs is supply voltage, Vl is led forward voltage and Il is the led running current.
So we know a standard led has a forward voltage of 1.3v and its running current is usually 20mA
Therefore at 12v supply, battery we get R= (12-1.3) / 0.020 =535 ohms
Hope this helps

 

[Ajaylat]

Ok thanks, uksparks! I wished I hadn't asked that question.

I bought this led strand:


I want to remove the individual leds and replace them with these ones:


My question is... Do I need to add a resistor for each cluster of leds? Or does the battery pack already contain a resistor. If yes, how do I know if the battery pack's resistor is appropriate for the ebay leds?

Hope my question makes sense.

 

[Marvo]

Hope my question makes sense.

Unfortunately no it doesn't.

You want to replace the LED's in a battery operated fairy light with LED car lamps?

It's not possible to answer your question without more information, can you post some technical specifications for both of these items.

 

[Ajaylat]

Yes, I want to replace them with the LED car lamps. I don't have any more technical specifications besides what's posted on the web sites.

Would the battery operated fairy light work if I replace the LEDs? That's really what I need to know.

 

[Ajaylat]

Here are the links to the websites.

Battery operated fairy light:
w w w.lightsforalloccasions.c o m/p-3484-silver-led-fairy-light-garland-23-foot-battery-operated-multicolor.aspx


3mm LEDs:
w w w.ebay.c a/itm/3mm-5mm-Flat-top-2pin-bright-waterclear-Bulb-Wide-Angle-long-leg-DIY-car-lamp-/360728040702?pt=LH_DefaultDomain_0&var=&hash=item53fd1114fe

 

[tazz]

what you have purchased, is a set of 48 leds set out in parallel and series config.....so would say 3-4 leds are in series at 4.5v (back to formula each led has forward voltage of 1.3v) so would say there are most likely 12 sets of leds in parallel with each set being made up of 4 leds in series..........you can only replace like for like...so leds must be standard 1.3v types

 

[Marvo]

The battery operated fairy lights are supplied through a '6 function switch' which probably has built-in dimming and switching and probably is also a powersupply. The LED's in the fairy light could also be connected in series groups to effect some kind of voltage control across them whereas the the car LED's will have built-in circuitry that requires a 12VDC input to each one and possibly far higher driving current.

Bottom line is they're not compatible.

 

[tazz]

Just seen post on 12v car led......deffo no, also possible damage to controller.....need to look at standard leds, from electronic stores

 

[Ajaylat]

Ok I get it! Thanks!! Most of the leds sold on eBay seem to have a forward voltage of 3.2 ~ 3.8.

 

[Ajaylat]

Hi guys,

I am bringing up an old topic, but I just started to work on this again, and I would like to ask a question.

I have not been able to find 1.3v (colorful) leds. I was wondering if these leds from ebay would be compatible with the battery operated fairy lights:

I would cut each individual leds and replace them with the micro leds that are on the battery operated fairy lights. Would this work?