Gary Murphy 1 Posted February 18, 2019 Share Posted February 18, 2019 I need to fit led bulbs in a project and never used them before. A diagram shows a resistor fitted at the bulb. I will need to bend really tight to the bulb. Can I fit the resistor away from the bulb? ideally if I could fit it 12 inches away that would be great. If that is not going to work how close will it need to be? Quote Link to comment Share on other sites More sharing options...
Fun Flyer Posted February 18, 2019 Share Posted February 18, 2019 You can now get LEDs with an inbuilt resistor. Quote Link to comment Share on other sites More sharing options...
Barryorbik Posted February 18, 2019 Share Posted February 18, 2019 The resistor can be fitted anywhere in the LED leads and is not polarity conscious, but do you know the value and Wattage of the resistor you need? To work this out you would need to know the LED voltage and current. If you post the supply voltage and the LED ratings on here, I am sure some kind person will calculate the resistor value and Wattage rating for you. Barry Quote Link to comment Share on other sites More sharing options...
Plummet Posted February 18, 2019 Share Posted February 18, 2019 But to answer your original question... It does not matter where in the circuit the resistor is as long as it is in series with the led. All the current must flow through the led and the resistor. (As opposed to in parallel with it. If they are in parallel then current can flow through either the led or the resistor.) What is important is that the led is fitted the right way round with respect to +ve and -ve. Plummet Edited By Plummet on 18/02/2019 17:30:07 Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted February 18, 2019 Share Posted February 18, 2019 We all know V = IR Gary, and they are rated 0.020A or 20ma for your calculation They are not bulbs as there is no filament, as they emit light when power is applied to the diode in the right direction For polarity on an unmarked LED, look through it and you will see a large flag shape, that is minus,so follow that shape down for the negative connection As a rule of thumb, you need 75 Ohms per volt protection So 2v is in the order of 150ohms resistor Quote Link to comment Share on other sites More sharing options...
Barryorbik Posted February 18, 2019 Share Posted February 18, 2019 Another important point Gary is that the manufacturers normally recommend not bending the legs too close (3mm away minimum) as it can break the seal to the LED, or in the worse case the wires may snap off, as these days they are made of steel, which is not as malleable as older copper ones. Barry Quote Link to comment Share on other sites More sharing options...
Plummet Posted February 18, 2019 Share Posted February 18, 2019 Posted by Barryorbik on 18/02/2019 17:23:16: <snip/> ... I am sure some kind person will calculate the resistor value and Wattage rating for you. Barry Um... An example showing my working method. From the spec of the led find the forward voltage drop and the working current. The volt drop is the voltage change across the led when it is shining. It varied depending on the colour and brightness of the led. The current rating will be higher for brighter leds. Lets call the volt drop D. It will be about 2.0V A typical led will be rated at 15mA that is 0.15 Amps Lets call it I What is your battery voltage? Lets call it Vb, and I'll pretend that it is 12volts The resistor needs to ensure that the led gets the correct voltage across it. The led is dropping D volts, so the resistor needs to drop Vb -D volts. Dear old Ohms law says V = I R The voltage (in Volts) across a resistor equals the current (in amps) multiplied by its resistance(in Ohms) Reorganising this gives us R = V / I The volt drop across the resistor is Vb - D, so R = (Vb - D) / I which with our numbers is R = (12 - 2) / 0.15 = 66.6 ish Ohms OK? Plummet Quote Link to comment Share on other sites More sharing options...
Former Member Posted February 18, 2019 Share Posted February 18, 2019 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted February 18, 2019 Share Posted February 18, 2019 Plummet, 666 ohms, as the 0.15 should be 0.015A Noughts always get me too Quote Link to comment Share on other sites More sharing options...
Gary Murphy 1 Posted February 18, 2019 Author Share Posted February 18, 2019 Once again you guys are great, all the answers I needed right there. Electrics of any sort is like black magic to me. I have gone to the darkside and building a Tamiya truck for my grandson. Tamiya do a sound/light unit but I have read you can get better and cheaper. The sound side I have worked out,just lights now. I do not know what voltage the leds need to be BUT it will run at 7.2/8.4v 6 cell nimh packs. Rather than run several bulbs this unit can vary the voltage to the led so one bulb can be dipped ,main and high beam.If I have read correctly. still trying to find mention of led voltage needed. Quote Link to comment Share on other sites More sharing options...
Plummet Posted February 18, 2019 Share Posted February 18, 2019 Posted by Denis Watkins on 18/02/2019 18:11:14: Plummet, 666 ohms, as the 0.15 should be 0.015A Noughts always get me too Ta, Expletive! Of course, instead of using Amps and Ohms you can use mA and kOhms and the sums come out correctly usually without the pesky decimal points. Plummet Quote Link to comment Share on other sites More sharing options...
Plummet Posted February 18, 2019 Share Posted February 18, 2019 This page has some nice stuff... There is a pink table some way down that gives some sample values. http://dangerousprototypes.com/docs/Basic_Light_Emitting_Diode_guide Plummet Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted February 18, 2019 Share Posted February 18, 2019 Nothing wrong with all the info given above, but to make things easy I use an online calculator to figure the correct resistor value. Quote Link to comment Share on other sites More sharing options...
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