# How To Choose The Right Resistor **URL:** \ **Category:** Guides\ **Created:** [18 July 2018 03:33 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117 "2018-07-18T03:33:49Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Clinton](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/clinton/32/1515_2.png) [@Clinton](https://forum.core-electronics.com.au/u/Clinton)\ **Post date:** [18 July 2018 03:33 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/1 "2018-07-18T03:33:49Z")
Clinton just shared a new tutorial: [**"How To Choose The Right Resistor."**](http://core-electronics.com.au/tutorials/how-to-choose-the-right-resistor.html) ![](https://canada1.discourse-cdn.com/flex028/uploads/core_electronics/original/2X/a/ac2e4ca44809f4c6dd9b3a39b368b898f1480efb.jpg) Resistors are the hardworking underdogs of the electrical circuit world. You have probably added a 4.7k or 330-ohm resistor into your circuit without thinking about why we use those values, let alone what the resistor is doing. We are going to have a… [**Read more**](http://core-electronics.com.au/tutorials/how-to-choose-the-right-resistor.html) ---
**Author:** ![Darryl132568](https://avatars.discourse-cdn.com/v4/letter/d/cdc98d/32.png) [@Darryl132568](https://forum.core-electronics.com.au/u/Darryl132568)\ **Post date:** [7 August 2020 07:28 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/2 "2020-08-07T07:28:57Z")
g’day, I’m looking at building a bridge (extension lead) between the trailer plug on my car and the led lights on my trailer. Plan is to mount a die cast box in the extension lead and place 150 ohm resistors in it so that the low power draw of the led lights stop registering as faulty. What sort of resistors would you recommend, and how much would they cost? ---
**Author:** ![Oliver](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/oliver/32/3315_2.png) [@Oliver](https://forum.core-electronics.com.au/u/Oliver)\ **Post date:** [11 August 2020 01:18 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/3 "2020-08-11T01:18:58Z")
Hi Darryl, It’s hard to give a definite answer without a lot more information - LED forward voltages, electrical specifications on the flasher relay/circuit, etc. If you want 150 ohm specifically, I’d suggest 3 of these 100 Ohm power resistors: > **[Power Resistor 100 Ohm 10W](https://core-electronics.com.au/power-resistor-100-ohm-10w.html)** > > High Power Package. Wired like so to get you 150 Ohms: ![image](https://canada1.discourse-cdn.com/flex028/uploads/core_electronics/original/2X/b/bba51b3e534e9f27e126a96fd3cf658d5926ab23.png) But to determine something more specific, you would need to know details of your LED bulb (particularly the forward voltage and rated current), and details of the flasher relay (or the power of your indicator bulbs as a proxy for this information). FYI In modern vehicles indicator flashing is often handled electronically, but traditionally it was done electro-mechanically using a flasher unit (a bi-metallic strip and a heater coil). In this case, flashing rate is dependent on the current draw of the indicator bulbs. The goal of adding a resistor to the circuit is to adjust the equivalent circuit resistance in order to get a similar enough current draw that the flasher unit behaves ‘normally’. Regards, Oliver Support | Core Electronics ---
**Author:** ![Evegenios188152](https://avatars.discourse-cdn.com/v4/letter/e/3d9bf3/32.png) [@Evegenios188152](https://forum.core-electronics.com.au/u/Evegenios188152)\ **Post date:** [19 January 2022 23:58 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/4 "2022-01-19T23:58:24Z")
Hi Darryl, I have a HX 1978 Holden Ute and I am using 12 volt 21 Watt load resistors, You need two but they have to be wired indipendently , one for the left the other for the right. \*\*_Do not wire them together_.\*\*Excuse my spelling, i’m greek. Attach to a heatsink ,away from plastic, they get hot. solder the wires and use heatshrink. They will stand out in the crowd. Mine came in the package. ---
**Author:** ![Robert93820](https://avatars.discourse-cdn.com/v4/letter/r/ac91a4/32.png) [@Robert93820](https://forum.core-electronics.com.au/u/Robert93820)\ **Post date:** [20 January 2022 03:30 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/5 "2022-01-20T03:30:11Z")
Hi Darryl Firstly what vintage is your car. If it like mine with computers everywhere (6) it will have a little microswitch in the trailer plug or something to tell the system a trailer is connected. This would include turning off the rear parking sensors. If this is the case it will probably have a CAN bus running around for inter computer communication and general vehicle well being. There are LEDs around which are “CAN bus compatible”. That is they don’t report a failed globe just because have fitted LEDs to replace incandescent bulbs. For instance Jaycar have a range both screw and double (stop and tail) bayonet and single (stop/turn etc) bayonet which are CAN compatible. If your car is older and does not have all these goodies connecting a trailer generally meant the flashing indicators just flashed slower, so fitting LEDs would in the trailer would just make them return to normal speed. Personally as long as the lights flashed I wouldn’t worry about it. If a failed bulb is going to be reported somewhere it means the presence of CAN bus so use the compatible LEDs. Cheers Bob ---
**Author:** ![Robert93820](https://avatars.discourse-cdn.com/v4/letter/r/ac91a4/32.png) [@Robert93820](https://forum.core-electronics.com.au/u/Robert93820)\ **Post date:** [20 January 2022 04:22 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/6 "2022-01-20T04:22:51Z")
Hi Evegenios > [@Evegenios188152](#): > > I am using 12 volt 21 Watt load resistors, I have never heard of a resistor being quoted in those terms. This would equate to 6.85Ω. Is that correct ?? Cheers Bob ---
**Author:** ![Jeff105671](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/jeff105671/32/3729_2.png) [@Jeff105671](https://forum.core-electronics.com.au/u/Jeff105671)\ **Post date:** [20 January 2022 05:14 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/7 "2022-01-20T05:14:07Z")
> [@Robert93820](#): > > have never heard of a resistor being quoted in those terms It’s a resistor in common automotive use. See: [Load Resistor - LED, 12V, 21W | Supercheap Auto](https://www.supercheapauto.com.au/p/narva-load-resistor---led-12v-21w/582919.html) ---
**Author:** ![Robert93820](https://avatars.discourse-cdn.com/v4/letter/r/ac91a4/32.png) [@Robert93820](https://forum.core-electronics.com.au/u/Robert93820)\ **Post date:** [20 January 2022 06:33 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/8 "2022-01-20T06:33:59Z")
Hi Jeff I realise it is a resistor to put across a LED bulb to simulate a 21W incandescent but that still does not specify exactly what it is. SupercheapAuto can probably call it what it likes and charge accordingly but still does not make it correct. It does not alter the fact that I started in this business in 1964 and the statement > [@Robert93820](#): > > I have never heard of a resistor being quoted in those terms is true. I haven’t. Cheers Bob ---
**Author:** ![Clinton](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/clinton/32/1515_2.png) [@Clinton](https://forum.core-electronics.com.au/u/Clinton)\ **Post date:** [21 January 2022 15:24 UTC](https://forum.core-electronics.com.au/t/how-to-choose-the-right-resistor/3117/9 "2022-01-21T15:24:50Z")
You can work this out from Ohms law and power relationship, I’ll skip the algebra but… Resistance = Voltage^2 / Power = 144/21 =6.8571 so probably a 10 Ohm resistor would do the trick. Just make sure you pick one that is over is rated over 21W or you will make fires, like a former content maker I will not name.