Trailer leds dim indicators

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Handle9

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#2424598 20-Feb-2020 16:21

Technofreak:
Handle9:

Resistance is always measured in ohms but a resistor can be described by other things.

Just pop into Jaycar or try to order a 12V 21 watt resistor through Element 14 etc. Don't think you'll find there's such a beast.

The wattage is the maximum amount of power the resistor can dissipate without damage to the resistor. Resistors are sold/named by their resistance and the wattage they can dissipate. Voltage doesn't come into the equation.

Lol. Ohms law is V=IR

Voltage is very firmly in the equation as that is how you determine resistance.

12V nominal resistors are very commonly used in this application and they are described as 12V 21W or 12V 50W etc

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Tonydrum

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#2424630 20-Feb-2020 18:49

Ok so I found the problem. I was trying to get everything going with the ignition on but not with the truck running. I started the truck and everything works all lights nice and bright. So Iv wasted a few days playing with it lol 😂
At least I learned a fair bit about load resistors 😂 thanks for all the help

Tony

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#2424690 20-Feb-2020 20:51

Tonydrum: Ok so I found the problem. I was trying to get everything going with the ignition on but not with the truck running. I started the truck and everything works all lights nice and bright. So Iv wasted a few days playing with it lol 😂
At least I learned a fair bit about load resistors 😂 thanks for all the help

Tony

I'm pleased you solved your issue. Very often the solution is very basic.

I think you mentioned you measured 9 volts at a bulb connection. Assuming this was also your battery voltage, that is a pretty low voltage for a battery. Either you flattened your battery to this level during your testing or your battery may be on the way out.

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Technofreak

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#2424698 20-Feb-2020 21:15

Handle9: Lol. Ohms law is V=IR

Voltage is very firmly in the equation as that is how you determine resistance.

12V nominal resistors are very commonly used in this application and they are described as 12V 21W or 12V 50W etc

You don't need to school me on Ohms Law. It's something I've been very familiar with for over 45 years. When you can go to an electronics component supplier like Element 14 or RS components and buy a "nominal 12V resistor" then I might take some notice of what you have to say on resistors. I rather suspect anyone asking for such a component would be laughed at.

Narva and the like have taken a standard 6.8 ohm, 25 watt resistor, a bit like this the one in the picture, but with pigtails and marketed it as a 21 watt load resistor when used in a 12 volt system. The same resistor when used in a 6 volt system could be marketed as a 5.3 watt load resistor.

This makes it easy for anyone without knowing how to work out what size resistor they need to wire up led lights in place of incandescent bulbs. I suspect it also allows Narva etc to sell a basic component at a very good mark up.

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Handle9

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#2424710 20-Feb-2020 22:13

<p>

Technofreak:
Handle9: Lol. Ohms law is V=IR

Voltage is very firmly in the equation as that is how you determine resistance.

12V nominal resistors are very commonly used in this application and they are described as 12V 21W or 12V 50W etc

You don't need to school me on Ohms Law. It's something I've been very familiar with for over 45 years. When you can go to an electronics component supplier like Element 14 or RS components and buy a "nominal 12V resistor" then I might take some notice of what you have to say on resistors. I rather suspect anyone asking for such a component would be laughed at.

Narva and the like have taken a standard 6.8 ohm, 25 watt resistor, a bit like this the one in the picture, but with pigtails and marketed it as a 21 watt load resistor when used in a 12 volt system. The same resistor when used in a 6 volt system could be marketed as a 5.3 watt load resistor.

This makes it easy for anyone without knowing how to work out what size resistor they need to wire up led lights in place of incandescent bulbs. I suspect it also allows Narva etc to sell a basic component at a very good mark up.

So you are saying that resistors are sold as 12V 21W after saying that there was no such thing?

What has buying from RS have to do with anything? That isn't the same channel as most people would buy from in an automotive application. If you buy an LED kit the industry standard way is to describe the load resistor as 12V 21W or 12V 50W. If you are buying through that channel, which the OP clearly did, this makes total sense for this application.

Different industries describe the same product in different ways. This type of resistor is described in automotive applications as a 21W 12V resistor. Of course it's ~7 ohms, that is what 12V 21W tells you. Knowing it is ~7 ohms is no real use in this application which is why it is described differently.

I'm speaking from the experience of having a degree in electrical engineering and 20 years of industry experience but hey what do I know.

Edited for BBcode weirdness

1101

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#2424906 21-Feb-2020 11:12

Handle9:

So you are saying that resistors are sold as 12V 21W after saying that there was no such thing?

Such labeling is obviously aimed at imbiscles , who really shouldn't be trying to use them as they dont know how they work and have no idea what to buy.
:-)
I jest, but its true .

Technofreak

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#2425193 21-Feb-2020 22:38

Handle9:

So you are saying that resistors are sold as 12V 21W after saying that there was no such thing?

These resistors are being sold by describing their function not by their electrical characteristics which would be the usual way to describe such a component. 1101 summed it up pretty well in his post.

As I pointed out in my previous post that a 6.8 ohm resistor would be provide a 21 watt load in a 12V circuit and a 5.3 watt load in a 6 volt circuit. How can a component be 12V in one installation and 6V in another?

Are you telling me as an electrical engineer designing such circuit you would specify a 12V 21W resistor rather than specifying a 6.8 Ohm 25 watt resistor? I somehow doubt that you'd do that. I can imagine the fun a logistics person would have trying to source a supply of 12V 21W resistors for your product.

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#2425196 21-Feb-2020 22:51

Whatever, I'm out - you aren't making any real sense.

k1w1k1d

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#2425548 22-Feb-2020 17:54

From the Narva catalogue.

12 VOLT 21 WATT L.E.D. LOAD RESISTOR

Part No. 90034BL

L.E.D load resistors are purpose designed for installation in vehicles where problems result from L.E.D replacement of standard incandescent globes. Suitable for indicator and brake light circuits, L.E.D load resistors are designed to draw the same current as a 21 watt incandescent globe, correcting any problems with low current draw

Technical Information

INPUT VOLTAGE RANGE

12 Volt

POWER

21 Watt

CANDELA RATING

OTHER

Resistance: 8 ohms

They are marketed for the automotive industry to cure a specific issue, so don't strictly follow the correct electronic conventions.

RunningMan

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#2425577 22-Feb-2020 19:25

Technofreak:[snip]Are you telling me as an electrical engineer designing such circuit you would specify a 12V 21W resistor rather than specifying a 6.8 Ohm 25 watt resistor?

Not at all. I'd expect an electrical engineer to specify resistance and power dissipation at a given ambient temperature. But what @Handle9 is explaining is that the target market for the product is not electrical engineers; it's consumers who want a black box that replaces a 12v 21w incandescent globe. As such, that's how the product is described to the consumer. Of course it's not how it would be specified for manufacture, but is simply the marketing description. @k1w1k1d has beautifully illustrated this with the excerpt from the Narva catalogue.

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