Changing 21w brake bulb to 21w/5w

Andy From Sligo

Hello all, anyone good with electronics (resistors and diodes etc) ?

Here's a little project i want to see if I can do , change a 21w BA15s (Red) brake light bulb to a 21/5w without having to change the bulb holder.

So its a single contact 21w single filament bulb. once powered by 12v+ DC it burns at 21w . I would like to adapt it to burn at 5w for sidelights and 21w for brake light.

I have done this diagram as to what components I need to do this , a resistor to reduce the power to 5w and a Diode to make the positive go in one direction only from brake light to the bulb so the positive does not travel down the tail light circuit and potentially blow the fuse for either the brake light circuit or the tail light circuit of the car.

So, the things I have to know (I am am intermediate electronics knowledge lets say, I know what a resistor does and i know what a diode does and I am handy with a soldering iron) - here's what i need help on:

1. What value resistor will I need? (ohm / k/ohms and watt ie quarter or
0.5w) to reduce the 21w brightness down to 5w
2. How hot will the resistor get? - will it get really hot? - could it be
covered with some heatshrink?
3. are the Diodes in the right place and what value diodes would they
need to be
4. Can you see any problems or any irregularities looking at the diagram
that could cause problems or the fuses to blow.

I realise it will more than likely shorten the life of the brake light, it being one filament being used as a brake & tail bulb .. but that's no biggy.

Thank you for any help .... apart from "what do you want to do that for?" and "you will be breaking the law" and one shot answers like "it wont work" without adding any context why it wont technically work - many thanks.

cml387

I don't see anything wrong with the theory.
However it might take a bit of experimentation to get a correct setting for the lower brightness. Lower wattage bulbs have a smaller filament that will burn white at less wattage. Your existing 21W bulb will glow redder and have much less luminance than you might think.

newirishman

It has been a few years (ok, quite a few years) but should that be a 22 ohm resistor?
Given P=U*I and R=U/I, with your inputs we calculate

21w, 12V = 1.75 Ampere, 6.85 Ohms
5w, 12V = 0.4167 Ampere, 28.8 Ohms

Difference: 22 Ohm, so you need a 22 Ohm resistor

To be honest, not sure if you need the Diodes. I doubt the resistor needs a heat sink.

cml387

newirishman wrote: »

It has been a few years (ok, quite a few years) but should that be a 22 ohm resistor?
Given P=U*I and R=U/I, with your inputs we calculate

21w, 12V = 1.75 Ampere, 6.85 Ohms
5w, 12V = 0.4167 Ampere, 28.8 Ohms

Difference: 22 Ohm, so you need a 22 Ohm resistor

To be honest, not sure if you need the Diodes. I doubt the resistor needs a heat sink.

Yes, that's what I came up with.A 5W resistor shouldn't get hot.
I think the diodes are a wise move, otherwise you'd be feeding back into the circuit with unknown consequences. Any rectifier diode rated at about 2A would do. I still think that the loss of luminance with that setup would be too great.

cyborg

The 21w bulb resistance is ~7ohms , adding 7 ohms in series halves the power but may actually 1/4 the brightness

25 watt res required with a value prob somewhere between 3 and 10 ohms but it will take trial and error to see how bright you need it.

dkd21

If you have a variable resistor handy it would make it easy to play the brightness at mockup

Spook_ie

You have two supplies, I assume one is fed from the brakes and the other from the sidelight switch why not just use the correct 5/21w bulb with the correct holder?

zg3409

What car? Many cars now have blown bulb detectiom, CAN messaging to lamps and it might throw up errors. You can also get ABS warnings if brake lamps are thought to be blown. Most resistors are rated 1/4 watt, so may overheat. As said before the maths may not work as cold bulbs have different resistance to hot lamps, so cooler tail light will not be sane resistance as hotter brake.

A good trick I did in the past was to use a car bulb as a resistor. They have a resistance and are designed to take 21 watts. I recommend checking if car has blown bulb detection. Then try without diodes, but use them if you have them, wire a second 21w car lamp in series with tail wiring as a resistor, and try it. You can add extra lamps in parallel or series in order to brighten or dim lamp in tail light mode.

If you have blown bulb detection or can messaging to tail lamps it might be safer to keep old light hidden in the boot, and run new light from relays or a trailer light break out box..

In the past I turned on rear fog lights based on a slowing down sensor, which was a form of tilt switch, so rear red fog lights came on with brake lights but only when very hard braking.

Steamed Clams

Worth noting that your diodes will drop the voltage going to the bulb. Not a big deal on the tail circuit as you can just use a lower value resistor to compensate. But, your brake circuit brightness will drop as you're supplying ~1V less than +VE. And filament bulb brightness vs voltage is an exponential relationship, so you'll be losing approx 10% of your peak brightness. The diodes are necessary, or you'll find that putting on the side light circuit powers the brake circuit and vice versa.

Your 22 ohm resistor will get much hotter than you're expecting, it'll be dissipating about 9W of power. Leave a 10W incandescent bulb switched on for a few mins, touch it and see how much heat 10W produces Definitely don't wrap it in tape or heat shrink, that's a fire waiting to happen. Ideally it needs to be mounted outside the car, with a heatsink.

Using a pair of resistors to make a voltage divider will consume a lot less power (ie, a lot less heat!). In the example below, you get a 14/4 voltage division, which is close to the 21/5 ratio you're after. In this configuration, R1 dissipates 76 mW and R2 dissipates 191 mW. The usual 1/4 W resistors will work fine here.




Another idea might be to use a PWM controller to pulse the bulb for tail light usage. Just bypass the controller for brake circuit brightness. This is how OEMs do this for cars with a single filament doing both functions.

zg3409

Steamed Clams wrote: »

Using a pair of resistors to make a voltage divider will consume a lot less power (ie, a lot less heat!). In the example below, you get a 14/4 voltage division, which is close to the 21/5 ratio you're after. In this configuration, R1 dissipates 76 mW and R2 dissipates 191 mW. The usual 1/4 W resistors will work fine here.




.

A voltage divider won't work. The lamp if 21W and 14v has a resistance about 9 Ohm or less if cold, so it will mess up the voltage divider sums. I would recommend to fully test any trial and check for overheating diodes, resistors, and lamp brightness along with unexpected effects if brake is pressed when tail is on or off. You can get high power resistors but you would need to know value needed first. High power resistors need to be mounted to metal chassis such as car body to let the heat away.

And more importantly YouTube video or it did not happen

colm_mcm

Change the bulb holder. Car wiring is multiplexed so don’t be pricking around too much with it.

cml387

colm_mcm wrote: »

Change the bulb holder. Car wiring is multiplexed so don’t be pricking around too much with it.

I don't know what type of car. If this is anything later than about 1990 forget it.
But on the other hand if it's an old Morris Minor..

colm_mcm

cml387 wrote: »

I don't know what type of car. If this is anything later than about 1990 forget it.
But on the other hand if it's an old Morris Minor..

I think it’s a fairly recent Clio or Sandero he has.

mickdw

colm_mcm wrote: »

I think it’s a fairly recent Clio or Sandero he has.

Could be a good way to get your money back on a sandero