Thread: Another one for the sparkies?

Originally Posted by BradC

Most (no, pretty much all) "led replacement" bulbs are one or more leds with series resistors. They can be dimmed just the same way a traditional incandescent filament is dimmed by reducing the voltage, and traditionally in a car that was done by a rheostat (a variable resistor). Slightly more modern cars used something a bit more clever (like a variable linear regulator), and it's only really quite recently that PWM became common.

No need to over-complicate it. Connect the LEDs up to the existing dash dimmer, add another one or use some other form of rheostat. No PWM, no RFI, no hassle.
Yes, if you go to Jaycar you'll need to know the current consumption to ensure they give you a wirewound pot with the correct approximate value and power rating. You can measure this with a multi-meter in series with the lamps.

Thanks BradC

Unfortunately there is no existing dimmer of perhaps I wouldn’t have this challenge.

Maybe I need to pull the dash apart and put a multimeter across as you say.

You need to get in series with one of the supplies to the LEDs and measure the current.

Are the LEDs currently on all the time or switched with the head/dash lights?

Let's do a worked example based on 6 gauges and an estimated maximum LED current of 20mA (pretty typical).
6 x 20mA = 120mA.

So flat out, those LEDs are going to draw 120mA. As you drop the voltage, the current will also fall in a non-linear manner, but for the sake of the exercise, let's go with half brightness, half voltage.

So we assume at half brightness, we'll want half voltage (6V) and the LEDs are going to draw 60mA. You need to drop 6V at 60mA so you'll want a 100 Ohm resistor which is going to dissipate ~360mW.

You'll want a >=3W wirewound pot. Something like : R2156A - Taiwan Alpha 100R Lin D Shaft 24mm Single 3W Wire-wound Pot - AltronicsAlthough ideally you'd probably want more like 250 Ohm to dim them to almost nothing.

The reason for the 3W pot is to do with the current handling of the element. The power rating of a pot is for the entire element, so the closer the wiper is to the start the lower the power it can handle. Easiest way to work that out is to calculate the current the entire element can handle and not exceed that. Power = the square of the current multiplied by the resistance (P=I2R).

For a 3W 100 Ohm pot that is the square root of 3W/100 Ohm (173mA). A 1W pot would only be able to handle 100mA, so when you just crack the knob to "slightly less bright" you've already exceeded the element power handling and it will eventually experience a "life limiting event" and release the magic smoke. Once you get down into carbon trace pots (which barring one useless exception are all Jaycar stock) then you'll cook the element in short order.

If you can get a current measurement for a group of LEDs, it's easy to work out the entire load. If you don't have a decent meter with a good low current resolution, put a 100Ohm 1W resistor in series with the LEDs and measure the voltage drop. Current can be calculated from that I=V/R.

You can always play with fixed resistors until you get a brightness that you're happy with and work from there.

Originally Posted by BradC

You need to get in series with one of the supplies to the LEDs and measure the current.

Are the LEDs currently on all the time or switched with the head/dash lights?

Let's do a worked example based on 6 gauges and an estimated maximum LED current of 20mA (pretty typical).
6 x 20mA = 120mA.

So flat out, those LEDs are going to draw 120mA. As you drop the voltage, the current will also fall in a non-linear manner, but for the sake of the exercise, let's go with half brightness, half voltage.

So we assume at half brightness, we'll want half voltage (6V) and the LEDs are going to draw 60mA. You need to drop 6V at 60mA so you'll want a 100 Ohm resistor which is going to dissipate ~360mW.

You'll want a >=3W wirewound pot. Something like : R2156A - Taiwan Alpha 100R Lin D Shaft 24mm Single 3W Wire-wound Pot - AltronicsAlthough ideally you'd probably want more like 250 Ohm to dim them to almost nothing.

The reason for the 3W pot is to do with the current handling of the element. The power rating of a pot is for the entire element, so the closer the wiper is to the start the lower the power it can handle. Easiest way to work that out is to calculate the current the entire element can handle and not exceed that. Power = the square of the current multiplied by the resistance (P=I2R).

For a 3W 100 Ohm pot that is the square root of 3W/100 Ohm (173mA). A 1W pot would only be able to handle 100mA, so when you just crack the knob to "slightly less bright" you've already exceeded the element power handling and it will eventually experience a "life limiting event" and release the magic smoke. Once you get down into carbon trace pots (which barring one useless exception are all Jaycar stock) then you'll cook the element in short order.

If you can get a current measurement for a group of LEDs, it's easy to work out the entire load. If you don't have a decent meter with a good low current resolution, put a 100Ohm 1W resistor in series with the LEDs and measure the voltage drop. Current can be calculated from that I=V/R.

You can always play with fixed resistors until you get a brightness that you're happy with and work from there.

Thanks BradC

I won’t pretend that I understand half of that. Yes they are wired via the head/dash lights. From memory they are all on the same relay.
I would be happy with between a 1/3 - 1/2 less brightness. It’s ok during the day but at night is just too much. Im off to the auto sparkie for other things this morning so will discuss with him.

Thanks again.

Originally Posted by travelrover

Thanks BradC

I won’t pretend that I understand half of that. Yes they are wired via the head/dash lights. From memory they are all on the same relay.
I would be happy with between a 1/3 - 1/2 less brightness. It’s ok during the day but at night is just too much. Im off to the auto sparkie for other things this morning so will discuss with him.

Thanks again.

See if you can get your auto sparkie to retro fit a rheostat (dimmer) for you

Originally Posted by trout1105

See if you can get your auto sparkie to retro fit a rheostat (dimmer) for you

He said, hmmm a good question! We did discuss rheostats. Anyway it’s booked in for some other work in a couple of weeks and he will have another look then.

G'day Simon,
Out of interest and research I purchased a PWM dimmer on Ebay for $8.68 post free. (See attached photo.). It arrived today and pulled it apart to see what technology is used. The design is a low frequency PWM using a 555 IC timer. It will not generate RFI. The circuit design, PWM's the negative supply lead to the LED's. This means it will control the LED's in the Carlin switches as well because the input and output positive leads in the dimmer are connected together on the circuit board. However, the dimmer will only control 12 volt LED's from a 12 volt supply. (Caution. If you use 24 volts supply you must use 24 volt LED's.)

I made up a test rig after re soldering 2 dodgy joints on circuit board and it controlled a 12 volt gauge LED from Off to Full brightness. (The standby current is 15mA without a LED on the output.)
This information is only for the dimmer in the photo and a search on Ebay will find it.

Happy Days!
Hope this helps
Chriss-l1600.jpg

Originally Posted by Busted Syncro

G'day Simon,
Out of interest and research I purchased a PWM dimmer on Ebay for $8.68 post free. (See attached photo.). It arrived today and pulled it apart to see what technology is used. The design is a low frequency PWM using a 555 IC timer. It will not generate RFI. The circuit design, PWM"s the negative supply lead to the LED's. This means it will control the LED's in the Carlin switches as well because the input and output positive leads in the dimmer are connected together on the circuit board. However, the dimmer will only control 12 volt LED's from a 12 volt supply. (Caution. If you use 24 volts supply you must use 24 volt LED's.)

I made up a test rig after re soldering 2 dodgy joints on circuit board and it controlled a 12 volt gauge LED from Off to Full brightness. (The standby current is 15mA without a LED on the output.)
This information is only for the dimmer in the photo and a search on Ebay will find it.

Happy Day!
Hope this helps
Chriss-l1600.jpg

Brilliant

Thanks Chris! All 12 volts here, I’ll track it down

CAUTION. The negative leads from the gauges and switch LED's must go directly to the dimmer output negative terminal. NOT thru the vehicle earth or body system. Also the existing positive lead that comes from the side light/headlight vehicle switch to your LED lights can stay connected however you need to connect a wire to the INPUT positive terminal on the dimmer from this side light/headlight switch. (Reason. The dimmer input positive terminal and output positive terminal are connected together on the circuit board.)
This means you don't have to pull cables out and will make install easier.

NOTE: the dimmer NEGATIVE INPUT terminal goes to battery negative/ vehicle negative.

Hope this makes sense.

Chris