LED sidelights / day time running lights
Nov 7, 2015 12:39:59 GMT
Roy of the Rovers, Phil Nottingham, and 1 more like this
Post by djm16 on Nov 7, 2015 12:39:59 GMT
Am I really posting in the Modified Forum. Yes, it really is me, Mr Originality. But I haven't actually changed anything except the bulb, no new wiring, still +ve earth etc. So read on.
The thread on LED lights reminded me that I had meant to put in some bright sidelights to be used as daytime running lights in the manner of Volvos et al.
I have had these planar COB 3W LEDs for months, just waiting for time to think about it. Well I have now done it. COB stands for "chip on board". GOK what that actually means, but in practice, the LED disks have 9 mini white LEDs with a cream colored diffuser melted over the top so that the impression is of a single 15mm round light source. The LEDs are well lit with a forward voltage of 9 to 9.5V at a current of 100 to 200 mA. So to prevent them instantly frying when connected to 12.5V you need a resistor. I chose 15 Ohms by trial and error. If you do the calculations (200 mA at 14.5V, or 100 mA at 12.5V) you come up with a value rather larger than 15 Ohms. But these disks get hot, and when resistors heat up their resistance increases. When mounted in the sidelight bulb holder, the heat sink gets up to about 40 degrees, which is only 14 degrees above ambient. There is some merit in the resistor getting hot, as it acts as an instrument of non-linear negative feedback to reduce the current in the device and prevent overheating.
The parts I needed were:
Here you can see the made up LED super power bulb, front and back.
Yes, it fits pretty much exactly:
Lastly, what do they look like when they are on? The following two pictures are of the original 4W filament bulb followed by the newly made 2W LED bulb, behind their lenses. The images are directly comparable as they are taken with the same manual exposure setting.
The thread on LED lights reminded me that I had meant to put in some bright sidelights to be used as daytime running lights in the manner of Volvos et al.
I have had these planar COB 3W LEDs for months, just waiting for time to think about it. Well I have now done it. COB stands for "chip on board". GOK what that actually means, but in practice, the LED disks have 9 mini white LEDs with a cream colored diffuser melted over the top so that the impression is of a single 15mm round light source. The LEDs are well lit with a forward voltage of 9 to 9.5V at a current of 100 to 200 mA. So to prevent them instantly frying when connected to 12.5V you need a resistor. I chose 15 Ohms by trial and error. If you do the calculations (200 mA at 14.5V, or 100 mA at 12.5V) you come up with a value rather larger than 15 Ohms. But these disks get hot, and when resistors heat up their resistance increases. When mounted in the sidelight bulb holder, the heat sink gets up to about 40 degrees, which is only 14 degrees above ambient. There is some merit in the resistor getting hot, as it acts as an instrument of non-linear negative feedback to reduce the current in the device and prevent overheating.
The parts I needed were:
- an 50 mm aluminium disk to act as a heat sink - much higher thermal conductivity than steel, copper would do too, but I have a large sheet of 1mm gauge ally.
- an old mini bayonet bulb (came out of the sidelight)
- resistor
- cold weld epoxy glue.
Here you can see the made up LED super power bulb, front and back.
Yes, it fits pretty much exactly:
Lastly, what do they look like when they are on? The following two pictures are of the original 4W filament bulb followed by the newly made 2W LED bulb, behind their lenses. The images are directly comparable as they are taken with the same manual exposure setting.
a nice cheap practical project 
anyone wanting to follow there are COB chips that are 9v-12v on Ebay for 99p! or 4 for 99p China again! I would still fit a limiting resistor to protect the LED even though they say 12v!! we are as djm says running on 14.5v ish!