One For The Techies - Electrical/electronic

October 19, 2008

Right,

I fitted some heated seats in the RAV4 and added an bi-colour LED so it will show which of the two heat settings each seat is set at - Green for setting 1 and Red for setting 2.

All was OK, but first one led failed and then the other led failed and I suspect I used the wrong resisitor values to drop the voltage (the heated seats still work fine)

So I am going to get a new set of LEDs and would appreciate it if someone could confirm what I need to do ....

LEDs are These ones

specs are in link, but key info (I think) is they are 2.5V, 30mA, 105mW

Connecting to +12V supply, so I have bought 0.6W 470R Resistors and believe I need to connect as follows:

Switched 12V ====== 470R ====== Cathode

GND ====== Anode

Does that sound reasonable? it is how I connected the last ones but can't remember the Resistor values used that time, but am I forgetting something? (been nearly 20 years since I last did this kind of stuff properly so very rusty)

October 19, 2008

Well, the red absolute max rating is 30mA. The green is 25mA. Generally, it's a good idea to keep away from absolute max values. I'd run 'em at about 20mA or lower if there is sufficient light output for you.

Now, if the vehicle supply was really 12V and the forward diode drop was 2.5V, a 470R would give you about 20mA. However, the vehicle supply with the engine running is more like a nominal 14.2V, but see below. You'll be running the green led just above or at least very close to its absolute max current = short life and pop.

At 20mA the diode drop is about 2.0V.

(14.2V-2.0V)/20E-3 = 610. So I'd go for a 620R Power = (20E-3)^2*620=0.25W, so go for your 0.6W again.

However, the vehicle supply is anything far from "clean". It can have all sorts of nasty transients on it, which in ECU design we have to protect for. Load dump for instance:

http://en.wikipedia.org/wiki/Load_dump

(reasonable-ish explanation)

can produce 80V to 120V transients with decay times up to half a second. That will certainly pop your leds. Now, I'm not suggesting for one moment this is happening, Battery disconnect is a fairly rare occurrence. You can get smaller transients will smaller load switching, perhaps a seat heater being switched off for instance. How much current in those heaters? Even so, I wouldn't expect those to cause too much of a disturbance to be honest.

Try it with the 620R. You may get away with it. If you don't, well the leds aren't that expensive and we'll have to come up with a bit of protection circuitry for you, or a constant current drive or something. Just means you'll be redoing it yet again! Perhaps experiment with a few resistors values and select the highest value that gives an acceptable light intensity output for you.

Don't want to suggest a protection scheme just yet, because to be honest, if we clamp at a voltage lower than every other bit of electronics on the vehicle, we can end up having to deal with the full "belt" and it would be better to let some of the existing stuff on the vehicle do that. I'm not convinced you need it anyway, the leds may just be running a bit close to max.

Oh, and if I understand your wiring shown correctly, I think there's a slip of the keyboard, with your diagram. led anode is usually most positive, but you knew that anyway!

October 19, 2008

Stick a 1k in there mate. If the LED isn't bright enough for you then just reduce the resistor value slightly

The lower the resistance, the brighter the LED. As you're only using it as a visual indicator and not a specific light source then there's no point going too bright !

October 19, 2008

Hurraw there ma man (Scots in Glasgae fur Hullo )

Ahm a simple man, as ye aw ken (know)

And ahm using hunners of LEDS a week (I am a moderate user of LEDs to manufacture unique lighting design)

My first reaction would be for you to buy 12V leds.

Otherwise, reduce it down using resisitors which in a car setting is just adding more joints to go wrong. The stuff we are doing has got to be weatherprrof cos lots of it is outside. I've also done some stuff for cars and then you build in vibration along with all that shcm mentioned. Witness the RAV4 clock of old - naff circuit board.

I also buy from RS Components - perhaps in bulk right enough but if you're looking for a small number I could probably supply by post for a few halfpennies. At the moment we are using 3.6v leds bunched and to good effect for lighting.

Or you could go adventurous and buy the electro-luminescent panels !!

However, you could also fit a radio wireless sensor to yer backside and use a receiver to know when the bums up to temperature :lol: We use them as well - sensors not bums!!

October 19, 2008

Thanks guys - the LEDs when working were very bright (too bright really) which is why I suspected too low a value.

My Scanguage does report voltage when running engine at 14.2V :thumbsup:

I did check if maplins did 12V LEDs when I nipped in there originally to buy but they did not - are 5mm Bi-Colour 12V LEDs available? Don't want to use two seperate, but Bi-Colour, you see.

I have the 470Rs already so I may go for a pair in series for each LED s that will give a little bit of extra protection along lines of Boro Seras comment.

Thanks :)