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I posted a bit of this in WHYDTYTT recently, but think it deserves a thread to pull the bits together. And, in order to do that I'm going to restate some things from there.
First, the issue is whether or not LED replacement for the #194 bulbs can be dimmed. Matthew said:
Originally Posted by FordFETruck
I used 5 SMD bulbs off eBay and they dim just fine....
But others have said that their LED's don't dim or don't dim enough. So, I want to find out why there seems to be such a difference of opinion.
Given that, I ordered two sets of LED's - a 5-SMD set and a 6-SMD set. But, you might ask, what is an SMD? It is a surface-mount device, and in this case it is a surface-mount LED. So, one set has 5 SMD's and one has 6 SMD's. As you can see in the picture below, the one on the left has 1 SMD on the top and 1 on each side, and the other has 2 on the top and 1 on each side. Therefore, one is a 5-SMD and the other is a 6-SMD LED.
But, there are other differences. The 5-SMD unit emits white light and the other emits blue light. And, there are differences in size. Yes, the 6-SMD is actually smaller since its "devices" are smaller.
How about the amount of light they provide? Well, a standard #194 bulb puts out 25 lumens at 14v. In contrast, the white 5-SMD's I bought put out 125 lumens each, so they'll put out 5 times as much light. And the 6-SMD's put out 300 lumens, so they'll put out 12 times as much light!
Alright, they'll be BRIGHT. But, can they be dimmed? Some say yes and some say no. So, how do we find out. I guess the best way is to determine the voltage at which the LED's dim appreciably. To do that I put my camera on Aperture mode so it could adjust the shutter speed to keep the amount of light hitting the lens constant. In other words, if the light is cut in half the shutter speed will double.
The results are shown below. Note that the 5-SMD white LED's basically turn off below about 8 volts, but the 6-SMD blue ones are still giving 4 times as much light as the incandescent #194's at that voltage. (4*25 = 100 lumens, and 35% times 300 = 100 lumens) So, if the dimmer could reduce the voltage to the white LED's down to maybe 9 volts they would dim appreciably. But, the blue LED's would probably still be quite bright.
The next question is whether or not the dimmer rheostat can drop the voltage to maybe 9 volts. And the quick answer is no. From my calculations the lowest voltage the LED's will see with the rheostat fully on will be 12 volts (voltage drop = resistance * current = 8 ohms times 5 LED's times 60 ma/LED = 2.4 volts.) And at that voltage the white LED's will still be kicking out 80% of full brilliance.
Now, this is all hypothetical, and maybe I'm missing something. So, David/1986F150six has agreed for his truck to be a guinea pig this weekend at the Okie GTG and we'll find out if my math works. If the LED's dim acceptably I'll take a number of readings and report back. And if they don't we are going to install the 8 ohm 50 watt resistor I got in today, and that'll let us drop the voltage to the LED's down to between 6 and 7 volts, which is essentially off for either LED.
I replaced the 194 bulbs in my instrument cluster with green LEDs a few years ago and I have pretty much the same control with the dimmer compared to the regular bulbs. At full power they are MUCH brighter than the 194 bulbs, but at full dim they are considerably dimmer.
I have had trouble with one on the right side of the cluster from day one though. It will often not come on at all or it will sometimes flicker. I don't know if its a bad connection or a bad LED...I never bothered to tear it apart again because I rarely drive the truck at night anyway.
Well researched info, but I'm curious about something. In this application, what is the primary advantage of converting to LED illumination? Is it for brighter output? Reduced amp draw? Increased reliability?
I've converted my camper's interior lighting to LED. I dry camp (no external hookups) so the reduced amp draw is a huge advantage when running off the coach battery. I haven't changed the external lights as those are only in use with the truck's engine running, so the alternator can supply an essentially 'unlimited' source of electricity as far as the incandescent bulbs are concerned.
Also, what has been your experience with reliability? Some of the LED bulbs I tried were of very poor quality and didn't last long at all. I finally found a reliable brand and am very happy with them. (In fact, I'm typing by the light of one now.)
Slightly off-topic, but I wouldn't suggest converting to LED on any newer vehicle that has a bulb failure warning system. I think the system operatesby sensing amp draw under load and continuity at rest. The monitor circuitry expects to see an incandescent filament and anything else will cause a false warning. I've even seen LED bulbs with extra circuitry to mimic an incandescent bulb, but that just seems way too complicated, plus you lose the failure warning capability in the stock arrangement.
Rowdy - Could you post what LED's you used? There is such a huge variation in what you get that I'd like to understand which ones work and which don't. As for the flickering, I suspect you have a bad LED.
KR98664 - Thanks. I think the push to go LED is to get more light as the instruments in these trucks are really hard to read at night with the stock bulbs. And at 12 times the light output the blue ones I have should certainly do that. Ditto the backup lights, dome lights, bed light, etc. By today's standards these trucks have marginal lighting and LED's promise to resolve that.
As for reliability, I don't have any experience yet on automotive LED's. But I do have on household LED's and there I've been disappointed as several big-name modules have failed within a month or two of installation. So, if big-name units fail there I can only imagine what will happen on the cheap ones for automotive use.
They cost a lot more than the ones you find on ebay but I assumed they were higher quality. I'm not sure what my problem is though...I might tear back into it this winter while its stored. Other than the intermittent flickering/not working in that one socket I have no complaints on them.
I had a project in college where I was making a digital clock(long time ago) and I tried using a cds cell to dim the display at night. A cds cell is a light sensitive resistor.
Found out when it dimmed, some of the segments where bright, some were dim. The teacher came to the conclusion that during the manufacturing process, the junction in the LED varied enough that the voltage when they started conducting forward would be different for different segments. So some would be off while others would be on.
The most reliable way to dim LED's is with pulse width modulation. In other words, turn them completely off and back on many times a second. When the off time is more than the on time, they will start to dim. The more off time, the dimmer they get.
Rowdy - Thanks. Those LED's certainly are more expensive - like almost 10 times more. And yet they don't put out as much light as it says "Up to 90 Lumens" while the white ones I bought are 125 lumens and the blue ones are 300 lumens. Also, yours say they operate on "10~30 VDC", and that may be the key - if they quit below 10v then that might explain why the dimmer works.
Jim - You are absolutely right, the best way to dim LED's is via PWM. But that's not easily done on our trucks for two reasons. First, you need the rheostat to vary the voltage significantly to control the pulse-width modulator, and that means you still need the 8 ohm resistor. Second, you have to source the power to the LED's and, as you know, solid state electronics is much better at sinking the power.
Anyway, while dimming via voltage reduction won't give completely uniform illumination, I'm hoping it will be satisfactory.
Ok, here you go. Sorry its a little out of focus. The low battery light was flashing on my camera so I had to shoot the video quickly before the battery went dead. I went through the full range with the rheostat and as you can see the dimming is quite controllable. The flickering only occurs while I'm actually turning the rheostat. Once I stop turning it in any position, the flickering stops. When I first turned the lights on the problem light on the right side under the fuel/amp gauges did not come on, but after about 30 seconds it flickered a few times and finally lit up so I'm pretty much convinced its a bad LED in that socket. The other four have always worked without any issues. These are the green LEDs sourced from the link I posted above. I also removed the blue diffuser caps inside the cluster.
I think its the headlight switch design that determines this. I have the same set of bulbs in my '82 F150 and in my '56 Fairlane only difference is my truck has the blue bulbs and my '56 has the white. Well the '56 will dim all the way off but my truck will just barely dim and with the **** rotated to full off the lights are still on in my case.
Does your Fairlane take more bulbs than the trucks? If so, that would suggest the rheostat would have less range of resistance due to the increased current.
If the number and type of bulbs are the same then I would expect the rheostat's range to be the same - wait a minute! Is the '56 still a 6 volt system?
09-13-2016, 08:25 PM
Obviously your LED's dim nicely and, oddly enough, quite uniformly in spite of not being pulsed.
