Single filament 1156 bulbs have low and high power mode?

[och]

Tail light bulbs and turn signal bulbs often use double filament 1157 bulbs, with common ground but separate power tabs to each of the filament, which allows them to have low power mode on one filament, and high power mode on the other filament. I recently found out that there are some vehicles that use single filament 1156 bulbs, and these bulbs have low and high power modes. How is this accomplished? Does the vehicle send lower voltage for low powder mode?

 

[hamhanded]

It's probably highly implementation specific. I would guess PWM since it would be cheaper than voltage dividing.

 

[-Virgil-]

arrgh is right; usually it's done with PWM, though resistor setups also exist. It's usually not 1156 bulbs, which were obsolete before this kind of operational strategy came into wide use...if you see it on a vehicle with metal-base bulbs, it's probably a European vehicle that uses P21W bulbs which are not the same as 1156. More common these days are the all-glass wedge base bulbs (W21W / 7440) or the bulbs with a plastic wedge base (P27W / 3156, 3456, or WT21W). There have even been lamps that use the W16W, similar to the US 921, in this manner -- such as the '12-'18 mk6 VW Golf stop-tail light.

Usually this kind of system is one more reason why "LED bulbs" don't work.

 

[Alaric Darconville]

A very slight advantage to this is that when the tail lamp function is on, the rise from "tail" to "stop" is shortened thanks to the filament already having been heated to that lower state of incandescence. It also reduces the rise "shock" from full off to full on (such advantage lost if the tail lamps aren't lit, although very clever PWM could keep the filament heated below the point of incandescence).

A very real DISadvantage is that one filament is doing all the work and when it fails you've lost both the "tail" and "stop" functions for that lamp.

 

[-Virgil-]

I wouldn't be quite so sure to call that advantage "very slight"; it is a real one. Research here and here. How much practical improvement that might bring in terms of crash avoidance is more of an open question, research here.

Yes, using one filament for both functions increases the effects of a failure...but on the other hand, the number one filament killer is inrush current, so operation between dim and bright is a lot easier on the filament than operation between off and bright, which means reduced tendency toward a failure.

 

[och]

arrgh is right; usually it's done with PWM, though resistor setups also exist. It's usually not 1156 bulbs, which were obsolete before this kind of operational strategy came into wide use...if you see it on a vehicle with metal-base bulbs, it's probably a European vehicle that uses P21W bulbs which are not the same as 1156. More common these days are the all-glass wedge base bulbs (W21W / 7440) or the bulbs with a plastic wedge base (P27W / 3156, 3456, or WT21W). There have even been lamps that use the W16W, similar to the US 921, in this manner -- such as the '12-'18 mk6 VW Golf stop-tail light.

Usually this kind of system is one more reason why "LED bulbs" don't work.

Yeah, its a European vehicle, my work van which is a 2018 Mercedes Metris. I have recently discovered this when I replaced its rear bulbs with LEDs. I am not sure if the original bulbs are P21W, but I installed Sylvania Zevo LED 1156 bulbs - they fit just fine and operate properly. There was first an error on the dashboard saying brake/turn signal bulb is out, but it went away after a few minutes, I guess the CANBUS module recognized LED bulbs, and they are operating properly, no fast flash or anything.

I was just very surprised that any modern vehicle would use the same bulb for three different functions.

 

[och]

A very real DISadvantage is that one filament is doing all the work and when it fails you've lost both the "tail" and "stop" functions for that lamp.

And running light as well, all three functions are on the same filament.

 

[-Virgil-]

Yeah, its a European vehicle, my work van which is a 2018 Mercedes Metris. I have recently discovered this when I replaced its rear bulbs with LEDs. I am not sure if the original bulbs are P21W, but I installed Sylvania Zevo LED 1156 bulbs - they fit just fine and operate properly

Time-out! This is really important: how did you figure out that they operate properly? A whole lot more goes into it than "I checked and they light up". Did you do the comparative test described here?

I was just very surprised that any modern vehicle would use the same bulb for three different functions.

It's still legal as per Federal Motor Vehicle Safety Standard 108 to use a single bulb to provide the stop, tail, turn signal, and side marker functions. Shouldn't be, but is.

 

[och]

Time-out! This is really important: how did you figure out that they operate properly? A whole lot more goes into it than "I checked and they light up". Did you do the comparative test described here?

Thank you, I will check this out, but at a brief inspection the pattern seems pretty close to the incandescent bulb it replaced. These are Sylvania Zevo LED bulbs, they are far better designed than the cheap LED stuff from ebay/amazon, and they produce a beam pattern pretty close to incandescent bulbs.

This being said, your test is missing one very important criteria that you should add - low/high mode contrast. I understand most incandescent bulbs have roughly 1:5 low to high ratio, while most cheap amazon LED bulbs have 1:2 ratio at best. On a motorcycle this can potentially be fatal.

There are only two direct replacement 1156/1157 bulbs with correct low/high contrast ratio on the market that I know of - the Sylvania Zevo and the XP80 from Diode Dynamics.

 

[och]

It's still legal as per Federal Motor Vehicle Safety Standard 108 to use a single bulb to provide the stop, tail, turn signal, and side marker functions. Shouldn't be, but is.

Yes, and my Metris seems to burn these bulbs out after 2-3 month, and I was tired of replacing them, and so that's why I installed the LEDs, they should last longer. So far its been about 3 month and they are still working.

 

[-Virgil-]

your test is missing one very important criteria that you should add - low/high mode contrast.

That's a good point -- it is missing, and should/will be added.

I understand most incandescent bulbs have roughly 1:5 low to high ratio

Much more than that. An 1157 or 3157 is a little over 10:1. A P21/5W is a little under 13:1. A 2057 is 16:1. A P21/4W is a little over 29:1. Etc.

There are only two direct replacement 1156/1157 bulbs with correct low/high contrast ratio on the market that I know of - the Sylvania Zevo and the XP80 from Diode Dynamics.

The thing is, though, the intensity ratio at the bulb is a usable proxy/predictor of the intensity ratio from the lamp if the bulb and the lamp are an as-designed match (that is: an incandescent bulb in a lamp designed to use an incandescent bulb). With LED bulbs in lamps designed for incandescent bulbs, the optical mismatch means you can wind up with all kinds of improper light distribution -- including inadequate bright/dim intensity ratio.

my Metris seems to burn these bulbs out after 2-3 month, and I was tired of replacing them

Well, there's always this...!

 

[och]

Thank you for all this. I didnt realize how great the contrast ratio really is, I simply thought low power is 4-5w vs high power 20-21w, so 1:5 ratio - but this would be power ratio, not necessarily brightness intensity ratio.

Both Sylvania Zevo and Diode Dynamics XP80 claim correct ratio, and they are both very reputable companies. The XP80 is a $45 bulb, the Zevo is usually $25ish for a pair, so not cheap ebay junk, and when I had the Zevo bulb installed in just one side of my van, it looked very similar to the stock incandescent on the other side brightness/viewing angle wise, and high/low contrast. In one of my motorcycles I have the XP80 bulb, and its glorious, much brighter in every way than the old incandescent.

But now I am thinking, maybe the bulbs in my van were burning fast because I was replacing them with regular 1156 and they require the P21W?

 

[-Virgil-]

An 1156 bulb really does not belong where a P21W is specified. Electrical and photometric specs are different, but also the P21W specification requires a nickel-plated base, while many/most 1156s are bare brass -- a good recipe for bad trouble in a socket that is expecting a nickel-plated bulb.

I agree with you that the Sylvania Zevo (= Osram LEDriving) signal bulbs are among the few legitimate products in the space. They're optically compatible with some (just some, not all, not even "most") of the lamps they can be installed in. In many cases, the optical configuration of the lamp and/or the bulb angle relative to the reflector means it's a zero.

I don't agree with you that Sylvania is a reputable company, in general. Among other reasons: a large proportion of their automotive product line is junk, and many/most of their "upgrade" claims are BS.

 

[jzchen]

Just an FYI the ZEVO bulbs that as far as I know do not have emitters facing towards the lens, and at least the one I tested in our '12 Prius v reverse lamp, made a smaller patch of light on the wall in the dark, failing that part of the comparison.

Since the filament is shooting directly out the front it gets a wider angle spread…