Rectifier Diode in automotive lighting application

[Pipe Wrench]

I have noticed that my car's door warning light features a rectifier diode. I "think" rectifier diodes are meant to block high voltages. Why would the door warning light (red light in the door panel that comes on when door is opened), which runs on 12-14.5 Volt be equipped with such a diode?

The light itself is an incandescent festoon bulb. The car has an electronic "comfort control module" to which this door warning light is connected. The "comfort control module" does such things as dim the light instead of turning it abruptly off, and it also knows the door status, which is important for the alarm system and door locks.

Does the diode maybe somehow prevent the door light in the other door from coming on when one door is being opened?

The "comfort control module" knows when a door is open thanks to micro-switches in the door lock control modules.

This is how the door warning light is wired:

 

[gungatim]

diodes do not block high voltages, they are a one way valve: they block voltage in one direction but allow it in the other. they can be bridged together to turn AC into DC. in applications like yours, the diode is likely used to prevent backfeeding the circuit since many lights are daisy-chained and only come on when there is an imbalance (think alternator warning light)...hope that helps.

 

[Falcon67]

IMHO it's in place to block a voltage spike. Why - ask the EE that specified configuration.

 

[ForceFed70]

Falcon67 is on the right trail.

It's there to prevent voltage spikes. Spikes from what? Static Electricity.

There will be protection circuitry built into the ECU, but on circuits like this where it's common for a human to touch the door and produce voltage spike due to static electricity having an extra layer of protection is a good idea.

Plus it reduces the change of a static discharge happening in the 1st place which makes it more comfortable for us humans.

 

[Stuff]

You are showing two components out of hundreds that make up the system so anything would be speculation. Diodes generally used for isolation as you mentioned - one door ajar light while each door controls separate courtesy lights. Also used to prevent one circuit from leaking current and having a light unintentionally glow at partial brightness.

 

[dogdog]

OP, sure that the diode is not parallel with the bulb?

 

[Alchymist]

In the circuit shown, the diode is useless. There must be more to the circuit. And in that configuration, it will do nothing to stop voltage spikes.

 

[Captain Spaulding]

The diode is there to allow multiple controls to activate the light or multiple sensors to see that the light is on without interfering with each other.


You'll find that there are multiple diodes connected together to allow any one door switch to be detected by the alarm/computer without activating all of them at the same time.

It has nothing to do with surges or spikes. Light bulbs make excellent surge suppressors themselves.

 

[AffableCurmudgeon]

As Captain Spaulding stated, there must be more diodes connected to other circuits which connect to the dash light.

I have had to insert didoes in a turn signal circuit when a single lamp turn signal indicator was back feeding turn signal lamps on the opposite side that was not supposed to be blinking.

 

[erswill]

OP : with your wiring diagram the lamp will not turn on. For the lamp to work you have to either reverse the supply polarity or the diode.

 

[theoldwizard1]

Falcon67 is on the right trail.

It's there to prevent voltage spikes. Spikes from what? Static Electricity.

Here is my S.W.A.G. !

You mentioned a "comfort control module" to gradually dim the lights. This is likely don by pulse width modulation (PWM). Remember an incandescent light bulb is both a resistor and and inductor. If the "load" (amount of current used by all of the light bulbs) is sufficiently high enough when it turns off, there will be a back (reverse) EMF (voltage ) spike that could damage the power transistor being used to turn the bulb(s) on and off. (On things like an ignition coil or a fuel injector, this could be 100s of volts !)

A little surprising if this is the case, because the power transistor is likely a MOSFET and it has a builtin "body" diode.

 

[Pipe Wrench]

You are showing two components out of hundreds that make up the system so anything would be speculation.

I understand I am only showing a tiny part of the system, but I was hoping that someone was familiar enough with this type setup to know the function. Apparently, VWs have used this system for at least the last 17 years.

Diodes generally used for isolation as you mentioned - one door ajar light while each door controls separate courtesy lights. Also used to prevent one circuit from leaking current and having a light unintentionally glow at partial brightness.

That's what I have been suspecting this diode's purpose is, but it is merely a suspicion.

 

[Pipe Wrench]

OP : with your wiring diagram the lamp will not turn on. For the lamp to work you have to either reverse the supply polarity or the diode.

I will double-check polarity on the terminals. remove the light and take some pictures of the assembly. It's possible that I made a mistake.

Thanks for everybody's input!

 

[walta]

My guess is the is diode and lamp are part of a matrix circuit.

That would allow a computer to operate every combination of the 64 different lamps with only 16 wires back to the computer.



Walta

 

[Pipe Wrench]

OP : with your wiring diagram the lamp will not turn on. For the lamp to work you have to either reverse the supply polarity or the diode.

You are correct. I mixed up polarity. Here is the correct diagram:

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[Pipe Wrench]

My guess is the is diode and lamp are part of a matrix circuit.

That would allow a computer to operate every combination of the 64 different lamps with only 16 wires back to the computer.



Walta

That makes sense. I know my vehicle uses at least partially a CAN bus to save wires.

What exactly is the function of a rectifier diode in a matrix circuit?

 

[theoldwizard1]

What exactly is the function of a rectifier diode in a matrix circuit?

All diodes have only one function, to only allow current flow in one direction.

The point of the matrix is, that only power needs to be applied on one of the "Column" wires and ground on one of the "Row" wires to light one light bulb.

Most automotive circuits "switch" the ground side to complete the circuit (the positive side goes back to the battery via a fuse and possibly a relay) because this is the most cost effective circuit.

 

[dogdog]

My guess is the is diode and lamp are part of a matrix circuit.

That would allow a computer to operate every combination of the 64 different lamps with only 16 wires back to the computer.



Walta

Never understand how this matrix works.....

so if I have lamp 1 on. and also lamp 64 on at the same time.....what happened to 8 and 57 ?

 

[gungatim]

if rd/bn is grounded, and 1 is on, 8 will only be on if red/gry is grounded as well. 57 will only be on if 1 is on (or rd/brn grounded) and yel/gy is hot. can control both grnd and hots to power multiple banks or single bulbs at the same time...kind of like a logic puzzle or the game clue...except not only can individual combinations exist, but all combinations can also exist at the same time if so desired.

 

[penright]

Probably more info than you care, if so look away now.

Rectifier Diode, is usually just called a diode. From a water flowing perspective think of them as a check valve. As long as the pressure is more on one side than the other water will flow. It does not matter what that pressure is, well actually it does, if it is too much difference than the valve is designed to handle, it will fail. Also it takes a certain amount before it opens. In case of diodes, too much and it can't handle the current, and the break over depends on the substrate, but usually it is in the .3 volt range. For basic discussions, just think of it as a check valve. It will not flow "backward" no matter the voltage difference, until you get an arc, but that would be a failure.

There are Zener diodes and they work the same, say .3 volts in the forward direction. But they have also have a value in the back flow direction. Say it is a 15 volt zener, then when the reverse voltage hits 15 volts, it will start flowing current, keeping the voltage at 15, until too much current burns it up.

So in the case of a diode (Rectified), looking at the expand view, current flows from positive to negative, ok let's not confused things with hole flow, then when the arrow is more positive than the bar side, you have current flowing.

BTW, the more positive (arrow) is the anode and the negative (bar) is the cathode. Physically looking at a diode, there will be an indicator, and it will indicate the cathode side.

Never understand how this matrix works.....
so if I have lamp 1 on. and also lamp 64 on at the same time.....what happened to 8 and 57 ?

So let's say all the rows and columns are at +12 volts. Then there is no current flow. But if you drop row 3 to ground(-) and keep the other rows at +, then diodes (3, 11, 19, 27, 35, 43, 51, and 58) would turn on and lights light up. Now lets say you drop all columns except 3 to ground also. Now 19 would light up. At this point if you did not have diodes 3, 11, 27, 35, 43, 51 and 58 blocking you would have some shorts.

Now to the pulsing comment. This all works find if I want just one bulb. But what if I want 19 and 28 lite up. We know 19 matrix is only row 3 (-) and only column 3 (+). For light 28 it would be only row 4 (-) and only column 4 (+). If you did row 3 and 4 (-), then column 3 and 4 (+), you would lite up 19, 27, 20, and 28. That's why you have to do it one at a time or pulse.

One other note, you could reverse the voltages and columns would be - and rows +. So it would depend on the design.

Clear as mud ...
I just hope, I got my example numbers correct. I did double check, but there been times when I double checked a circuit and still caused smoke.


Edit: @gungatim posted, while I was typing. I guess his was a lot simpler and easier to understand.

 

[dogdog]

LOL : 8x8 is not really 64 lamps now isn't it there are some limitations.

 

[American Locomotive]

You can totally control 64 lamps with an 8x8 grid of wires. The trick is you only turn one lamp on at a time, and if you switch from one lamp to another fast enough, you can give the illusion of multiple lamps being on at the same time. It's called "multiplexing", and is how the majority of displays on small consumer electronics function.

 

[Alchymist]

You can totally control 64 lamps with an 8x8 grid of wires. The trick is you only turn one lamp on at a time, and if you switch from one lamp to another fast enough, you can give the illusion of multiple lamps being on at the same time. It's called "multiplexing", and is how the majority of displays on small consumer electronics function.

Half the wires can be eliminated by using a decoding module. With the proliferation of electronics in today's vehicles, silicon is cheaper than copper.

 

[walta]

In a matrix the lamps are never on 100% the 8 column wires are pulsed 1 at a time.
If the lamp is pulsed on 1/8 of the time the filament in the lamp will get and stay hot long enough that the human eye cannot perceive the flickering.

So each column wire is pulsed on to +12 volts one at a time several times a second one after another.

When the computer want a lamp on it will ground the row wire for the moments in time when its column is sending out power.

So to turn on lamp #1 row 1 is grounded when column 1 is turned on

And to turn on # 64 row 8 is grounded when column 8 is turned on

If we need lamps 1,8 and 64 Row 1 is still grounded when column 1 is on, and row 8 is grounded when both columns 1 and 8 are on.

Things are happening pretty fast.

The diodes are necessary to keep the 12 volts from going backward thru a lamp and affecting other columns.


Walta

 

[Alchymist]

No perceptible flicker at frequencies above 50-60 Hz. Said frequencies are child's play.And electronics as I said before are cheaper than wire. Thus we have CAN, AFDX. Byteflight, ARINC 429, IDB-C, etc. OBD, Bluetooth, etc provide convenient user interfaces. Integrated system are the norm in new vehicles, combining HVAC, entertainment, navigation,etc controls in one panel. Today's typical vehicle has more computing power than the original space shuttles.

 

[Pipe Wrench]

I only got to take a closer look at the door warning light, because I wanted to wire a real puddle light to each front doot - which I did.

Now, a few weeks ago, I replaced the OEM incandescent trunk light, which was abysmally weak, with an 18-LED strip light, which is made for automotive use and which comes with bulb adaptors that replace the OEM bulb. So the LED strip light gets its power via the bulb adaptor form the OEM bulb socket. At that point I did not check the OEM trunk light assembly, so I don't know if it is also equipped with a diode. I did however make an observation:

When I unlock the trunk, the LED strip light comes on at full power, but after maybe 30 seconds it dims to maybe 50% brightness. If I then open one of the vehicles doors, the trunk light will go back to full power. No big deal, because ecven at 50% brightness output it's bright enough. The question is, why does the LED strip light get dimmed after a short while?

So I proceeded to install the same type LED strip light as puddle lights under each door panel. However, because I wanted to maintain the door warning lights, I did wire the puddle lights into the + and - wires going to the door warning light (pre-diode). This arrangement works perfectly, and the LED strip lights do not get dimmed but stay fully bright until the door is closed.

I did notice one thing: If I remove the the incandescent door warning bulb, the strip light will flicker rapidly with barely any light being emitted. So this must have to do with the comfort control module pulsing the power suplly to the light?

 

[dogdog]

I think , only you and your meter can tell or answer that question for your car....

some multi meters are capable of measuring duty cycle (PWM) or if you have any oscilloscope.

 

[Pipe Wrench]

I'll be the first to admit that electric systems are not my forté. My meter cannot measure PMW.

 

[Alchymist]

I only got to take a closer look at the door warning light, because I wanted to wire a real puddle light to each front doot - which I did.

Now, a few weeks ago, I replaced the OEM incandescent trunk light, which was abysmally weak, with an 18-LED strip light, which is made for automotive use and which comes with bulb adaptors that replace the OEM bulb. So the LED strip light gets its power via the bulb adaptor form the OEM bulb socket. At that point I did not check the OEM trunk light assembly, so I don't know if it is also equipped with a diode. I did however make an observation:

When I unlock the trunk, the LED strip light comes on at full power, but after maybe 30 seconds it dims to maybe 50% brightness. If I then open one of the vehicles doors, the trunk light will go back to full power. No big deal, because ecven at 50% brightness output it's bright enough. The question is, why does the LED strip light get dimmed after a short while?

So I proceeded to install the same type LED strip light as puddle lights under each door panel. However, because I wanted to maintain the door warning lights, I did wire the puddle lights into the + and - wires going to the door warning light (pre-diode). This arrangement works perfectly, and the LED strip lights do not get dimmed but stay fully bright until the door is closed.

I did notice one thing: If I remove the the incandescent door warning bulb, the sripl light will flicker rapidly with barely any light being emitted. So this must have to do with the comfort control module pulsing the power suplly to the light?

SCHEMATICS are made to answer these questions.

 

[Pipe Wrench]

SCHEMATICS are made to answer these questions.

I have only partial schematics for this vehicle.

I was just hoping someone had already dealt with the same system and was in the know.

 

[theoldwizard1]

You can totally control 64 lamps with an 8x8 grid of wires. The trick is you only turn one lamp on at a time, and if you switch from one lamp to another fast enough, you can give the illusion of multiple lamps being on at the same time. It's called "multiplexing", and is how the majority of displays on small consumer electronics function.

That is not I was taught in college 40+ years ago. !

Digital multiplexing reduces the wire count by using "encode" wire to select o output. In this case, 8 wires/input would sent into a mux. The output would be 3 binary encode wires (000, 001, 010, 011 ...), Those 3 wire would travel to a demuxer and out would come your 1 of 8 input. The binary encode wires could also be easily generated by a μprocessor.

Turning a light on and off quickly so that the eye does not detect is simply "persistence" both inside the bulb and in your eye.