Need help choosing a resistor again.

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
I have a circuit board from an electronic 4WD switch.
It had two "regular" green LEDs, not superbright or anything, just your average 3mm indicator LED. I don't know what the voltage of these LEDs are but the system voltage is 14.5V, and the LEDs use 690 Ohm resistors.

I replaced the green LEDs with super bright blue LEDs (3.4V, 20mA) but let the resistors in place.

The problem I'm experiencing now is when the system voltage is 12-14.5 V the blue LEDs I swapped in remain lightly lit when they should be off.

Which resistor value should I replace the 690 Ohm resistors with so that these blue LEDs are fully off when they're supposed to be off.
I may just experiment with it and drop in a higher value resistor. The LEDs do NOT need to be at full brightness when they're "on", they just need to be completely off when they're supposed to be off.
 

dl324

Joined Mar 30, 2015
11,833
We need to see a schematic of the circuit.

Substituting superbright LEDS for standard can cause problems because the former are much brighter at low current levels.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
it worked everywhere else, I wasn't aware that there was any current draw on this circuit when it's supposed to be off until I put it back and turned on the vehicle.

Which is why I need to replace the resistors with a higher value.
 

AnalogKid

Joined Aug 1, 2013
8,676
The problem I'm experiencing now is when the system voltage is 12-14.5 V the blue LEDs I swapped in remain lightly lit when they should be off.
Why should they be off? 12 V, resistor, LED, ground. Sounds like normal operation to me. A blue LED has a higher forward voltage than a green one, but not nearly high enough to be off with 12 V applied through a reasonable resistor. In your case there is about 12 mA through the blue LED, more than enough for it to shine. Maybe a wiring diagram sketch would help.

ak
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
It's a 4WD switch, I don't exactly have a schematic. there are 3 LEDs in total. a Green one which is on when in 2WD mode. The other two LEDs (the blue ones) shouldn't be on unless either 4HI or 4LO is selected, in which case the green one turns off, and whichever mode is selected has it's LED lit to indicate which mode you're in, leaving two of the LEDs off.

So at any given time there should only ever be one LED lit depending on which mode you're in. I didn't replace the green 2WD LED.

also I was incorrect, the two I replaced were orange not green. My mistake. It originally had 1 green, and 2 orange LEDs. I replaced thw two orange with blue.
 

ElectricSpidey

Joined Dec 2, 2017
1,322
I would measure the voltage at the diodes when they are supposed to be off, and then place a Zener diode with a voltage of about 1 volt higher in series with the resistor and LED. (in reverse polarity)
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
There's no space for that, my singular option is to swap the resistors with a higher value.

I can't power the circuit out of the truck because multiple pins need to have power and I don't have a spare harness, but most importantly the rotary switch is part of the case and without it making contact the board won't work properly to test the LEDs.....so it has to be in it's case to work, but can't be tested when it's in the case....catch 22.

The two resistors I need to swap are the horizontal one that has a black mark above it, and the vertical one directly to the right of it.

They're very dim when they're "off" so I shouldn't need a much higher value resistor. It wouldn't be bad for them to be dimmer at full power....currently they're less "indicator" and more "spotlight" lol.
IMG_7434[1].JPG
 

ElectricSpidey

Joined Dec 2, 2017
1,322
Did you buy spares?

If you did, just place one on a breadboard and see how high you can go with the resistance, I would start with 2.2k and go up from there.

Be sure to use the correct voltage, when they are "on" when you test.
 

ebeowulf17

Joined Aug 12, 2014
3,274
There's no space for that, my singular option is to swap the resistors with a higher value.

I can't power the circuit out of the truck because multiple pins need to have power and I don't have a spare harness, but most importantly the rotary switch is part of the case and without it making contact the board won't work properly to test the LEDs.....so it has to be in it's case to work, but can't be tested when it's in the case....catch 22.

The two resistors I need to swap are the horizontal one that has a black mark above it, and the vertical one directly to the right of it.

They're very dim when they're "off" so I shouldn't need a much higher value resistor. It wouldn't be bad for them to be dimmer at full power....currently they're less "indicator" and more "spotlight" lol.
View attachment 153908
If it feels like a spotlight right now, you can pretty safely increase the resistance a LOT!!!

Doubling resistance won't look like much change. Even 4-5x resistance probably isn't going to be night and day.

Unless you've got a neighbourhood electronics shop to buy from, I'd recommend getting a variety of potential resistor options all in one order (shipping for each order costs more than all the resistors, so you only want to order once.) I'd order a progression of values starting at 2-4x the current value, and ending around 10-20x the current value, then just try some out till you're satisfied.

The previous comment about breadboard testing is wise too - would save a lot of soldering during experimentation. So you might want one spare LED if you don't already have it.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
I've got almost every value of resistor known to man from my college kit, and there is actually an amazing shop here that has all of these parts an a massive shop full of other equipment even salvaged/auctioned hospital lasers and such.

Anyway interesting problem. I dropped in 7.5K resistors, and it didn't actually make much of a change, on top of that the darn things are still dimly lit when they're supposed to be completely off.

I'll have to see if I can find the orange ones I removed and test them or something.

Every other LED I swapped out has worked great so far. This is the only one that's causing a "problem".

I'm going to see if the pinout says anything useful.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
Looks like there's a 5V supply to the switch constantly..... which I personally find a little odd in an automotive application..... this was just supposed to be a simple resistor swap. Perhaps then this should be moved to the automotive section at this point.

TRANSFER CASE SELECTOR SWITCH - 12 WAY


CAV CIRCUIT FUNCTION
1 T322 20DG/GY 5 VOLT SELECTOR SWITCH SUPPLY
2 G2 20BR/WT IGNITION SWITCH SENSE
3 - -
4 G95 20VT/BR NEUTRAL INDICATOR
5 K977 20BR/WT MODE SELECT
6 E11 20OR/DB PANEL LAMPS DRIVER
7 F21 20PK/DG FUSED IGNITION SWITCH OUTPUT (RUN-START)
8 G92 20VT/OR 4WD LOW INDICATOR
9 G91 20VT/WT 4WD HIGH INDICATOR
10 - -
11 Z905 20BK GROUND
12 -
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
so applying full power to the panel lamps indicator pin lights up the green 2WD LED, but not the blue 4HI and 4LO LEDs......

applying 5V to the supply pin does nothing at all.
if I apply power to the 4LO and 4HI indicator pins, the LEDs don't light up.

......... so this is a rather interesting problem. I may just have to live with it for now.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
Just to confirm:
1. Everything worked correctly before changing the LEDs?
2. The only change was the LEDs?

ak
ya it was just LEDs that were replaced. The switch itself still works fine and has no issues switching modes and everthing changes over as it should.

It's just the LEDs staying on when they should be off.

in all other cases it was a simple matter of just removing an LED, and popping in a new one.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
you know, id even settle for just "regular" blue LEDs instead of ultrabright ones. Ideally ultrabright aren't the best choice for these indicator lights, and perhaps using a regular version would prevent this constant illumination problem.
 

AnalogKid

Joined Aug 1, 2013
8,676
It might be that the LEDs are fine, but the dis-assembly/re-assembly caused an issue with the switch contacts, the wiper, connecting wires, etc.

ak
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
I don't really think that's the case, at least at this point anyway.

If you follow through the traces it looks like there are multiple paths to illuminate the leds. Auto manufacturers do some really odd things sometimes, but I'm sure they have their reasons.

Do you happen to know where I could buy some plain blue LEDs for indicators? through hole, 3mm? I'm going to pickup a second 4WD switch to put in the truck while I mess about with this one and see if non-superbright LEDs would have a different result.

as it stands the switch still functions properly, I'm able to switch into every available mode as normal, 2WD, 4Hi, 4Lo, and 4 Neutral.

I'm half tempted to replace every resistor on the board with a different value, but given that there were only 4 (I think) 690 Ohm resistors, and 4 LEDs (1x green, 2x orange and one red for neutral mode)
The other resistors are a different value but don't have the switch handy to check the value.

so yeah....any source for "normal" blue LEDs ? I see lots on google at various places, just not sure which might be the best.

Also is there anyway to test the stock orange LEDs to determine their operating voltage?
 

dl324

Joined Mar 30, 2015
11,833
Also is there anyway to test the stock orange LEDs to determine their operating voltage?
This question doesn't make sense. Their operating voltage will be less then the 12V from the vehicle. You control the current using a current limiting resistor and allow current to vary as the nominal 12V supply changes.

If you want a specific brightness, you drive them with a current source.
 

Thread Starter

GRNDPNDR

Joined Mar 1, 2012
540
ya I know that, but super bright LEDs are brighter at lower currents are they not?

At this point what's to lose by trying regular LEDs?? I wouldn't be at all surprised if they worked as intended..... I told you, auto manufacturers do weird things, and it at least appears that there are multiple paths to provide power to the LEDs on the board. so any one of them could keep the superbrights lit up.....and I could be wrong on that.

So why not just use a regular LED without replacing any resistors and see what happens? Worst thing that could happen is that they still remain lit.

This still wouldn't provide any explanation as to why the stock orange LEDs do not remain lit up.
 

BobTPH

Joined Jun 5, 2013
2,576
You could try rewiring the LED between the indicator output and the +5V supply. It sounds like maybe the LED is switched between 5V and 12V.

Bob
 
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