PNP transistor circuit switch help

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
30
Hello,

I neeed help figuring out this circuit. Originaly all of the LED's and their current limiting resistors where 12V lamps, but I put LED's with resistors in their place. What it should do: if SW4 is closed, LED1 should be on, if SW3 (regardless of SW4) is closed, all LED's should be on and change brightness when R7 pot is being turned. The problem is, when SW4 is closed and SW3 is open, LED1 is on, but others glow dimly too. When there are 12V filament light bulbs instead of LEDs, there is no problem. Any ideas?

problem.png

Things to ignore: high current for LED's 2-4, as they can withstand 75mA each no problem, and limited brightness dimming range because of small value of R7.
 

dl324

Joined Mar 30, 2015
8,918
When SW4 is closed, LEDs 2-4 are on because the bias current for Q1 has to flow through them. It's low, but LEDs these days are efficient enough to emit light at very low currents.
 

GopherT

Joined Nov 23, 2012
7,983
First, a 220 ohm resistor in series with simple indicator LEDs is not big enough to limit current to below 30 mA. You now have about 45 mA. Increase those resistors to 330 or 470 ohms.

to solve your problem...
Try putting a 1k resistor in parallel with your LED2. That should make an easier path for your bias current to get to ground and the LEDs should turn off.
 

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
30
Big thanks, looks like 1k will work. :)

High current for LEDs are not a problem, because those are OSPW5111P, specs are 75mA typical, 100mA max, 150mA pulse max, and are used for lighting not indicators.
 

AnalogKid

Joined Aug 1, 2013
8,103
Do you want to resolve the issues with this circuit, or work out a different circuit that works the way you want?

ak
 

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
30
1k in paralel with LED's worked on the test bench with only LED1 and LED2 installed, it didn't work when there are 10+ LEDs in application, LED's stil glows.

I need to resolve issues with this circuit, as I can't change PCB.
 

GopherT

Joined Nov 23, 2012
7,983
1k in paralel with LED's worked on the test bench with only LED1 and LED2 installed, it didn't work when there are 10+ LEDs in application, LED's stil glows.

I need to resolve issues with this circuit, as I can't change PCB.
Change R2 to 50k and R1 to 20k. Then the bias current will, hopefully, be so small that the LEDs will not light. You will still have enough base current to light LED 1.
 

dl324

Joined Mar 30, 2015
8,918
You don't even need a transistor. Don't know how varying brightness will work when both switches are closed, but it's no worse than what you have now.
upload_2016-1-7_12-33-7.png
BTW, using R7 as a rheostat and pumping a lot of current through it could fry it when you're only using a portion of the resistance.
 

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
30
You don't even need a transistor. Don't know how varying brightness will work when both switches are closed, but it's no worse than what you have now.
With this circuit, the LED1 will not react to R7 changing.
BTW, using R7 as a rheostat and pumping a lot of current through it could fry it when you're only using a portion of the resistance.
R7 is a rheostat, not normal potentiometer, it's designed to dim about 10-15 1.2w 12V light bulbs, don't worry about it, it's not a problem.

Just checked new rules, automotive mods talk is allowed, yay. So I can't scrap PCB and start over, the schematic is a part of Audi 80 B4 instrument cluster PCB, only the cluster lighting part. So I'm adapting instrument cluster to take LED's in place of bulbs. That's why I can't just make a new PCB.

Also, as it's a car, the voltage could rise to 14V easy, when alternator is running.
 

dl324

Joined Mar 30, 2015
8,918
With this circuit, the LED1 will not react to R7 changing.
Does dimming need to work when S4 is closed?
R7 is a rheostat, not normal potentiometer, it's designed to dim about 10-15 1.2w 12V light bulbs, don't worry about it, it's not a problem.
You neglected to mention that very important detail. I guess I shouldn't worry about it more than you do...
 

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
30
Dimming have to work, when SW3 is closed, regardless of position of SW4. If only SW4 is closed, dimming should not be working. Sorry about rheostat. :)
 

dl324

Joined Mar 30, 2015
8,918
Change R2 to 50k and R1 to 20k. Then the bias current will, hopefully, be so small that the LEDs will not light. You will still have enough base current to light LED 1.
If bias current is still an issue, replace the PNP transistor with a P channel MOSFET.
 

GopherT

Joined Nov 23, 2012
7,983
If bias current is still an issue, replace the PNP transistor with a P channel MOSFET.
Or see post 10. I don't understand the need for the transistor if the switch is already there and only one LED is turned on/off.
 

AnalogKid

Joined Aug 1, 2013
8,103
When SW3 is closed, LED1 responds to variable brightness regardless of the state of SW4. This is achieved by SW3 turning off Q1, so all of LED3's current now comes through R7 via D2.

Although it is connected "backwards" (clockwise rotation decreases circuit resistance), there is nothing different or special about R7 other than its power rating.

Replacing Q1 with a P-channel MOSFET and increasing R1 and R2 looks like a good way to fix the problem without a board edit.

A small current was peed through the incandescent bulbs in their "off" state on purpose. Keeping the filaments just a little bit warm makes them supple, and increases long-term reliability in a high shock/vibration environment.

ak
 
Last edited:

dannyf

Joined Sep 13, 2015
2,196
but others glow dimly too.
They are powered on by R1/R2/R7....

When there are 12V filament light bulbs instead of LEDs, there is no problem. Any ideas?
Depends on the differences in the forward voltage drops of the various LEDs used.

The circuit is far more complicated than it needs to be, and R7 can be expensive. If I were you, I would think about changing it.
 
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