# The transistor and led driver problems.

Discussion in 'General Electronics Chat' started by FroceMaster, Dec 18, 2014.

1. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
hi
now it look odd. but will this Work
when i aktivate t2-t4 will the leds light up ?
t2-t3-t4 have 60 pair of leds and by selecting T1-t5-t6 each Group will light up

what i eant to know is if i can connect the transistors in series like i did ?

rgds
Sten

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2. ### ScottWang Moderator

Aug 23, 2012
4,782
762
1. Please modify the title as : The transistor and led driver problems.
2. This circuit is not a completed circuit, all the Base of transistor need to in series with a current limiting resistor.
3. T1, T5,T6 are the in parallel control, the input voltages should be great than 0.7V+0.2V or some more.
4. T2, T3, T4 are the in series control, the input voltages should be great than 0.7V.
5. Whatever you want to control T2 or T3 or T4, the first thing that you have to do is to turn on the common input of T1 , T5, T6.
6. If you choosing the correct values of R1~R15 then the circuit should be work.

3. ### #12 Expert

Nov 30, 2010
16,023
6,537
Yes, you can use 2 transistors in series to make switches. Just be careful to limit their base current with one resistor each.

4. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,522
2,342
Yes, you can connect transistors in series, but, why would you want to do that? Personally I'd get rid of T2, T3 and T4, (other than the fact that you shouldn't leave their gates floating, as shown in your diagram).
Connect a 2K resistor in series with the common line to the gates of T1, T5 and T6 and that should limit the current going into them.
Also, if you have 60 pairs of LEDs (do you mean 120 total?) make sure that your 5V power supply can deliver enough current.... ie, if you were to use a value of 270Ω for each LED, and considering a voltage drop per LED of 1.8V, then each led would consume around 12ma, and this is not considering what the transistors would consume themselves. So if you're talking about 120 LEDs, the total current drawn would be at least about an Amp and a half... that's 7.5 watts!
Finally, assuming that you're drawing that current from a 12V wall wart, and that you have it connected to a 7805 regulator, I'd suggest two things:
1. That you make sure that your 12V wall wart is capable of delivering at least 2 amps
2. That you use at least four 7805 connected in parallel so that they won't heat up too much, or you could use only three of them if you were to install heatsinks on them

5. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,522
2,342
Also, keep in mind that an ordinary BC337 transistor can only handle about 500ma of current. But personally, I'd design my circuit to handle half of that, tops, or they could overheat. So under the described conditions each transistor would be limited to lighting up to 40 LEDs

6. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
Ok, have alter the Circuit a Little.
By turning on T1 , i select 1 row of led.
By turning on Q1 i turn on the first 5 leds.
By turning on Q2 or Q3 i get each other sets of LED.
If turning on T2 i select 2. row of LEDS, ( will have up to 60 sets of LEDS,, only one set on at a time )

Is this useable or how should i connect the Q1 , to control the +5V

Total max leds turn on at a time will be 30. and never more. sometimes even just 15,

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7. ### ScottWang Moderator

Aug 23, 2012
4,782
762
If you used so many LEDs then you better using mosfet to reducing the power consuming on bjt.

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8. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
It will be controlled by multiplexing. The pnp will only be on with max total of 15 led (15×10 mA) 150 mA
depending of the Npn's

the Npn's will only draw 50 mA

The resistor base will be 1k5 or 2k2

not quite sure how to connect the pnp's base. To 0v =on and 5v= off ??? OR ??

9. ### ScottWang Moderator

Aug 23, 2012
4,782
762
If you using uC then you have to adding the Rbe for each pnp to turn off the pnp quickly, and the initial program should be send high for each pnp inputs and send low for each npn inputs.

Send a low level to the inputs of Q1, Q2, Q3 will turn them off, and send a high level will turn them on.

10. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
Have now this schematic., have changes the LED to be in series. due to Space on PCB.
if i connect 12v to the Collector i will get 12v into my MCU. ( Not good).
see "PNP and MCU"
How can that be fixed ?

could i connect like in "instaed of PNP.png"

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• ###### PNP and MCU.png
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Apr 5, 2008
15,532
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Hello,

In your schematic "PNP and MCU" the PNP transistor will always be in conduction.
The difference between 0 and 5 volts of the MCU will be minor.
To switch off the PNP the voltage on the base resistor of the PNP should be 12 Volts.

Bertus

12. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
ok, how should i then do it ?
Use a relay instead of Q1-3 or ? ( not solution , noisy )

Apr 5, 2008
15,532
2,300
Hello,

You will need a NPN transistor to act as level shifter.
You already posted a circuit, I added an extra resistor to have the PNP shut off.

Bertus

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14. ### ScottWang Moderator

Aug 23, 2012
4,782
762
Please see the circuit as Fig-07, or you want to moving to become Rbe_Q2.

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15. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
The drawing bertus altered and fig 07 is not completly the same. Wich to use ??

16. ### ScottWang Moderator

Aug 23, 2012
4,782
762
I Already said that both can be used, they all can be avoid the pnp become a floating status.

17. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
Remodeling Again,, will the multiplexing thing with the LED/ transistor Work. ?

Ex. if i want to turn on the LEDS in the middle, i set output 2 and 5 high from my MCU. ?

seems like a waiste of outputs, but when i need to light on 180 different sets of LEDs it will be ok to control it by just 63 outputs.

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18. ### ScottWang Moderator

Aug 23, 2012
4,782
762
The circuit seems ok now, but since you already used 4 npn bjts that you may want to using uln2003 to replacing them.

19. ### FroceMaster Thread Starter Member

Jan 28, 2012
398
3
I know. And in final layout i will be using that. 8 out from mcu. 3 to select wich is 64 outs. (60 using) and then the 3 pnp to multiply to 180 different sets of LED.
8+3+3 is 14 outs from mcu.

I just needed to be sure that i Could connect it that way.