2 transistors in series

Thread Starter

FroceMaster

Joined Jan 28, 2012
702
Hi.
Have this setup where i will need up to 1,5A 5vDC.
I need to control this with a PWM from MCU,
But how can i connect just one transistor ? as i can see the transistor need at least 5v on base to operate. therefor i have connected 2 in series.
But will that work, or is there a better way ?
Udklip.JPG
 

BobTPH

Joined Jun 5, 2013
9,189
With a BJT, the base must be about 0.7V higher than the emitter. So you would need 5.7V on the base to have 5V at the emitter.

That s why your circuit cannot output 5V. Your first transistor can outout only 4.3V to its emitter. The second transistor has 4.3V on its base, so its output is 3.6V at most.
 

WBahn

Joined Mar 31, 2012
30,255
Hi.
Have this setup where i will need up to 1,5A 5vDC.
I need to control this with a PWM from MCU,
But how can i connect just one transistor ? as i can see the transistor need at least 5v on base to operate. therefor i have connected 2 in series.
But will that work, or is there a better way ?
View attachment 317193
What are the values of R11 and R12?

The max voltage that you get to your load will be you MCU pin output voltage minus ~1.4 V (possibly more in the case of 5 A from T2 (Q is the usual prefix for BJT transistors, T is usually used for transformers) minus the sum of the voltage drops across those two resistors.
 

ericgibbs

Joined Jan 29, 2010
19,026
Hi FM,
Regarding the P-MOSFET circuit, the MCU Gate voltage will have to be 0V to operate the relay, also to turn the Relay Off the MCU Gate drive output voltage will have to be equal to Vdd.

E
 

BobTPH

Joined Jun 5, 2013
9,189
If there is not a reason you cannot use a low side switch, use a low side switch and an N-channel logic level MOSFET.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
702
test.pngok, lets try to explain a little better. I have a MCU, need to control this : The TLC9225 is controlling all my LEDS, have many.
Each TLC have 7 Leds connected to EACH Out. But ony one at a time do need to light up. I am i cycling between all the 7 from 7 transistors. turning on the POWER to each row of lights. and then i need to control the "contrast level" by PWM also. Dont know how i should connect it, or what to use., i been testing a lot online sim, but do not seem to can handle it.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
702
can someone come with a transistor that will be able to do the job , controlled from MCU, 5vDC onto base. and have 5vDC on collector, and out to LED on Emitter. Can be up to 50-60 in parellel(not same resistor). and 10-20mA, so total MAX 1200mA and 6W.
 

ericgibbs

Joined Jan 29, 2010
19,026
hi FM,
A couple of questions.
Are all the LEDs the same colour?
Do you want a different current through each LED?
Will each LED have its own series resistor.?

Is there any reason why the LED's in parallel cannot be in the Collector circuit of the power transistor?


E
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
702
SAme color all LED. Same current in all. LED's resistor is controlled by TLC5925-to GND. and that's why i need to have load on Emitter . HAve been testing in sim with N-Channel Mosfet. but cant not completly understand mosfets
 

ericgibbs

Joined Jan 29, 2010
19,026
hi,
This is one option.
Choose a P MOS with a low Vg on and say at leat 3A Source to Drain current rating.
E
BTW: the 1k is for simulation ONLY, not used in final project.
 

Attachments

Last edited:

BobTPH

Joined Jun 5, 2013
9,189
Can be up to 50-60 in parellel(not same resistor). and 10-20mA, so total MAX 1200mA and 6W.
Did you miss this in the datasheet?

• Constant Output Current Range: 3mA to 45mA
You cannot have more than two LEDs at 20mA on any output. Also, connecting parallel LEDs a constant current driver does not divide the current evenly among the LEDs.

I think you are trying too hard to use a chip that is not suited for your application. 10 to 20 mA LEDs do not need a constant current driver, a simple resistor is fine.

In the OP you said there were 7 LEDs connected to each output and only one output can be on at a time. Now you saying there are 50 to 60 LEDs in parallel. Those two statements cannot both be true.

You need to state your entire problem clearly and let us help you find a solution instead of asking us to fix your design, which is either fatally flawed or not described accurately.
 
Top