Size of resistor and transistor

Thread Starter

FroceMaster

Joined Jan 28, 2012
699
Hi
Need to get this working.
have an output 12v 20ma max.
need to lid up 3 lights 3*21w
how to connect the transistor, and witch ?

Rgds.
Sten
 

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Jony130

Joined Feb 17, 2009
5,487
So use irf4905 instead.

1a.PNG
Also you 12V power supply for the bulps must be able to deliver at least 7A.
And also small heat sink for the MOSFET will also be needed.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
699
the input can only Draw 20mA. the powersupply is at least 300A.
why at least 7 A ? 3*21 = 63w. 63/12 is not more than 5,25.
 

Jony130

Joined Feb 17, 2009
5,487
the input can only Draw 20mA.
With R3 = 1K the input current is smaller than 12V/1K = 12mA.
Also you can increase R3 to 10K and R4 to 100K
But you can also remove Q2, R3, R4 , R2 but then low state (close to 0V) at the input will light the bulbs and high state (12V) will turn-off the bulbs

the powersupply is at least 300A.
Very good.

why at least 7 A ? 3*21 = 63w. 63/12 is not more than 5,25.
Just as a safety margin.
 

Alec_t

Joined Sep 17, 2013
14,280
Your new circuit should work. You could even increase the resistor value to, say, 3.3k to reduce the current draw.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
699
Use of diode . Yes good ? , really dont remember why. Its for use to turn on taillights on new cars. When deliver from factory there is no taillight in the day. Many custumor will have them on. I think it is so that left/right/licenceplate light still is on each fuse
 

Papabravo

Joined Feb 24, 2006
21,159
So the transistor will be sinking about 33 mA in the on state for 12 mA of base current. You don't need that much base current to turn the transistor on as has already been noted. You should have a resistor from the base to GND in case the driving source becomes disconnected.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
699
ok if i chance the 1k to 3.3k and where to put the extra resistor. before the base resistor or between base and resistor. and what value ?
 

Papabravo

Joined Feb 24, 2006
21,159
ok if i chance the 1k to 3.3k and where to put the extra resistor. before the base resistor or between base and resistor. and what value ?
Between the base resistor and the base of the transistor. Check the earlier schematic that was offered in this thread for the value.
 
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