hi all
ive been playing around with this design for a while and
am ready to commit to a PCB. but first could you chaps
just advise on my final setup please. basically the front
end of the circuit originally was a sound to light vellerman
kit that drove 4 LED's in series on the emitter of the BC558.
what i have done is replace them with a BD139 to a 2N3055
to drive a parallel pair of 12V 24W bulbs. the circuit will of
supplied by 12V DC 45Ahr car battery. originally i was using
a pair of TIP41C transistors driven by the BD139 with 1 bulb
driven by one TIP41 each to share the load. but i have now
found the 2N3055 which is rated a lot higher and can handle
the load with a more generous reserve. i have run the circuit
on a national instruments simulator and it seems ok. my problem
is my maths is, frankly, not up to snuff with this sort of thing, i
can do ohms law but i lack the intuitive application of said data.
so given the supplied data on the attached pic is everything ok?
also im trying to work out heatsink size, the 2N3055 will be mounted
on the exterior of the case so my effort at calculating it is as follows;
3.8A x 12V = 45.6W
Max juncion temp is 200C
total Dissapation @ Tc < 25C is 115W
junction to case Th r is 1.5C\W + 0.5C\W for the insulator and paste.
therefore
200C - 40C (worst case safety margin) / 46W (my calculation)
= 3.49C\W - 1.5C\W + 0.5C\W = 1.49C\W
so i need a heatsink rated at :ltoet: 1.49C\W
the heatsink i have found is rated at 1.65C\W
so a little higher than recommended but given the safety margin
probably ok. have i done the maths ok?
i have breadboarded the circuit but only with a 1Ahr 12V
battery so although it worked with the 1 TIP41: 1bulb pairing
design, the run time due to Amps drawn was only 35mins. but
was consistant with each attempt after recharging. a car
battery should run for at least a day. obviously as the bulbs
are flashing at a variable rate then the Amps drawn at any given
point will vary a lot so hopefully the worst case scenario is accounted
for.
all help advice gratefully received. cheers
Fenris
ive been playing around with this design for a while and
am ready to commit to a PCB. but first could you chaps
just advise on my final setup please. basically the front
end of the circuit originally was a sound to light vellerman
kit that drove 4 LED's in series on the emitter of the BC558.
what i have done is replace them with a BD139 to a 2N3055
to drive a parallel pair of 12V 24W bulbs. the circuit will of
supplied by 12V DC 45Ahr car battery. originally i was using
a pair of TIP41C transistors driven by the BD139 with 1 bulb
driven by one TIP41 each to share the load. but i have now
found the 2N3055 which is rated a lot higher and can handle
the load with a more generous reserve. i have run the circuit
on a national instruments simulator and it seems ok. my problem
is my maths is, frankly, not up to snuff with this sort of thing, i
can do ohms law but i lack the intuitive application of said data.
so given the supplied data on the attached pic is everything ok?
also im trying to work out heatsink size, the 2N3055 will be mounted
on the exterior of the case so my effort at calculating it is as follows;
3.8A x 12V = 45.6W
Max juncion temp is 200C
total Dissapation @ Tc < 25C is 115W
junction to case Th r is 1.5C\W + 0.5C\W for the insulator and paste.
therefore
200C - 40C (worst case safety margin) / 46W (my calculation)
= 3.49C\W - 1.5C\W + 0.5C\W = 1.49C\W
so i need a heatsink rated at :ltoet: 1.49C\W
the heatsink i have found is rated at 1.65C\W
so a little higher than recommended but given the safety margin
probably ok. have i done the maths ok?
i have breadboarded the circuit but only with a 1Ahr 12V
battery so although it worked with the 1 TIP41: 1bulb pairing
design, the run time due to Amps drawn was only 35mins. but
was consistant with each attempt after recharging. a car
battery should run for at least a day. obviously as the bulbs
are flashing at a variable rate then the Amps drawn at any given
point will vary a lot so hopefully the worst case scenario is accounted
for.
all help advice gratefully received. cheers
Fenris